Human Evolution - biology.

Human Evolution - biology. I INTRODUCTION Human Evolution, lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people evolved over a period of at least 6 million years. One of the earliest defining human traits, bipedalism--walking on two legs as the primary form of locomotion--evolved more than 4 million years ago. Other important human characteristics--such as a large and complex brain, the ability to make and use tools, and the capacity for language--developed more recently. Many advanced traits--including complex symbolic expression, such as art, and elaborate cultural diversity--emerged mainly during the past 100,000 years. Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa--chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas--share a common ancestor that lived sometime between 8 million and 6 million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 million and 2 million years ago come entirely from Africa. Most scientists distinguish among 12 to 19 different species of early humans. Scientists do not all agree, however, about how the species are related or which ones simply died out. Many early human species--probably the majority of them--left no descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species. Early humans first migrated out of Africa into Asia probably between 2 million and 1.7 million years ago. They entered Europe somewhat later, generally within the past 1 million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years. The scientific study of human evolution is called paleoanthropology. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, as well as the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world came to be. Yet many people have come to reconcile such beliefs with the scientific evidence. II THE PROCESS OF EVOLUTION All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring--that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens. The mechanism for evolutionary change resides in genes--the basic units of heredity. Genes affect how the body and behavior of an organism develop during its life. The information contained in genes can change--a process known as mutation. The way particular genes are expressed--how they affect the body or behavior of an organism--can also change. Over time, genetic change can alter a species's overall way of life, such as what it eats, how it grows, and where it can live. Genetic changes can improve the ability of organisms to survive, reproduce, and, in animals, raise offspring. This process is called adaptation. Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population--a group of organisms of the same species that share a particular local habitat. Many factors can favor new adaptations, but changes in the environment often play a role. Ancestral human species adapted to new environments as their genes changed, altering their anatomy (physical body structure), physiology (bodily functions, such as digestion), and behavior. Over long periods, evolution dramatically transformed humans and their ways of life. Geneticists estimate that the human line began to diverge from that of the African apes between 8 million and 5 million years ago (paleontologists have dated the earliest human fossils to at least 6 million years ago). This figure comes from comparing differences in the genetic makeup of humans and apes, and then calculating how long it probably took for those differences to develop. Using similar techniques and comparing the genetic variations among human populations around the world, scientists have calculated that all people may share common genetic ancestors that lived sometime between 290,000 and 130,000 years ago. III CHARACTERISTICS, CLASSIFICATION, AND EVOLUTION OF THE PRIMATES Humans belong to the scientific order named Primates, a group of over 230 species of mammals that also includes lemurs, lorises, tarsiers, monkeys, and apes. Modern humans, early humans, and other species of primates all have many similarities as well as some important differences. Knowledge of these similarities and differences helps scientists to understand the roots of many human traits, as well as the significance of each step in human evolution. All primates, including humans, share at least part of a set of common characteristics that distinguish them from other mammals. Many of these characteristics evolved as adaptations for life in the trees, the environment in which earlier primates evolved. These include more reliance on sight than smell; overlapping fields of vision, allowing stereoscopic (three-dimensional) sight; limbs and hands adapted for clinging on, leaping from, and swinging on tree trunks and branches; the ability to grasp and manipulate small objects (using fingers with nails instead of claws); large brains in relation to body size; and complex social lives. The scientific classification of primates reflects evolutionary relationships among individual species and groups of species. Strepsirhine (meaning 'turned-nosed') primates--of which the living representatives include lemurs, lorises, and other groups of species all commonly known as prosimians--evolved earliest and are the most primitive forms of primates. The earliest monkeys and apes evolved from ancestral haplorhine (meaning 'simple-nosed') primates, of which the most primitive living representative is the tarsier. Humans evolved from ape ancestors. Tarsiers have traditionally been grouped with prosimians, but many scientists now recognize that tarsiers, monkeys, and apes share some distinct traits, and group the three together. Monkeys, apes, and humans--who share many traits not found in other primates--together make up the suborder Anthropoidea. Apes and humans together make up the superfamily Hominoidea, a grouping that emphasizes the close relationship among the species of these two groups. A Strepsirhines Strepsirhines are the most primitive types of living primates. The last common ancestors of strepsirhines and other mammals--creatures similar to tree shrews and classified as Plesiadapiformes--evolved at least 65 million years ago. The earliest primates evolved by about 55 million years ago, and fossil species similar to lemurs evolved during the Eocene Epoch (about 55 million to 38 million years ago). Strepsirhines share all of the basic characteristics of primates, although their brains are not particularly large or complex and they have a more elaborate and sensitive olfactory system (sense of smell) than do other primates. B Haplorhines B1 Tarsiers Tarsiers are the only living representatives of a primitive group of primates that ultimately led to monkeys, apes, and humans. Fossil species called omomyids, with some traits similar to those of tarsiers, evolved near the beginning of the Eocene, followed by early tarsier-like primates. While the omomyids and tarsiers are separate evolutionary branches (and there are no living omomyids), they both share features having to do with a reduction in the olfactory system, a trait shared by all haplorhine primates, including humans. B2 Anthropoids The anthropoid primates are divided into New World (South America, Central America, and the Caribbean Islands) and Old World (Africa and Asia) groups. New World monkeys--such as marmosets, capuchins, and spider monkeys--belong to the infraorder of platyrrhine (broad-nosed) anthropoids. Old World monkeys and apes belong to the infraorder of catarrhine (downward-nosed) anthropoids. Since humans and apes together make up the hominoids, humans are also catarrhine anthropoids. B2a The First Catarrhine Primates The first catarrhine primates evolved between 50 million and 33 million years ago. Most primate fossils from this period have been found in a region of northern Egypt known as Al Fayy? m (or the Fayum). A primate group known as Propliopithecus, one lineage of which is sometimes called Aegyptopithecus, had primitive catarrhine features--that is, it had many of the basic features that Old World monkeys, apes, and humans share today. Scientists believe, therefore, that Propliopithecus resembles the common ancestor of all later Old World monkeys and apes. Thus, Propliopithecus may also be considered an ancestor or a close relative of an ancestor of humans. B2b Hominoids Hominoids evolved during the Miocene Epoch (24 million to 5 million years ago). Among the oldest known hominoids is a group of primates known by its genus name, Proconsul. Species of Proconsul had features that suggest a close link to the common ancestor of apes and humans--for example, the lack of a tail. The species Proconsul heseloni lived in the trees of dense forests in eastern Africa about 20 million years ago. An agile climber, it had the flexible backbone and narrow chest characteristic of monkeys, but also a wide range of movement in the hip and thumb, traits characteristic of apes and humans. Large ape species had originated in Africa by 23 million or 22 million years ago. By 15 million years ago, some of these species had migrated to Asia and Europe over a land bridge formed between the Africa-Arabian and Eurasian continents, which had previously been separated. See also Plate Tectonics: Continental Drift. Early in their evolution, the large apes underwent several radiations--periods when new and diverse species branched off from common ancestors. Following Proconsul, the ape genus Afropithecus evolved about 18 million years ago in Arabia and Africa and diversified into several species. Soon afterward, three other ape genera evolved--Griphopithecus of western Asia about 16.5 million years ago, the earliest ape to have spread from Africa; Kenyapithecus of Africa about 15 million years ago; and Dryopithecus of Europe about 12 million years ago. Scientists have not yet determined which of these groups of apes may have given rise to the common ancestor of modern African apes and humans. Scientists do not all agree about the appropriate classification of hominoids. They group the living hominoids into either two or three families: Hylobatidae, Hominidae, and sometimes Pongidae. Hylobatidae consists of the small or so-called lesser apes of Southeast Asia, commonly known as gibbons and siamangs. The Hominidae (hominids) include humans and, according to some scientists, the great apes. For those who include only humans among the Hominidae, all of the great apes, including the orangutans of Southeast Asia, belong to the family Pongidae. In the past only humans were considered to belong to the family Hominidae, and the term hominid referred only to species of humans. Today, however, genetic studies support placing all of the great apes and humans together in this family and the placing of African apes--chimpanzees and gorillas--together with humans at an even lower level, or subfamily. According to this reasoning, the evolutionary branch of Asian apes leading to orangutans, which separated from the other hominid branches by about 13 million years ago, belongs to the subfamily Ponginae. The ancestral and living representatives of the African ape and human branches together belong to the subfamily Homininae (sometimes called hominines). Lastly, the line of early and modern humans belongs to the tribe (classificatory level above genus) Hominini, or hominins. This order of classification corresponds with the genetic relationships among ape and human species. It groups humans and the African apes together at the same level in which scientists group together, for example, all types of foxes, all buffalo, or all flying squirrels. Within each of these groups, the species are very closely related. However, in the classification of apes and humans the similarities among the names hominoid, hominid, hominine, and hominin can be confusing. In this article the term early human refers to all species of the human family tree since the divergence from a common ancestor with the African apes. Popular writing often still uses the term hominid to mean the same thing. C Humans as Primates About 98.5 percent of the genes in people and chimpanzees are identical, making chimps the closest living biological relatives of humans. This does not mean that humans evolved from chimpanzees, but it does indicate that both species evolved from a common ape ancestor. Orangutans, the great apes of Southeast Asia, differ much more from humans genetically, indicating a more distant evolutionary relationship. Modern humans have a number of physical characteristics reflective of an ape ancestry. For instance, people have shoulders with a wide range of movement and fingers capable of strong grasping. In apes, these characteristics are highly developed as adaptations for brachiation--swinging from branch to branch in trees. Although humans do not brachiate, the general anatomy from that earlier adaptation remains. Both people and apes also have larger brains and greater cognitive abilities than do most other mammals. Human social life, too, shares similarities with that of African apes and other primates--such as baboons and rhesus monkeys--that live in large and complex social groups. Group behavior among chimpanzees, in particular, strongly resembles that of humans. For instance, chimps form long-lasting attachments with each other; participate in social bonding activities, such as grooming, feeding, and hunting; and form strategic coalitions with each other in order to increase their status and power. Early humans also probably had this kind of elaborate social life. However, modern humans fundamentally differ from apes in many significant ways. For example, as intelligent as apes are, people's brains are much larger and more complex, and people have a unique intellectual capacity and elaborate forms of culture and communication. In addition, only people habitually walk upright, can precisely manipulate very small objects, and have a throat structure that makes speech possible. IV THE FIRST HUMANS: AUSTRALOPITHECINES By around 6 million years ago in Africa, an apelike species had evolved with two important traits that distinguished it from apes: (1) small canine, or eye, teeth (teeth next to the four incisors, or front teeth) and (2) bipedalism--that is, walking on two legs as the primary form of locomotion. Scientists refer to these earliest human species as australopithecines, or australopiths for short. The earliest australopith species known today belong to three genera: Sahelanthropus, Orrorin, and Ardipithecus. Other species belong to the genus Australopithecus and, by some classifications, Paranthropus. The name australopithecine translates literally as "southern ape," in reference to South Africa, where the first known australopith fossils were found. The Great Rift Valley, a region in eastern Africa in which past movements in Earth's crust have exposed ancient deposits of fossils, has become famous for its australopith finds. Countries in which scientists have found australopith fossils include Ethiopia, Tanzania, Kenya, South Africa, and Chad. Thus, australopiths ranged widely over the African continent. A From Ape to Human Fossils from several different early australopith species that lived between 4 million and 2 million years ago clearly show a variety of adaptations that mark the transition from ape to human. The very early period of this transition, prior to 4 million years ago, remains poorly documented in the fossil record, but those fossils that do exist show the most primitive combinations of ape and human features. Fossils reveal much about the physical build and activities of early australopiths, but not everything about outward physical features such as the color and texture of skin and hair, or about certain behaviors, such as methods of obtaining food or patterns of social interaction. For these reasons, scientists study the living great apes--particularly the African apes--to better understand how early australopiths might have looked and behaved, and how the transition from ape to human might have occurred. For example, australopiths probably resembled the great apes in characteristics such as the shape of the face and the amount of hair on the body. Australopiths also had brains roughly equal in size to those of the great apes, so they probably had apelike mental abilities. Their social life probably resembled that of chimpanzees. B Australopith Characteristics Most of the distinctly human physical qualities in australopiths related to their bipedal stance. Before australopiths, no mammal had ever evolved an anatomy for habitual upright walking. Australopiths also had small canine teeth, as compared with long canines found in almost all other catarrhine primates. Other characteristics of australopiths reflected their ape ancestry. They had a low cranium behind a projecting face, and a brain size of 390 to 550 cu cm (24 to 34 cu in)--in the range of an ape's brain. The body weight of australopiths, as estimated from their bones, ranged from 27 to 49 kg (60 to 108 lb), and they stood 1.1 to 1.5 m (3.5 to 5 ft) tall. Their weight and height compare closely to those of chimpanzees (chimp height measured standing). Some australopith species had a large degree of sexual dimorphism--males were much larger than females--a trait also found in gorillas, orangutans, and some other primates. Australopiths also had curved fingers and long thumbs with a wide range of movement. In comparison, the fingers of apes are longer, more powerful, and more curved, making them extremely well adapted for hanging and swinging from branches. Apes also have very short thumbs, which limits their ability to manipulate small objects. Paleoanthropologists speculate as to whether the long and dexterous thumbs of australopiths allowed them to use tools more efficiently than do apes. B1 Bipedalism The anatomy of australopiths shows a number of adaptations for bipedalism, in both the upper and lower body. Adaptations in the lower body included the following: The australopith ilium, or pelvic bone, which rises above the hip joint, was much shorter and broader than it is in apes. This shape enabled the hip muscles to steady the body during each step. The australopith pelvis also had a bowl-like shape, which supported the internal organs in an upright stance. The upper legs angled inward from the hip joints, which positioned the knees to better support the body during upright walking. The legs of apes, on the other hand, are positioned almost straight down from the hip, so that when an ape walks upright for a short distance, its body sways from side to side. Australopiths also had shorter and less flexible toes than do apes. The toes worked as rigid levers for pushing off the ground during each bipedal step. Other adaptations occurred above the pelvis. The australopith spine had an S-shaped curve, which shortened the overall length of the torso and gave it rigidity and balance when standing. By contrast, apes have a relatively straight spine. The australopith skull also had an important adaptation related to bipedalism. The opening at the bottom of the skull through which the spinal cord attaches to the brain, called the foramen magnum, was positioned more forward than it is in apes. This position set the head in balance over the upright spine. Australopiths clearly walked upright on the ground, but paleoanthropologists debate whether the earliest humans also spent a significant amount of time in the trees. Certain physical features indicate that they spent at least some of their time climbing in trees. Such features include their curved and elongated fingers and elongated arms. However, their fingers, unlike those of apes, may not have been long enough to allow them to brachiate through the treetops. Study of fossil wrist bones suggests that early australopiths had the ability to lock their wrists, preventing backward bending at the wrist when the body weight was placed on the knuckles of the hand. This could mean that the earliest bipeds had an ancestor that walked on its knuckles, as African apes do. B2 Small Canine Teeth Compared with apes, humans have very small canine teeth. Apes--particularly males--have thick, projecting, sharp canines that they use for displays of aggression and as weapons to defend themselves. The oldest known bipeds, who lived at least 6 million years ago, still had large canines by human standards, though not as large as in apes. By 4 million years ago australopiths had developed the human characteristic of having smaller, flatter canines. Canine reduction might have related to an increase in social cooperation among humans and an accompanying decrease in the need for males to make aggressive displays. The australopiths can be divided into an early group of species, known as gracile australopiths, which arose prior to 3 million years ago; and a later group, known as robust australopiths, which evolved after 3 million years ago. The gracile australopiths--of which several species evolved between 4.5 million and 3 million years ago--generally had smaller teeth and jaws. The later-evolving robusts had larger faces with large jaws and molars (cheek teeth). These traits indicate powerful and prolonged chewing of food, and analyses of wear on the chewing surface of robust australopith molar teeth support this idea. Some fossils of early australopiths have features resembling those of the later species, suggesting that the robusts evolved from one or more gracile ancestors. C Early Australopiths Paleoanthropologists recognize at least eight species of early australopiths. These include the three earliest established species, which belong to the genera Sahelanthropus, Orrorin, and Ardipithecus, a species of the genus Kenyanthropus, and four species of the genus Australopithecus. C1 Sahelanthropus tchadensis The oldest known australopith species is Sahelanthropus tchadensis. Fossils of this species were first discovered in 2001 in northern Chad, Central Africa, by a research team led by French paleontologist Michel Brunet. The researchers estimated the fossils to be between 7 million and 6 million years old. One of the fossils is a cracked yet nearly complete cranium that shows a combination of apelike and humanlike features. Apelike features include small brain size, an elongated brain case, and areas of bone where strong neck muscles would have attached. Humanlike features include small, flat canine teeth, a short middle part of the face, and a massive brow ridge (a bony, protruding ridge above the eyes) similar to that of later human fossils. The opening where the spinal cord attaches to the brain is tucked under the brain case, which suggests that the head was balanced on an upright body. It is not certain that Sahelanthropus walked bipedally, however, because bones from the rest of its skeleton have yet to be discovered. Nonetheless, its age and humanlike characteristics suggest that the human and African ape lineages had divided from one another by at least 6 million years ago. In addition to reigniting debate about human origins, the discovery of Sahelanthropus in Chad significantly expanded the known geographic range of the earliest humans. The Great Rift Valley and South Africa, from which almost all other discoveries of early human fossils came, are apparently not the only regions of the continent that preserve the oldest clues of human evolution. C2 Orrorin tugenensis Orrorin tugenensis lived about 6 million years ago. This species was discovered in 2000 by a research team led by French paleontologist Brigitte Senut and French geologist Martin Pickford in the Tugen Hills region of central Kenya. The researchers found more than a dozen early human fossils dating between 6.2 million and 6 million years old. Among the finds were two thighbones that possess a groove indicative of an upright stance and bipedal walking. Although the finds are still being studied, the researchers consider these thighbones to be the oldest evidence of habitual two-legged walking. Fossilized bones from other parts of the skeleton show apelike features, including long, curved finger bones useful for strong grasping and movement through trees, and apelike canine and premolar teeth. Because of this distinctive combination of ape and human traits, the researchers gave a new genus and species name to these fossils, Orrorin tugenensis, which in the local language means "original man in the Tugen region." The age of these fossils suggests that the divergence of humans from our common ancestor with chimpanzees occurred before 6 million years ago. C3 Ardipithecus ramidus In 1994 an Ethiopian member of a research team led by American paleoanthropologist Tim White discovered human fossils estimated to be about 4.4 million years old. White and his colleagues gave their discovery the name Ardipithecus ramidus. Ramid means "root" in the Afar language of Ethiopia and refers to the closeness of this new species to the roots of humanity. At the time of this discovery, the genus Australopithecus was scientifically well established. White devised the genus name Ardipithecus to distinguish this new species from other australopiths because its fossils had a very ancient combination of apelike and humanlike traits. More recent finds indicate that this species may have lived as early as 5.8 million to 5.2 million years ago. It has been suggested, however, that these older fossils may represent a related species called Ardipithecus kadabba. The teeth of Ardipithecus ramidus had a thin outer layer of enamel--a trait also seen in the African apes but not in other australopith species or most older fossil apes. This trait suggests a fairly close relationship with an ancestor of the African apes. In addition, the skeleton shows strong similarities to that of a chimpanzee but has slightly reduced canine teeth and adaptations for bipedalism. C4 Australopithecus anamensis In 1965 a research team from Harvard University discovered a single arm bone of an early human at the site of Kanapoi in northern Kenya. The researchers estimated this bone to be 4 million years old, but could not identify the species to which it belonged or return at the time to look for related fossils. It was not until 1994 that a research team, led by British-born Kenyan paleoanthropologist Meave Leakey, found numerous teeth and fragments of bone at the site that could be linked to the previously discovered fossil. Leakey and her colleagues determined that the fossils were those of a very primitive species of australopith, which was given the name Australopithecus anamensis. Researchers have since found other A. anamensis fossils at nearby sites, dating between about 4.2 million and 3.9 million years old. The skull of this species appears apelike, while its enlarged tibia (lower leg bone) indicates that it supported its full body weight on one leg at a time, as in regular bipedal walking. C5 Australopithecus afarensis Australopithecus anamensis was quite similar to another, much better-known species, A. afarensis, a gracile australopith that thrived in eastern Africa between about 3.8 million and 3 million years ago. The most celebrated fossil of this species, known as Lucy, is a partial skeleton of a female discovered by American paleoanthropologist Donald Johanson in 1974 at Hadar, Ethiopia. Lucy lived 3.2 million years ago. Scientists have identified several hundred fossils of A. afarensis from Hadar, including a collection representing at least 13 individuals of both sexes and various ages, all from a single site. One of the most complete specimens of A. afarensis found so far was announced in 2006. A team led by Ethiopian scientist Zeresenay Alemseged unearthed the partial skeleton of a three-year-old female at Dikika in the Afar region of Ethiopia. Nicknamed "Selam," the Dikika child dates from around 3.3 million years ago. The wellpreserved bones provide previously undocumented details of the skull and skeleton. Some features such as the shape of the shoulder blades, the long, curved fingers, and the semicircular ear canals involved in balance are more apelike, suggesting an adaptation for climbing trees. However, the leg bones and feet indicate an ability to walk upright even at an early age. The shape of the brain was preserved and its size indicates the species grew to adulthood more slowly than chimpanzees, a characteristic of later hominids, including modern humans. The hyoid bone that supports the tongue was found, as well. The bone is crucial to speech in modern humans but the shape in the Dikika child is like that found in modern great apes, and not humans. Researchers working in northern Tanzania have also found fossilized bones of A. afarensis at Laetoli. This site, dated at 3.6 million years old, is best known for its spectacular trails of bipedal human footprints. Preserved in hardened volcanic ash, these footprints were discovered in 1978 by a research team led by British paleoanthropologist Mary Leakey. They provide irrefutable evidence that australopiths regularly walked bipedally. Paleoanthropologists have debated interpretations of the characteristics of A. afarensis and its place in the human family tree. One controversy centers on the Laetoli footprints, which some scientists believe show that the foot anatomy and gait of A. afarensis did not exactly match those of modern humans. This observation may indicate that early australopiths did not live primarily on the ground or at least spent a significant amount of time in the trees. The skeleton of Lucy also indicates that A. afarensis had longer, more powerful arms than most later human species, suggesting that this species was adept at climbing trees. Another controversy has to do with the scientific classification of the A. afarensis fossils. Compared with Lucy, who stood only 1.1 m (3.5 ft) tall, other fossils identified as A. afarensis from Hadar and Laetoli came from individuals who stood up to 1.5 m (5 ft) tall. This great difference in size leads some scientists to suggest that the entire set of fossils now classified as A. afarensis actually represents two species. Most scientists, however, believe the fossils represent one highly dimorphic species--that is, a species that has two distinct forms (in this case, two sizes). Supporters of this view note that both large (presumably male) and small (presumably female) adults occur together in one site at Hadar. A third controversy arises from the claim that A. afarensis was the common ancestor of both later australopiths and the modern human genus, Homo. While this idea remains a strong possibility, the similarity between this and another australopith species--one from southern Africa, named Australopithecus africanus--makes it difficult to decide which of the two species gave rise to the genus Homo. C6 Australopithecus africanus Australopithecus africanus thrived in the Transvaal region of what is now South Africa between about 3.3 million and 2.5 million years ago. Australian-born anatomist Raymond Dart discovered this species--the first known australopith--in 1924 at Taung, South Africa. The specimen, that of a young child, came to be known as the Taung Child. For decades after this discovery, almost no one in the scientific community believed Dart's claim that the skull came from an ancestral human. In the late 1930s teams led by Scottish-born South African paleontologist Robert Broom unearthed many more A. africanus skulls and other bones from the Transvaal site of Sterkfontein. A. africanus generally had a more globular braincase and less primitive-looking face and teeth than did A. afarensis. Thus, some scientists consider the southern species of early australopith to be a likely ancestor of the genus Homo. According to other scientists, however, certain heavily built facial and cranial features of A. africanus from Sterkfontein identify it as an ancestor of the robust australopiths that lived later in the same region. In 1998 a research team led by South African paleoanthropologist Ronald Clarke discovered an almost complete early australopith skeleton at Sterkfontein. This important find may resolve some of the questions about where A. africanus fits in the story of human evolution. C7 Kenyanthropus platyops Working in the Lake Turkana region of northern Kenya, a research team led by paleontologist Meave Leakey uncovered in 1999 a cranium and other bone remains of an early human that showed a mixture of features unseen in previous discoveries of early human fossils. The remains were estimated to be 3.5 million years old, and the cranium's small brain and earhole were similar to those of the very earliest humans. Its cheekbone, however, joined the rest of the face in a forward position, and the region beneath the nose opening was flat. These are traits found in later human fossils from around 2 million years ago, typically those classified in the genus Homo. Noting this unusual combination of traits, researchers named a new genus and species, Kenyanthropus platyops, or "flat-faced human from Kenya." Before this discovery, it seemed that only a single early human species, Australopithecus afarensis, lived in East Africa between 4 million and 3 million years ago. Yet Kenyanthropus indicates that a diversity of species, including a more humanlike lineage than A. afarensis, lived in this time period, just as in most other eras in human prehistory. C8 Australopithecus garhi The human fossil record is poorly known between 3 million and 2 million years ago, which makes recent finds from the site of Bouri, Ethiopia, particularly important. From 1996 to 1998, a research team led by Ethiopian paleontologist Berhane Asfaw and American paleontologist Tim White found the skull and other skeletal remains of an early human specimen about 2.5 million years old. The researchers named it Australopithecus garhi; the word garhi means "surprise" in the Afar language. The specimen is unique in having large incisors and molars in combination with an elongated forearm and thighbone. Its powerful arm bones suggest a tree-living ancestry, but its longer legs indicate the ability to walk upright on the ground. Fossils of A. garhi are associated with some of the oldest known stone tools, along with animal bones that were cut and cracked with tools. It is possible, then, that this species was among the first to make the transition to stone toolmaking and to eating meat and bone marrow from large animals. D Late Australopiths By 2.7 million years ago the later, robust australopiths had evolved. These species had what scientists refer to as megadont cheek teeth--wide molars and premolars coated with thick enamel. Their incisors, by contrast, were small. The robusts also had an expanded, flattened, and more vertical face than did gracile australopiths. This face shape helped to absorb the stresses of strong chewing. On the top of the head, robust australopiths had a sagittal crest (ridge of bone along the top of the skull from front to back) to which thick jaw muscles attached. The zygomatic arches (which extend back from the cheek bones to the ears), curved out wide from the side of the face and cranium, forming very large openings for the massive chewing muscles to pass through near their attachment to the lower jaw. Altogether, these traits indicate that the robust australopiths chewed their food powerfully and for long periods. Other ancient animal species that specialized in eating plants, such as some types of wild pigs, had similar adaptations in their facial, dental, and cranial anatomy. Thus, scientists think that the robust australopiths had a diet consisting partly of tough, fibrous plant foods, such as seed pods and underground tubers. Analyses of microscopic wear on the teeth of some robust australopith specimens appear to support the idea of a vegetarian diet, although chemical studies of fossils suggest that the southern robust species may also have eaten meat. Scientists originally used the word robust to refer to the late australopiths out of the belief that they had much larger bodies than did the early, gracile australopiths. However, further research has revealed that the robust australopiths stood about the same height and weighed roughly the same amount as Australopithecus afarensis and A. africanus. D1 Australopithecus aethiopicus The earliest known robust species, Australopithecus aethiopicus, lived in eastern Africa by 2.7 million years ago. In 1985 at West Turkana, Kenya, American paleoanthropologist Alan Walker discovered a 2.5-million-year-old fossil skull that helped to define this species. It became known as the "black skull" because of the color it had absorbed from minerals in the ground. The skull had a tall sagittal crest toward the back of its cranium and a face that projected far outward from the forehead. A. aethiopicus shared some primitive features with A. afarensis--that is, features that originated in the earlier East African australopith. This may indicate that A. aethiopicus evolved from A. afarensis. D2 Australopithecus boisei Australopithecus boisei, the other well-known East African robust australopith, lived over a long period of time, between about 2.3 million and 1.4 million years ago. In 1959 Mary Leakey discovered the original fossil of this species--a nearly complete skull--at the site of Olduvai Gorge in Tanzania. Kenyan-born paleoanthropologist Louis Leakey, husband of Mary, originally named the new species Zinjanthropus boisei (Zinjanthropus translates as "East African man"). This skull--dating from 1.8 million years ago--has the most specialized features of all the robust species. It has a massive, wide and dished-in face capable of withstanding extreme chewing forces, and molars four times the size of those in modern humans. Since the discovery of Zinjanthropus, now recognized as an australopith, scientists have found great numbers of A. boisei fossils in Tanzania, Kenya, and Ethiopia. D3 Australopithecus robustus The southern robust species, called Australopithecus robustus, lived between about 1.8 million and 1.3 million years ago in the Transvaal, the same region that was home to A. africanus. In 1938 Robert Broom, who had found many A. africanus fossils, bought a fossil jaw and molar that looked distinctly different from those in A. africanus. After finding the site of Kromdraai, from which the fossil had come, Broom collected many more bones and teeth that together convinced him to name a new species, which he called Paranthropus robustus (Paranthropus meaning "beside man"). Later scientists dated this skull at about 1.5 million years old. In the late 1940s and 1950 Broom discovered many more fossils of this species at the Transvaal site of Swartkrans. D4 The Origins and Fate of Late Australopiths Many scientists believe that robust australopiths represent a distinct evolutionary group of early humans because these species share features associated with heavy chewing. According to this view, Australopithecus aethiopicus diverged from other australopiths and later gave rise to A. boisei and A. robustus. Paleoanthropologists who strongly support this view think that the robusts should be classified in the genus Paranthropus, the original name given to the southern species. Thus, these three species are sometimes referred to as P. aethiopicus, P. boisei, and P. robustus. Other paleoanthropologists believe that the eastern robust species, A. aethiopicus and A. boisei, may have evolved from an early australopith of the same region, perhaps A. afarensis. According to this view, A. africanus gave rise only to the southern species, A. robustus. Scientists refer to such a case--in which two or more independent species evolve similar characteristics in different places or at different times--as parallel evolution. If parallel evolution occurred in australopiths, the robust species would make up two separate branches of the human family tree. The last robust australopiths died out about 1.4 million years ago. At about this time, climate patterns around the world entered a period of fluctuation, and these changes may have reduced the food supply on which robusts depended. Interaction with larger-brained members of the genus Homo, such as Homo erectus, may also have contributed to the decline of late australopiths, although no compelling evidence exists of such direct contact. Competition with several other species of planteating monkeys and pigs, which thrived in Africa at the time, may have been an even more important factor. But the reasons why the robust australopiths became extinct after flourishing for such a long time are not yet known for sure. E Why Did Humans Evolve? Scientists have several ideas about why australopiths first split off from the apes, initiating the course of human evolution. Virtually all hypotheses suggest that environmental change was an important factor, specifically in influencing the evolution of bipedalism. Some well-established ideas about why humans first evolved include (1) the savanna hypothesis, (2) the woodland-mosaic hypothesis, and (3) the variability hypothesis. The global climate cooled and became drier between 8 million and 5 million years ago, near the end of the Miocene Epoch. According to the savanna hypothesis, this climate change broke up and reduced the area of African forests. As the forests shrunk, an ape population in eastern Africa became separated from other populations of apes in the more heavily forested areas of western Africa. The eastern population had to adapt to its drier environment, which contained larger areas of grassy savanna. The expansion of dry terrain favored the evolution of terrestrial living, and made it more difficult to survive by living in trees. Terrestrial apes might have formed large social groups in order to improve their ability to find and collect food and to fend off predators--activities that also may have required the ability to communicate well. The challenges of savanna life might also have promoted the rise of tool use, for purposes such as scavenging meat from the kills of predators. These important evolutionary changes would have depended on increased mental abilities and, therefore, may have correlated with the development of larger brains in early humans. Critics of the savanna hypothesis argue against it on several grounds, but particularly for two reasons. First, discoveries by a French scientific team of australopith fossils in Chad, in Central Africa, suggests that the environments of East Africa may not have been fully separated from those farther west. Second, recent research suggests that open savannas were not prominent in Africa until sometime after 2 million years ago. Criticism of the savanna hypothesis has spawned alternative ideas about early human evolution. The woodland-mosaic hypothesis proposes that the early australopiths evolved in patchily wooded areas--a mosaic of woodland and grassland--that offered opportunities for feeding both on the ground and in the trees, and that ground feeding favored bipedalism. The variability hypothesis suggests that early australopiths experienced many changes in environment and ended up living in a range of habitats, including forests, open-canopy woodlands, and savannas. In response, their populations became adapted to a variety of surroundings. Scientists have found that this range of habitats existed at the time when the early australopiths evolved. So the development of new anatomical characteristics--particularly bipedalism--combined with an ability to climb trees, may have given early humans the versatility to live in a variety of habitats. Scientists also have many ideas about which benefits of bipedalism may have influenced its evolution. Ideas about the benefits of regular bipedalism include that it freed the hands, making it easier to carry food and tools; allowed early humans to see over tall grass to watch for predators; reduced exposure of the body to hot sun and increased exposure to cooling winds; improved the ability to hunt or use weapons, which became easier with an upright posture; and made extensive feeding from bushes and low branches easier than it would have been for a quadruped. Scientists do not overwhelmingly support any one of these ideas. Recent studies of chimpanzees suggest, though, that the ability to feed more easily might have particular relevance. Chimps move on two legs most often when they feed from the ground on the leaves and fruits of bushes and low branches. Chimps cannot, however, walk in this way over long distances. Bipedalism in early humans would have enabled them to travel efficiently over long distances, giving them an advantage over quadrupedal apes in moving across barren open terrain between groves of trees. In addition, the earliest humans continued to have the advantage from their ape ancestry of being able to escape into the trees to avoid predators. The benefits of both bipedalism and agility in the trees may explain the unique anatomy of australopiths. Their long, powerful arms and curved fingers probably made them good climbers, while their pelvis and lower limb structure was reshaped for upright walking. V THE GENUS HOMO People belong to the genus Homo, which first evolved at least 2.3 million to 2.5 million years ago. The earliest members of this genus differed from the australopiths in at least one important respect--they had larger brains than did their predecessors. The evolution of the modern human genus can be divided roughly into three periods: early, middle, and late. Species of early Homo resembled gracile australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of middle Homo began perhaps between 2 million and 1.8 million years ago, overlapping with the end of early Homo. Species of middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from middle to late Homo probably occurred sometime around 200,000 years ago. Species of late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution. A Origins The origin of the genus Homo has long intrigued paleoanthropologists and prompted much debate. One of several known species of australopiths, or one not yet discovered, could have given rise to the first species of Homo. Scientists also do not know exactly what factors favored the evolution of a larger and more complex brain--the defining physical trait of modern humans. Louis Leakey originally argued that the origin of Homo related directly to the development of toolmaking--specifically, the making of stone tools. Toolmaking requires certain mental skills and fine hand manipulation that may exist only in members of our own genus. Indeed, the name Homo habilis (meaning "handy man") refers directly to the making and use of tools. However, several species of australopiths lived at the same time as early Homo, making it unclear which species produced the earliest stone tools. Recent studies of australopith hand bones have suggested that at least one of the robust species, Australopithecus robustus, could have made tools. In addition, during the 1960s and 1970s researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that australopiths and the apes that preceded them probably also made some kinds of tools. According to some scientists, however, early Homo probably did make the first stone tools. The ability to cut and pound foods would have been most useful to these smaller-toothed humans, whereas the robust australopiths could chew even very tough foods. Furthermore, early humans continued to make stone tools similar to the oldest known kinds for a time long after the gracile australopiths died out. Some scientists think that a period of environmental cooling and drying in Africa set the stage for the evolution of Homo. According to this idea, many types of animals suited to the challenges of a drier environment originated during the period between about 2.8 million and 2.4 million years ago, including the first species of Homo. A toolmaking human might have had an advantage in obtaining alternative food sources as vegetation became sparse in increasingly dry environments. The new foods might have included underground roots and tubers, as well as meat obtained through scavenging or hunting. However, some scientists disagree with this idea, arguing that the period during which Homo evolved fluctuated between drier and wetter conditions, rather than just becoming dry. In this case, the making and use of stone tools and an expansion of the diet in early Homo--as well as an increase in brain size--may all have been adaptations to unpredictable and fluctuating environments. In either case, more scientific documentation is necessary to strongly support or refute the idea that early Homo arose as part of a larger trend of rapid species extinction and the evolution of many new species during a period of environmental change. B Early Homo Paleoanthropologists generally recognize two species of early Homo--Homo habilis and H. rudolfensis (although other species may also have existed). The record is unclear because most of the early fossils that scientists have identified as species of Homo--rather than robust australopiths who lived at the same time--occur as isolated fragments. In many places, only teeth, jawbones, and pieces of skull--without any other skeletal remains--indicate that new species of smaller-toothed humans had evolved as early as 2.5 million years ago. Scientists cannot always tell whether these fossils belong to late-surviving gracile australopiths or early representatives of Homo. The two groups resemble each other because Homo likely descended directly from a species of gracile australopith. B1 Homo habilis Between 1960 and 1963, at Olduvai Gorge, Tanzania, a team led by Louis and Mary Leakey discovered the remains of an early human that seemed distinctly different from the australopiths. In 1964 Louis Leakey, South African paleoanthropologist Philip Tobias, and British primate researcher John Napier concluded that these remains represented a new species, which they named Homo habilis. The scientists placed the species in the genus Homo because its brain was estimated to be significantly larger than that of any known australopith. Other scientists questioned whether the amount of brain enlargement was sufficient for inclusion of the species in Homo, and even whether H. habilis was different from Australopithecus africanus, as the teeth of the two species look similar. However, scientists now widely accept both the genus and species names designated by the Olduvai team. According to recent estimates, H. habilis had a brain volume that ranged from 590 to 690 cu cm (36 to 42 cu in), well above the range for australopithecines. H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species somewhat resemble those of australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust australopiths. A fragmented skeleton of a female from Olduvai shows that she stood only about 1 m (3.3 ft) tall, and the ratio of the length of her arms to her legs was greater than that in the australopith Lucy. At least in the case of this individual, therefore, H. habilis had very apelike body proportions. However, H. habilis had more modernlooking feet and hands capable of producing tools. Some of the earliest stone tools from Olduvai have been found with H. habilis fossils, suggesting that this species made and used the tools at this site. Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have indicated that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with some other fossils--exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size actually exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa. B2 Homo rudolfensis This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana). The best-known fossils of H. rudolfensis come from the area surrounding this lake and date from about 1.9 million years ago. Paleoanthropologists have not determined the entire time range during which H. rudolfensis may have lived. This species had a larger face and body than did H. habilis. The cranial capacity of H. rudolfensis averaged about 750 cu cm (46 cu in). Scientists need more evidence to know whether the brain of H. rudolfensis in relation to its body size was larger than that proportion in H. habilis. A larger brain-to-body-size ratio can indicate increased mental abilities. H. rudolfensis also had fairly large teeth, approaching the size of those in robust australopiths. The discovery of even a partial fossil skeleton would reveal whether this larger form of early Homo had apelike or more modern body proportions. Scientists have found several modern-looking thighbones that date from between 2 million and 1.8 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m (5 ft) and 52 kg (114 lb). C Middle Homo By about 1.9 million years ago, the period of middle Homo had begun in Africa. Until recently, paleoanthropologists recognized one species in this period, Homo erectus. Many now recognize three species of middle Homo: H. ergaster, H. erectus, and H. heidelbergensis. However, some still think H. ergaster is an early African form of H. erectus, or that H. heidelbergensis is a late form of H. erectus. The skulls and teeth of early African populations of middle Homo differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. H. ergaster makes a better candidate for an ancestor of the modern human line because Asian H. erectus has some specialized features not seen in some later humans, including our own species. H. heidelbergensis has similarities to both H. erectus and the later species H. neanderthalensis, although it may have been a transitional species between middle Homo and the line to which modern humans belong. C1 Homo ergaster Homo ergaster probably first evolved in Africa around 2 million years ago. This species had a rounded cranium with a brain size of between 700 and 850 cu cm (49 to 52 cu in), a prominent brow ridge, small teeth, and many other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including relatively thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.5 million years ago. The most important fossil of this species yet found is a nearly complete skeleton of a young male from West Turkana, Kenya, which dates from about 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also determined from patterns of tooth eruption and bone growth that the boy had died when he was between 9 and 12 years old. The Turkana boy, as the skeleton is known, had elongated leg bones and arm, leg, and trunk proportions that essentially match those of a modern human, in sharp contrast with the apelike proportions of H. habilis and Australopithecus afarensis. He appears to have been quite tall and slender. Scientists estimate that, had he grown into adulthood, the boy would have reached a height of 1.8 m (6 ft) and a weight of 68 kg (150 lb). The anatomy of the Turkana boy indicates that H. ergaster was particularly well adapted for walking and perhaps for running long distances in a hot environment (a tall and slender body dissipates heat well) but not for any significant amount of tree climbing. The oldest humanlike fossils outside of Africa have also been classified as H. ergaster, dated around 1.75 million years old. These finds, from the Dmanisi site in the southern Caucasus Mountains of Georgia, consist of several crania, jaws, and other fossilized bones. Some of these are strikingly like East African H. ergaster, but others are smaller or larger than H. ergaster, suggesting a high degree of variation within a single population. H. ergaster, H. rudolfensis, and H. habilis, in addition to possibly two robust australopiths, all might have coexisted in Africa around 1.9 million years ago. This finding goes against a traditional paleoanthropological view that human evolution consisted of a single line that evolved progressively over time--an australopith species followed by early Homo, then middle Homo, and finally H. sapiens. It appears that periods of species diversity and extinction have been common during human evolution, and that modern H. sapiens has the rare distinction of being the only living human species today. Although H. ergaster appears to have coexisted with several other human species, they probably did not interbreed. Mating rarely succeeds between two species with significant skeletal differences, such as H. ergaster and H. habilis. Many paleoanthropologists now believe that H. ergaster descended from an earlier population of Homo--perhaps one of the two known species of early Homo--and that the modern human line descended from H. ergaster. C2 Homo erectus Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for a few million years. The earliest human species known to have spread in large numbers beyond the African continent was first discovered in Southeast Asia. In 1891 Dutch physician Eugène Dubois found the cranium of an early human on the Indonesian island of Java. He named this early human Pithecanthropus erectus, or "erect ape-man." Today paleoanthropologists refer to this species as Homo erectus. H. erectus appears to have evolved in Africa from earlier populations of H. ergaster, and then spread to Asia sometime between 1.8 million and 1.5 million years ago. The youngest known fossils of this species, from the Solo River in Java, may date from as recently as 53,000 to 27,000 years ago (although that dating is controversial). So H. erectus was a very successful species--both widespread, having lived in Africa and much of Asia, and long-lived, having survived for possibly more than 1.5 million years. H. erectus had a low and rounded braincase that was elongated from front to back, a prominent brow ridge, and an adult cranial capacity of 800 to 1,250 cu cm (50 to 80 cu in), an average twice that of the australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Although it had much smaller teeth than did the australopiths, it had a heavy and strong jaw. In the 1920s and 1930s German anatomist and physical anthropologist Franz Weidenreich excavated the most famous collections of H. erectus fossils from a cave at the site of Zhoukoudian (Chou-k'ou-tien), China, near Beijing (Peking). Scientists dubbed these fossil humans Sinanthropus pekinensis, or Peking Man, but others later reclassified them as H. erectus. The Zhoukoudian cave yielded the fragmentary remains of over 30 individuals, ranging from about 500,000 to 250,000 years old. These fossils were lost near the outbreak of World War II, but Weidenreich had made excellent casts of his finds. Further studies at the cave site have yielded more H. erectus remains. Other important fossil sites for this species in China include Lantian, Yuanmou, Yunxian, and Hexian. Researchers have also recovered many tools made by H. erectus in China at sites such as Nihewan and Bose, and other sites of similar age (at least 1 million to 250,000 years old). Ever since the discovery of H. erectus, scientists have debated whether this species was a direct ancestor of later humans, including H. sapiens. The last populations of H. erectus--such as those from the Solo River in Java--may have lived as recently as 53,000 to 27,000 years ago, at the same time as did populations of H. sapiens. Modern humans could not have evolved from these late populations of H. erectus, a much more primitive type of human. However, earlier East Asian populations could have given rise to H. sapiens. C3 Homo heidelbergensis Many paleoanthropologists believe that early humans migrated into Europe by 800,000 years ago, and that these populations were not Homo erectus. A growing number of scientists refer to these early migrants into Europe--who predated both Neandertals and H. sapiens in the region--as H. heidelbergensis. The species name comes from a 500,000-year-old jaw found near Heidelberg, Germany. Scientists have found few human fossils in Africa for the period between 1.2 million and 600,000 years ago, during which H. heidelbergensis or its ancestors first migrated into Europe. Populations of H. ergaster (or possibly H. erectus) appear to have lived until at least 800,000 years ago in Africa, and possibly until 500,000 years ago in northern Africa. When these populations disappeared, other massive-boned and larger-brained humans--possibly H. heidelbergensis--appear to have replaced them. Scientists have found fossils of these stockier humans at sites in Bodo, Ethiopia; Saldanha (also known as Elandsfontein), South Africa; Ndutu, Tanzania; and Kabwe, Zimbabwe. Scientists have come up with at least three different interpretations of these African fossils. Some scientists place the fossils in the species H. heidelbergensis and think that this species gave rise to both the Neandertals (in Europe) and H. sapiens (in Africa). Others think that the European and African fossils belong to two distinct species, and that the African populations--which, in this view, were not H. heidelbergensis but a separate species--gave rise to H. sapiens. Yet other scientists advocate a long-held view that H. erectus and H. sapiens belong to a single evolving lineage, and that the African fossils belong in the category of archaic H. sapiens (archaic meaning not fully anatomically modern). The fossil evidence does not clearly favor any of these three interpretations over another. A growing number of fossils from Asia, Africa, and Europe have features that are intermediate between early H. ergaster and H. sapiens. This kind of variation makes it hard to decide how to identify distinct species and to determine which group of fossils represents the most likely ancestor of later humans. C4 Why Did Humans Spread Out of Africa? Humans evolved in Africa and lived only there for as long as 4 million years or more, so scientists wonder what finally triggered the first human migration out of Africa (a movement that coincided with the spread of early human populations throughout the African continent). The answer to this question depends, in part, on knowing exactly when that first migration occurred. Some studies claim that sites in Asia and Europe contain crude stone tools and fossilized fragments of humanlike teeth that date from more than 1.8 million years ago. Although these claims remain unconfirmed, small populations of humans may have entered Asia prior to 1.8 million years ago, followed by a more substantial spread between 1.6 million and 1 million years ago. Early humans reached northeastern Asia by around 1.4 million years ago, inhabiting a region close to the perpetually dry deserts of northern China. The first major habitation of central and western Europe, on the other hand, does not appear to have occurred until between 1 million and 500,000 years ago. Scientists once thought that advances in stone tools could have enabled early humans such as Homo erectus to move into Asia and Europe, perhaps by helping them to obtain new kinds of food, such as the meat of large mammals. If African human populations had developed tools that allowed them to hunt large game effectively, they would have had a reliable source of food wherever they went. In this view, humans first migrated into Eurasia based on a unique cultural adaptation. By 1.5 million years ago, early humans had begun to make new kinds of tools, which scientists call Acheulean. Common Acheulean tools included large handaxes and cleavers. While these new tools might have helped early humans to hunt, the first known Acheulean tools in Africa date from later than the earliest known human presence in Asia. Also, most East Asian sites over 200,000 years old contain only simply shaped cobble and flake tools. In contrast, Acheulean tools were more finely crafted, larger, and more symmetrical. Thus, the earliest settlers of Eurasia did not have a true Acheulean technology, and advances in toolmaking alone cannot explain the spread out of Africa. Another possibility is that the early spread of humans to Eurasia was not unique, but rather part of a wider migration of meat-eating animals, such as lions and hyenas. The human migration out of Africa occurred during the early part of the Pleistocene Epoch, between 1.8 million and 780,000 years ago. Many African carnivores spread to Eurasia during the early Pleistocene, and humans could have moved along with them. In this view, H. erectus was one of many meat-eating species to expand into Eurasia from Africa, rather than a uniquely adapted species. Relying on meat as a primary food source might have allowed many meat-eating species, including humans, to move through many different environments without having to quickly learn about unfamiliar and potentially poisonous plants. However, the migration of humans to eastern Asia may have occurred gradually and through lower latitudes and environments similar to those of Africa. If East African populations of H. erectus moved at only 1.6 km (1 mi) every 20 years, they could have reached Southeast Asia in 150,000 years. Over this amount of time, humans could have learned about and begun relying on edible plant foods. Thus, eating meat may not have played a crucial role in the first human migrations to new continents. Careful comparison of animal fossils, stone tools, and early human fossils from Africa, Asia, and Europe will help scientists to better determine what factors motivated and allowed humans to venture out of Africa for the first time. D Late Homo The origin of our own species, Homo sapiens, is one of the most hotly debated topics in paleoanthropology. This debate centers on whether or not modern humans have a direct relationship to H. erectus or to the Neandertals, a well-known, more modern group of humans who evolved within the past 250,000 years. Paleoanthropologists commonly use the term anatomically modern Homo sapiens to distinguish people of today from these similar predecessors. Traditionally, paleoanthropologists classified as Homo sapiens any fossil human younger than 500,000 years old with a braincase larger than that of H. erectus. Thus, many scientists who believe that modern humans descend from a single line dating back to H. erectus use the name archaic Homo sapiens to refer to a wide variety of fossil humans that predate anatomically modern H. sapiens. The term archaic denotes a set of physical features typical of Neandertals and other species of late Homo prior to modern Homo sapiens. These features include a combination of a robust skeleton, a large but low braincase (positioned somewhat behind, rather than over, the face), and a lower jaw lacking a prominent chin. In this sense, Neandertals are sometimes classified as a subspecies of archaic H. sapiens--H. sapiens neanderthalensis. Other scientists think that the variation in archaic fossils actually falls into clearly identifiable sets of traits, and that any type of human fossil exhibiting a unique set of traits should have a new species name. According to this view, the Neandertals belong to their own species, H. neanderthalensis. D1 Neandertals The Neandertals lived in areas ranging from western Europe through central Asia from about 200,000 to about 28,000 years ago. The name Neandertal (sometimes spelled Neanderthal) comes from fossils found in 1856 in the Feldhofer Cave of the Neander Valley in Germany (tal--a modern form of thal--means "valley" in German). Scientists realized several years later that prior discoveries--at Engis, Belgium, in 1829 and at Forbes Quarry, Gibraltar, in 1848--also represented Neandertals. These two earlier discoveries were the first early human fossils ever found. In the past, scientists claimed that Neandertals differed greatly from modern humans. However, the basis for this claim came from a faulty reconstruction of a Neandertal skeleton that showed it with bent knees and a slouching gait. This reconstruction gave the common but mistaken impression that Neandertals were dimwitted brutes who lived a crude lifestyle. On the contrary, Neandertals, like the species that preceded them, walked fully upright without a slouch or bent knees. In addition, their cranial capacity was quite large at about 1,500 cu cm (about 90 cu in), slightly larger on average than that of modern humans. (The difference probably relates to the greater muscle mass of Neandertals as compared with modern humans, which usually correlates with a larger brain size.) Compared with earlier humans, Neandertals had a high degree of cultural sophistication. They appear to have performed symbolic rituals, such as the burial of their dead. Neandertal fossils--including a number of fairly complete skeletons--are quite common compared to those of earlier forms of Homo, in part because of the Neandertal practice of intentional burial. Neandertals also produced sophisticated types of stone tools known as Mousterian, which involved creating blanks (rough forms) from which several types of tools could be made. Along with many physical similarities, Neandertals differed from modern humans in several ways. The typical Neandertal skull had a low forehead, a large nasal area (suggesting a large nose), a forward-projecting nasal and cheek region, a prominent brow ridge with a bony arch over each eye, a nonprojecting chin, and an obvious space behind the third molar (in front of the upward turn of the lower jaw). Neandertals also had a more heavily built and large-boned skeleton than do modern humans. Other Neandertal skeletal features included a bowing of the limb bones in some individuals, broad scapulae (shoulder blades), hip joints turned outward, a long and thin pubic bone, short lower leg and arm bones relative to the upper bones, and large surfaces on the joints of the toes and limb bones. Together, these traits made a powerful, compact body of short stature--males averaged 1.7 m (5 ft 5 in) tall and 84 kg (185 lb), and females averaged 1.5 m (5 ft) tall and 80 kg (176 lb). The short, stocky build of Neandertals conserved heat and helped them withstand extremely cold conditions that prevailed in temperate regions beginning about 70,000 years ago. The last known Neandertal fossils come from western Europe and date from approximately 28,000 years ago. D2 Other Late Homo Populations At the same time as Neandertal populations grew in number in Europe and parts of Asia, other populations of nearly modern humans arose in Africa and Asia. Scientists also commonly refer to these fossils, which are distinct from but similar to those of Neandertals, as archaic. Fossils from the Chinese sites of Dali, Maba, and Xujiayao display the long, low cranium and large face typical of archaic humans, yet they also have features similar to those of modern people in the region. At the cave site of Jebel Irhoud, Morocco, scientists have found fossils with the long skull typical of archaic humans but also the modern traits of a somewhat higher forehead and flatter midface. Fossils of humans from East African sites older than 100,000 years--such as Ngaloba in Tanzania and Eliye Springs in Kenya--also seem to show a mixture of archaic and modern traits. One of the most unusual branches of the human family tree was discovered on the Indonesian island of Flores in 2003 and first described in 2004. A research team digging in a cave, Liang Bua, uncovered the nearly complete skeleton of what appeared to be a miniature human that lived as recently as 18,000 years ago. The specimen, believed to be an adult female, was estimated to stand only about 1 m (3.3 ft) tall. Its brain, estimated at 380 cu cm (23 cu in), was as small as those of chimpanzees and the smallest australopiths. It had fairly large brow ridges, and its teeth were large relative to the rest of the skull. Despite being extremely smallbrained, it apparently made simple stone tools. On the basis of these unique traits, the researchers assigned the skeleton to a new species, Homo floresiensis. The researchers concluded that H. floresiensis was probably descended from H. erectus, although this continues to be debated. The diminutive stature and small brain of H. floresiensis may have resulted from island dwarfism--an evolutionary process that results from long-term isolation on a small island with limited food resources and a lack of predators. Pygmy elephants on Flores, now extinct, showed the same adaptation. D3 Anatomically Modern Homo sapiens The oldest known fossils that possess skeletal features typical of modern humans date from 195,000 years ago. Several key features distinguish the skulls of modern humans from those of archaic species. These features include a much smaller brow ridge, if any; a globe-shaped braincase; and a flat or only slightly projecting face of reduced size, located under the front of the braincase. Among all mammals, only humans have a face positioned directly beneath the frontal lobe (forward-most area) of the brain. As a result, modern humans tend to have a higher forehead than did Neandertals and other archaic humans. The cranial capacity of modern humans ranges from about 1,000 to 2,000 cu cm (60 to 120 cu in), with the average being about 1,350 cu cm (80 cu in). Scientists have found both fragmentary and nearly complete cranial fossils of early anatomically modern Homo sapiens from the sites of Singha, Sudan; Omo, Ethiopia; Klasies River Mouth, South Africa; and Skh?l Cave, Israel. Based on these fossils, many scientists conclude that modern H. sapiens had evolved in Africa by 195,000 years ago and started spreading to diverse parts of the world beginning on a route through the Near East sometime before 90,000 years ago. E Theories of Modern Human Origins and Diversity Paleoanthropologists are engaged in an ongoing debate about where modern humans evolved and how they spread around the world. Differences in opinion rest on the question of whether the evolution of modern humans took place in a small region of Africa or over a broad area of Africa and Eurasia. By extension, opinions differ as to whether modern human populations from Africa displaced all existing populations of earlier humans, eventually resulting in their extinction. Those who think modern humans originated only in Africa and then spread around the world support what is known as the out of Africa hypothesis. Those who think modern humans evolved over a large region of Eurasia and Africa support the so-called multiregional hypothesis. Researchers have conducted many genetic studies and carefully assessed fossils to determine which of these hypotheses agrees more with scientific evidence. The results of this research do not entirely confirm or reject either one. Therefore, some scientists think a compromise between the two hypotheses is the best explanation. The debate between these views has implications for how scientists understand the concept of race in humans. The question raised is whether the physical differences among modern humans evolved deep in the past or relatively recently. E1 The Out of Africa Hypothesis According to the out of Africa hypothesis, also known as the replacement hypothesis, early populations of modern humans from Africa migrated to other regions and entirely replaced existing populations of archaic humans. The replaced populations would have included the Neandertals and any surviving groups of Homo erectus. Supporters of this view note that many modern human skeletal traits evolved relatively recently--within the past 200,000 years or so--suggesting a single, common origin. In addition, the anatomical similarities shared by all modern human populations far outweigh those shared by premodern and modern humans within particular geographic regions. Furthermore, biological research indicates that most new species of organisms, including mammals, arise from small, geographically isolated populations. E2 The Multiregional Hypothesis According to the multiregional hypothesis, also known as the continuity hypothesis, the evolution of modern humans began when Homo erectus spread throughout much of Eurasia around 1 million years ago. Regional populations retained some unique anatomical features for hundreds of thousands of years, but they also mated with populations from neighboring regions, exchanging heritable traits with each other. This exchange of heritable traits is known as gene flow. Through gene flow, populations of H. erectus passed on a variety of increasingly modern characteristics, such as increases in brain size, across their geographic range. Gradually this would have resulted in the evolution of more modern looking humans throughout Africa and Eurasia. The physical differences among people today, then, would result from hundreds of thousands of years of regional evolution. This is the concept of continuity. For instance, modern East Asian populations have some skull features that scientists also see in H. erectus fossils from that region. Some critics of the multiregional hypothesis claim that it wrongly advocates a scientific belief in race and could be used to encourage racism. Supporters of the theory point out, however, that their position does not imply that modern races evolved in isolation from each other, or that racial differences justify racism. Instead, the theory holds that gene flow linked different populations together. These links allowed progressively more modern features, no matter where they arose, to spread from region to region and eventually become universal among humans. E3 Testing the Two Theories Scientists have weighed the out of Africa and multiregional hypotheses against both genetic and fossil evidence. The results do not unanimously support either one, but weigh more heavily in favor of the out of Africa hypothesis. E3a Genetic Evidence Geneticists have studied the amount of difference in the DNA (deoxyribonucleic acid) of different populations of humans. DNA is the molecule that contains our heritable genetic code. Differences in human DNA result from mutations in DNA structure. Mutations may result from exposure to external elements such as solar radiation or certain chemical compounds, while others occur naturally at random. Geneticists have calculated rates at which mutations can be expected to occur over time. Dividing the total number of genetic differences between two populations by an expected rate of mutation provides an estimate of the time when the two shared a common ancestor. Many estimates of evolutionary ancestry rely on studies of the DNA in cell structures called mitochondria. This DNA is referred to as mtDNA (mitochondrial DNA). Unlike DNA from the nucleus of a cell, which codes for most of the traits an organism inherits from both parents, mtDNA inheritance passes only from a mother to her offspring. MtDNA also accumulates mutations about ten times faster than does DNA in the cell nucleus (the location of most DNA). The structure of mtDNA changes so quickly that scientists can easily measure the differences between one human population and another. Two closely related populations should have only minor differences in their mtDNA. Conversely, two very distantly related populations should have large differences in their mtDNA. MtDNA research into modern human origins has produced two major findings. First, the entire amount of variation in mtDNA across human populations is small in comparison with that of other animal species. This means that all human mtDNA originated from a single ancestral lineage--specifically, a single female--fairly recently and has been mutating ever since. Most estimates of the mutation rate of mtDNA suggest that this female ancestor lived about 200,000 years ago. In addition, the mtDNA of African populations varies more than that of peoples in other continents. This suggests that the mtDNA of African populations has changed for a longer time than it has in populations of any other region, and that all living people inherited their mtDNA from one woman in Africa, who is sometimes called the Mitochondrial Eve. Some geneticists and anthropologists have concluded from this evidence that modern humans originated in a small population in Africa and spread from there. MtDNA studies have weaknesses, however, including the following four. First, the estimated rate of mtDNA mutation varies from study to study, and some estimates put the date of origin closer to 850,000 years ago, the time of Homo erectus. Second, mtDNA makes up a small part of the total genetic material that humans inherit. The rest of our genetic material--about 400,000 times more than the amount of mtDNA--came from many individuals living at the time of the African Eve, conceivably from many different regions. Third, the time at which modern mtDNA began to diversify does not necessarily coincide with the origin of modern human biological traits and cultural abilities. Fourth, the smaller amount of modern mtDNA diversity outside of Africa could result from times when European and Asian populations declined in numbers, perhaps due to climate changes. Despite these criticisms, many geneticists continue to favor the out of Africa hypothesis of modern human origins. Studies of nuclear DNA also suggest an African origin for a variety of genes. Furthermore, in a remarkable series of studies in the late 1990s, scientists recovered mtDNA from the first Neandertal fossil found in Germany and two other Neandertal fossils. In each case, the mtDNA does not closely match that of modern humans. This finding suggests that at least some Neandertal populations had diverged from the line to modern humans by 500,000 to 600,000 years ago. This also suggests that Neandertals represent a separate species from modern H. sapiens. In another study, however, mtDNA extracted from a 62,000-year-old Australian H. sapiens fossil was found to differ significantly from modern human mtDNA, suggesting a much wider range of mtDNA variation within H. sapiens than was previously believed. According to the Australian researchers, this finding lends support to the multiregional hypothesis because it shows that different populations of H. sapiens, possibly including Neandertals, could have evolved independently in different parts of the world. E3b Fossil Evidence As with genetic research, fossil evidence also does not entirely support or refute either of the competing hypotheses of modern human origins. However, many scientists see the balance of evidence favoring an African origin of modern H. sapiens within the past 200,000 years. The oldest known modern-looking skulls come from Africa and date from perhaps 195,000 years ago. The next oldest come from the Near East, where they date from about 90,000 years ago. Fossils of modern humans in Europe do not exist from before about 40,000 years ago. In addition, the first modern humans in Europe--often referred to as Cro-Magnon people--had elongated lower leg bones, as did African populations that were adapted to warm, tropical climates. This suggests that populations from warmer regions replaced those in colder European regions, such as the Neandertals. Fossils also show that populations of modern humans lived at the same time and in the same regions as did populations of Neandertals and Homo erectus, but that each retained its distinctive physical features. The different groups overlapped in the Near East and Southeast Asia for between about 30,000 and 50,000 years. The maintenance of physical differences for this amount of time implies that archaic and modern humans either could not or generally did not interbreed. To some scientists, this also means that the Neandertals belong to a separate species, H. neanderthalensis, and that migratory populations of modern humans entirely replaced archaic humans in both Europe and eastern Asia. On the other hand, fossils of archaic and modern humans in some regions show continuity in certain physical characteristics. These similarities may indicate multiregional evolution. For example, both archaic and modern skulls of eastern Asia have flatter cheek and nasal areas than do skulls from other regions. By contrast, the same parts of the face project forward in the skulls of both archaic and modern humans of Europe. Assuming that these traits were influenced primarily by genetic inheritance rather than environmental factors, archaic humans may have given rise to modern humans in some regions or at least interbred with migrant modern-looking humans. E4 A Compromise Theory Each of the competing major hypotheses of modern human origins has its strengths and weaknesses. Genetic evidence appears to support the out of Africa hypothesis. In the western half of Eurasia and in Africa, this hypothesis also seems the better explanation, particularly in regard to the apparent replacement of Neandertals by modern populations. At the same time, the multiregional hypothesis appears to explain some of the regional continuity found in East Asian populations. Therefore, many paleoanthropologists advocate a theory of modern human origins that combines elements of the out of Africa and the multiregional hypotheses. Humans with modern features may have first emerged in Africa or come together there as a result of gene flow with populations from other regions. These African populations may then have replaced archaic humans in certain regions, such as western Europe and the Near East. But elsewhere--especially in East Asia--gene flow may have occurred among local populations of archaic and modern humans, resulting in distinct and enduring regional characteristics. All three of these views--the two competing positions and the compromise--acknowledge the strong biological unity of all people. In the multiregional hypothesis, this unity results from hundreds of thousands of years of continued gene flow among all human populations. According to the out of Africa hypothesis, on the other hand, similarities among all living human populations result from a recent common origin. The compromise position accepts both of these as reasonable and compatible explanations of modern human origins. VI THE EVOLUTION OF CULTURAL BEHAVIOR The story of human evolution is as much about the development of cultural behavior as it is about changes in physical appearance. The term culture, in anthropology, traditionally refers to all human creations and activities governed by social customs and rules. It includes elements such as technology, language, and art. Human cultural behavior depends on the social transfer of information from one generation to the next, which itself depends on a sophisticated system of communication, such as language. The term culture has often been used to distinguish the behavior of humans from that of other animals. However, some nonhuman animals also appear to have forms of learned cultural behavior. For instance, different groups of chimpanzees use different techniques to capture termites for food using sticks. Also, in some regions chimps use stones or pieces of wood for cracking open nuts. Chimps in other regions do not practice this behavior, although their forests have similar nut trees and materials for making tools. These regional differences resemble traditions that people pass from generation to generation. Traditions are a fundamental aspect of culture, and paleoanthropologists assume that the earliest humans also had some types of traditions. However, modern humans differ from other animals, and probably many early human species, in that they actively teach each other and can pass on and accumulate unusually large amounts of knowledge. People also have a uniquely long period of learning before adulthood, and the physical and mental capacity for language. Language of all forms--spoken, signed, and written--provides a medium for communicating vast amounts of information, much more than any other animal appears to be able to transmit through gestures and vocalizations. Scientists have traced the evolution of human cultural behavior through the study of archaeological artifacts, such as tools, and related evidence, such as the charred remains of cooked food. Artifacts show that throughout much of human evolution, culture has developed slowly. During the Paleolithic, or early Stone Age, basic techniques for making stone tools changed very little for periods of well over a million years. See also Archaeology: Prehistoric Archaeology. Human fossils also provide information about how culture has evolved and what effects it has had on human life. For example, over the past 30,000 years, the basic anatomy of humans has undergone only one prominent change: The bones of the average human skeleton have become much smaller and thinner. Innovations in the making and use of tools and in obtaining food--results of cultural evolution--may have led to more efficient and less physically taxing lifestyles, and thus caused changes in the skeleton. Culture has played a prominent role in the evolution of Homo sapiens. Within the last 60,000 years, people have migrated to settle almost all unoccupied regions of the world, such as small island chains and the continents of Australia and the Americas. These migrations depended on developments in transportation, hunting and fishing tools, shelter, and clothing. Within the past 30,000 years, cultural evolution has sped up dramatically. This change shows up in the archaeological record as a rapid expansion of stone tool types and toolmaking techniques, and in works of art and indications of evolving religion, such as burials. By 10,000 years ago, people first began to harvest and cultivate grains and to domesticate animals--a fundamental change in the ecological relationship between human beings and other life on Earth. The development of agriculture provided people with larger quantities and more stable supplies of food, which set the stage for the rise of the first civilizations. Today, culture--and particularly technology--dominates human life. Paleoanthropologists and archaeologists have studied many topics in the evolution of human cultural behavior. These have included the evolution of (1) social life; (2) subsistence (the acquisition and production of food); (3) the making and using of tools; (4) environmental adaptation; (5) symbolic thought and its expression through language, art, and religion; and (6) the development of agriculture and the rise of civilizations. A Social Life Most primate species, including the African apes, live in social groups of varying size and complexity. Within their groups, individuals often have multifaceted roles, based on age, sex, status, social skills, and personality. The discovery in 1975 at Hadar, Ethiopia, of a group of several Australopithecus afarensis individuals who died together 3.2 million years ago appears to confirm that early humans lived in social groups. Scientists have referred to this collection of fossils as The First Family. One of the first physical changes in the evolution of humans from apes--a decrease in the size of male canine teeth--also indicates a change in social relations. Male apes sometimes use their large canines to threaten (or sometimes fight with) other males of their species, usually over access to females, territory, or food. The evolution of small canines in australopiths implies that males had either developed other methods of threatening each other or become more cooperative. In addition, both male and female australopiths had small canines, indicating a reduction of sexual dimorphism from that in apes. Yet, although sexual dimorphism in canine size decreased in australopiths, males were still much larger than females. Thus, male australopiths might have competed aggressively with each other based on sheer size and strength, and the social life of humans may not have differed much from that of apes until later times. Scientists believe that several of the most important changes from apelike to characteristically human social life occurred in species of the genus Homo, whose members show even less sexual dimorphism. These changes, which may have occurred at different times, included (1) prolonged maturation of infants, including an extended period during which they required intensive care from their parents; (2) special bonds of sharing and exclusive mating between particular males and females, called pair-bonding; and (3) the focus of social activity at a home base, a safe refuge in a special location known to family or group members. A1 Parental Care Humans, who have a large brain, have a prolonged period of infant development and childhood because the brain takes a long time to mature. Since the australopith brain was not much larger than that of a chimp, some scientists think that the earliest humans had a more apelike rate of growth, which is far more rapid than that of modern humans. This view is supported by studies of australopith fossils looking at tooth development--a good indicator of overall body development. In addition, the human brain becomes very large as it develops, so a woman must give birth to a baby at an early stage of development in order for the infant's head to fit through her birth canal. Thus, human babies require a long period of care to reach a stage of development at which they depend less on their parents. In contrast with a modern female, a female australopith could give birth to a baby at an advanced stage of development because its brain would not be too large to pass through the birth canal. The need to give birth early--and therefore to provide more infant care--may have evolved around the time of the middle Homo species Homo ergaster. This species had a brain significantly larger than that of the australopiths, but a narrow birth canal. A2 Pair-Bonding Pair-bonding, usually of a fairly short duration, occurs in a variety of primate species. Some scientists speculate that prolonged bonds developed in humans along with increased sharing of food. Among primates, humans have a distinct type of food-sharing behavior. People will delay eating food until they have returned with it to the location of other members of their social group. This type of food sharing may have arisen at the same time as the need for intensive infant care, probably by the time of H. ergaster. By devoting himself to a particular female and sharing food with her, a male could increase the chances of survival for his own offspring. A3 The Home Base Humans have lived as foragers for millions of years. Foragers obtain food when and where it is available over a broad territory. Modern-day foragers (also known as hunter-gatherers)--such as the San people in the Kalahari Desert of southern Africa--also set up central campsites, or home bases, and divide work duties among men and women. Women gather readily available plant and animal foods, while men take on the often less successful task of hunting. Female and male family members and relatives bring together their food to share at their home base. The modern form of the home base--which also serves as a haven for raising children and caring for the sick and elderly--may have first developed with middle Homo after about 1.7 million years ago. However, the first evidence of hearths and shelters--common to all modern home bases--comes from only after 500,000 years ago. Thus, a modern form of social life may not have developed until late in human evolution. B Subsistence Human subsistence refers to the types of food humans eat, the technology used in and methods of obtaining or producing food, and the ways in which social groups or societies organize themselves for getting, making, and distributing food. For millions of years, humans probably fed on-the-go, much as other primates do. The lifestyle associated with this feeding strategy is generally organized around small, family-based social groups that take advantage of different food sources at different times of year. The early human diet probably resembled that of closely related primate species. The great apes eat mostly plant foods. Many primates also eat easily obtained animal foods such as insects and bird eggs. Among the few primates that hunt, chimpanzees will prey on monkeys and even small gazelles. The first humans probably also had a diet based mostly on plant foods. In addition, they undoubtedly ate some animal foods and might have done some hunting. Human subsistence began to diverge from that of other primates with the production and use of the first stone tools. With this development, the meat and marrow (the inner, fat-rich tissue of bones) of large mammals became a part of the human diet. Thus, with the advent of stone tools, the diet of early humans became distinguished in an important way from that of apes. Scientists have found broken and butchered fossil bones of antelopes, zebras, and other comparably sized animals at the oldest archaeological sites, which date from about 2.5 million years ago. With the evolution of late Homo, humans began to hunt even the largest animals on Earth, including mastodons and mammoths, members of the elephant family. Agriculture and the domestication of animals arose only in the recent past, with H. sapiens. B1 Models of Subsistence in Early Homo Paleoanthropologists have debated whether early members of the modern human genus were aggressive hunters, peaceful plant gatherers, or opportunistic scavengers. Many scientists once thought that predation and the eating of meat had strong effects on early human evolution. This hunting hypothesis suggested that early humans in Africa survived particularly arid periods by aggressively hunting animals with primitive stone or bone tools. Supporters of this hypothesis thought that hunting and competition with carnivores powerfully influenced the evolution of human social organization and behavior; toolmaking; anatomy, such as the unique structure of the human hand; and intelligence. Beginning in the 1960s, studies of apes cast doubt on the hunting hypothesis. Researchers discovered that chimpanzees cooperate in hunts of at least small animals, such as monkeys. Hunting did not, therefore, entirely distinguish early humans from apes, and therefore hunting alone may not have determined the path of early human evolution. Some scientists instead argued in favor of the importance of food-sharing in early human life. According to a food-sharing hypothesis, cooperation and sharing within family groups--instead of aggressive hunting--strongly influenced the path of human evolution. Scientists once thought that archaeological sites as much as 2 million years old provided evidence to support the food-sharing hypothesis. Some of the oldest archaeological sites were places where humans brought food and stone tools together. Scientists thought that these sites represented home bases, with many of the social features of modern hunter-gatherer campsites, including the sharing of food between pair-bonded males and females. Critique of the food-sharing hypothesis resulted from more careful study of animal bones from the early archaeological sites. Microscopic analysis of these bones revealed the marks of human tools and carnivore teeth, indicating that both humans and potential predators--such as hyenas, cats, and jackals--were active at these sites. This evidence suggested that what scientists had thought were home bases where early humans shared food were in fact food-processing sites that humans abandoned to predators. Thus, evidence did not clearly support the idea of food-sharing among early humans. The new research also suggested a different view of early human subsistence--that early humans scavenged meat and bone marrow from dead animals and did little hunting. According to this scavenging hypothesis, early humans opportunistically took parts of animal carcasses left by predators, and then used stone tools to remove marrow from the bones. Observations that many animals, such as antelope, often die off in the dry season make the scavenging hypothesis quite plausible. Early toolmakers would have had plenty of opportunity to scavenge animal fat and meat during dry times of the year. However, other archaeological studies--and a better appreciation of the importance of hunting among chimpanzees--suggest that the scavenging hypothesis is too narrow. Many scientists now believe that early humans both scavenged and hunted. Evidence of carnivore tooth marks on bones cut by early human toolmakers suggests that the humans scavenged at least the larger of the animals they ate. They also ate a variety of plant foods. Some disagreement remains, however, as to how much early humans relied on hunting, especially the hunting of smaller animals. B2 The Rise of Hunting Scientists debate about when humans first began hunting on a regular basis. For instance, elephant fossils found with tools made by middle Homo once led researchers to the idea that members of this species were hunters of big game. However, the simple association of animal bones and tools at the same site does not necessarily mean that early humans had killed the animals or eaten their meat. Animals may die in many ways, and natural forces can accidentally place fossils next to tools. Recent excavations at Olorgesailie, Kenya, show that H. erectus cut meat from elephant carcasses but do not reveal whether these humans were regular or specialized hunters. Humans who lived outside of Africa--especially in colder temperate climates--almost certainly needed to eat more meat than their African counterparts. Humans in temperate Eurasia would have had to learn about which plants they could safely eat, and the number of available plant foods would drop significantly during the winter. Still, although scientists have found very few fossils of edible or eaten plants at early human sites, early inhabitants of Europe and Asia probably did eat plant foods in addition to meat. Sites that provide the clearest evidence of early hunting include Boxgrove, England, where about 500,000 years ago people trapped a great number of large game animals between a watering hole and the side of a cliff and then slaughtered them. At Schöningen, Germany, a site about 400,000 years old, scientists have found wooden spears with sharp ends that were well designed for throwing and probably used in hunting large animals. Neandertals and other archaic humans seem to have eaten whatever animals were available at a particular time and place. So, for example, in European Neandertal sites, the number of bones of reindeer (a cold-weather animal) and red deer (a warm-weather animal) changed depending on what the climate had been like. Neandertals probably also combined hunting and scavenging to obtain animal protein and fat. For at least the past 100,000 years, various human groups have eaten foods from the ocean or coast, such as shellfish and some sea mammals and birds. Others began fishing in interior rivers and lakes. Between probably 90,000 and 80,000 years ago people in Katanda, in what is now the Democratic Republic of the Congo, caught large catfish using a set of barbed bone points, the oldest known specialized fishing implements. The oldest stone tips for arrows or spears date from about 50,000 to 40,000 years ago. These technological advances, probably first developed by early modern humans, indicate an expansion in the kinds of foods humans could obtain. Beginning 40,000 years ago humans began making even more significant advances in hunting dangerous animals and large herds, and in exploiting ocean resources. People cooperated in large hunting expeditions in which they killed great numbers of reindeer, bison, horses, and other animals of the expansive grasslands that existed at that time. In some regions, people became specialists in hunting certain kinds of animals. The familiarity these people had with the animals they hunted appears in sketches and paintings on cave walls, dating from as much as 32,000 years ago. Hunters also used the bones, ivory, and antlers of their prey to create art and beautiful tools. In some areas, such as the central plains of North America that once teemed with a now-extinct type of large bison (Bison occidentalis), hunting may have contributed to the extinction of entire species. C Tools The making and use of tools alone probably did not distinguish early humans from their ape predecessors. Instead, humans made the important breakthrough of using one tool to make another. Specifically, they developed the technique of precisely hitting one stone against another, known as knapping. Stone toolmaking characterized the period sometimes referred to as the Stone Age, which began at least 2.5 million years ago in Africa and lasted until the development of metal tools within the last 7,000 years (at different times in different parts of the world). Although early humans may have made stone tools before 2.5 million years ago, toolmakers may not have remained long enough in one spot to leave clusters of tools that an archaeologist would notice today. The earliest simple form of stone toolmaking involved breaking and shaping an angular rock by hitting it with a palm-sized round rock known as a hammerstone. Scientists refer to tools made in this way as Oldowan, after Olduvai Gorge in Tanzania, a site from which many such tools have come. The Oldowan tradition lasted for about 1 million years. Oldowan tools include large stones with a chopping edge, and small, sharp flakes that could be used to scrape and slice. Sometimes Oldowan toolmakers used anvil stones (flat rocks found or placed on the ground) on which hard fruits or nuts could be broken open. Chimpanzees are known to do this today. Scientists once thought that Oldowan toolmakers intentionally produced several different types of tools. It now appears that differences in the shapes of larger tools were a byproduct of detaching flakes from a variety of natural rock shapes. Learning the skill of Oldowan toolmaking certainly required observation, but not necessarily instruction or language. Thus, Oldowan tools were simple, and their makers used them for such purposes as cutting up animal carcasses, breaking bones to obtain marrow, cleaning hides, and sharpening sticks for digging up edible roots and tubers. Oldowan toolmakers sought out the best stones for making tools and carried them to food-processing sites. At these sites, the toolmakers would butcher carcasses and eat the meat and marrow, thus avoiding any predators that might return to a kill. This behavior of bringing food and tools together contrasts with an eat-as-you-go strategy of feeding commonly seen in other primates. The Acheulean toolmaking tradition, which began sometime between 1.7 million and 1.5 million years ago, consisted of increasingly symmetrical tools, most of which scientists refer to as handaxes and cleavers. Acheulean toolmakers, such as Homo erectus, also worked with much larger pieces of stone than did Oldowan toolmakers. The symmetry and size of later Acheulean tools shows increased planning and design--and thus probably increased intelligence--on the part of the toolmakers. The Acheulean tradition continued for over 1.35 million years. The next significant advances in stone toolmaking were made by at least 200,000 years ago. One of these methods of toolmaking, known as the prepared core technique (and Levallois in Europe), involved carefully and exactingly knocking off small flakes around one surface of a stone and then striking it from the side to produce a preformed tool blank, which could then be worked further. Within the past 40,000 years, modern humans developed the most advanced stone toolmaking techniques. The so-called prismatic-blade core toolmaking technique involved removing the top from a stone, leaving a flat platform, and then breaking off multiple blades down the sides of the stone. Each blade had a triangular cross-section, giving it excellent strength. Using these blades as blanks, people made exquisitely shaped spearheads, knives, and numerous other kinds of tools. The most advanced stone tools also exhibit distinct and consistent regional differences in style, indicating a high degree of cultural diversity. D Environmental Adaptation Early humans experienced dramatic shifts in their environments over time. Fossilized plant pollen and animal bones, along with the chemistry of soils and sediments, reveal much about the environmental conditions to which humans had to adapt. By 8 million years ago, the continents of the world, which move over very long periods, had come to the positions they now occupy. But the crust of the Earth has continued to move since that time. These movements have dramatically altered landscapes around the world. Important geological changes that affected the course of human evolution include those in southern Asia that formed the Himalayan mountain chain and the Tibetan Plateau, and those in eastern Africa that formed the Great Rift Valley. The formation of major mountain ranges and valleys led to changes in wind and rainfall patterns. In many areas dry seasons became more pronounced, and in Africa conditions became generally cooler and drier. By 5 million years ago, the amount of fluctuation in global climate had increased. Temperature fluctuations became quite pronounced during the Pliocene Epoch (5 million to 1.6 million years ago). During this time the world entered a period of intense cooling called an ice age, which began around 2.8 million years ago. Ice ages cycle through colder phases known as glacials (times when glaciers form) and warmer phases known as interglacials (during which glaciers melt). During the Pliocene, glacials and interglacials each lasted about 40,000 years each. The Pleistocene Epoch (1.6 million to 10,000 years ago), in contrast, had much larger and longer ice age fluctuations. For instance, beginning about 700,000 years ago, these fluctuations repeated roughly every 100,000 years. Between 5 million and 2 million years ago, a mixture of forests, woodlands, and grassy habitats covered most of Africa. Eastern Africa entered a significant drying period around 1.7 million years ago, and after 1 million years ago large parts of the African landscape turned to grassland. So the early australopiths and early Homo lived in relatively wooded places, whereas Homo ergaster and H. erectus lived in areas of Africa that were more open. Early human populations encountered many new and different environments when they spread beyond Africa, including colder temperatures in the Near East and bamboo forests in Southeast Asia. By about 1.4 million years ago, populations had moved into the temperate zone of northeast Asia, and by 800,000 years ago they had dispersed into the temperate latitudes of Europe. Although these first excursions to latitudes of 40° north and higher may have occurred during warm climate phases, these populations also must have encountered long seasons of cold weather. All of these changes--dramatic shifts in the landscape, changing rainfall and drying patterns, and temperature fluctuations--posed challenges to the immediate and long-term survival of early human populations. Populations in different environments evolved different adaptations, which in part explains why more than one species existed at the same time during much of human evolution. Some early human adaptations to new climates involved changes in physical (anatomical) form. For example, the physical adaptation of having a tall, lean body such as that of H. ergaster--with lots of skin exposed to cooling winds--would have dissipated heat very well. This adaptation probably helped the species to survive in the hotter, more open environments of Africa around 1.7 million years ago. Conversely, the short, wide bodies of the Neandertals would have conserved heat, helping them to survive in the ice age climates of Europe and western Asia. Increases in the size and complexity of the brain, however, made early humans progressively better at adapting through changes in cultural behavior. The largest of these brain-size increases occurred over the past 700,000 years, a period during which global climates and environments fluctuated dramatically. Human cultural behavior also evolved more quickly during this period, most likely in response to the challenges of coping with unpredictable and changeable surroundings. Humans have always adapted to their environments by adjusting their behavior. For instance, early australopiths moved both in the trees and on the ground, which probably helped them survive environmental fluctuations between wooded and more open habitats. Early Homo adapted by making stone tools and transporting their food over long distances, thereby increasing the variety and quantities of different foods they could eat. An expanded and flexible diet would have helped these toolmakers survive unexpected changes in their environment and food supply. When populations of H. erectus moved into the temperate regions of Eurasia, they faced new challenges to survival. During the colder seasons they had to either move away or seek shelter, such as in caves. Some of the earliest definitive evidence of cave dwellers dates from around 800,000 years ago at the site of Atapuerca in northern Spain. This site may have been home to early H. heidelbergensis populations. H. erectus also used caves for shelter. Eventually, early humans learned to control fire and to use it to create warmth, cook food, and protect themselves from other animals. The oldest known fire hearths date from between 450,000 and 300,000 years ago, at sites such as Bilzingsleben, Germany; Verteszöllös, Hungary; and Zhoukoudian (Chou-k'ou-tien), China. African sites as old as 1.6 million to 1.2 million years contain burned bones and reddened sediments, but many scientists find such evidence too ambiguous to prove that humans controlled fire. Early populations in Europe and Asia may also have worn animal hides for warmth during glacial periods. The oldest known bone needles, which indicate the development of sewing and tailored clothing, date from about 30,000 to 26,000 years ago. E Symbolic Thought--Language, Art, and Religion The evolution of cultural behavior relates directly to the development of the human brain, and particularly the cerebral cortex, the part of the brain that allows abstract thought, beliefs, and expression through language. Humans communicate through the use of symbols--ways of referring to things, ideas, and feelings that communicate meaning from one individual to another but that need not have any direct connection to what they identify. For instance, a word--one type of symbol--does not usually relate directly to the thing or idea it represents; it is abstract. English-speaking people use the word lion to describe a lion, not because a dangerous feline looks like the letters l-i-o-n, but because these letters together have a meaning created and understood by people. See also Culture: Culture Is Symbolic. People can also paint abstract pictures or play pieces of music that evoke emotions or ideas, even though emotions and ideas have no form or sound. In addition, people can conceive of and believe in supernatural beings and powers--abstract concepts that symbolize real-world events such as the creation of Earth and the universe, the weather, and the healing of the sick. Thus, symbolic thought lies at the heart of three hallmarks of modern human culture: language, art, and religion. E1 Language In language, people creatively join words together in an endless variety of sentences--each with a distinct meaning--according to a set of mental rules, or grammar. Language provides the ability to communicate complex concepts. It also allows people to exchange information about both past and future events, about objects that are not present, and about complex philosophical or technical concepts. Language gives people many adaptive advantages, including the ability to plan for the future, to communicate the location of food or dangers to other members of a social group, and to tell stories that unify a group, such as mythologies and histories. However, words, sentences, and languages cannot be preserved like bones or tools, so the evolution of language is one of the most difficult topics to investigate through scientific study. It appears that modern humans have an inborn instinct for language. Under normal conditions it is almost impossible for a person not to develop language, and people everywhere go through the same stages of increasing language skill at about the same ages. While people appear to have inborn genetic information for developing language, they learn specific languages based on the cultures from which they come and the experiences they have in life. The ability of humans to have language depends on the complex structure of the modern brain, which has many interconnected, specific areas dedicated to the development and control of language. The complexity of the brain structures necessary for language suggests that it probably took a long time to evolve. While paleoanthropologists would like to know when these important parts of the brain evolved, endocasts (inside impressions) of early human skulls do not provide enough detail to show this. Some scientists think that even the early australopiths had some ability to understand and use symbols. Support for this view comes from studies with chimpanzees. A few chimps and other apes have been taught to use picture symbols or American Sign Language for simple communication. Nevertheless, it appears that language--as well as art and religious ritual--became vital aspects of human life only during the past 100,000 years, primarily within our own species. E2 Art Humans also express symbolic thought through many forms of art, including painting, sculpture, and music. An apparent stone human figurine painted with red ocher was found at Tan-Tan, Morocco, in 1999. The object, which is at least 300,000 years old and possibly as old as 400,000 years, may be a naturally formed stone that was reworked by humans to emphasize its humanlike shape, making it the earliest sculpture known. Another object found at a site in Berekhat Ram, Israel, and made of red volcanic rock, has been interpreted as representing the outline of a female body. The piece dates from about 250,000 years ago and is controversial--some experts see it as the result of natural geological processes rather than human handiwork. Claims for the earliest art made by modern humans also come from Africa and the Middle East. Small, perforated shells found in Algeria, Israel, and South Africa, dating from between 100,000 and 75,000 years ago, may represent beads worn as personal ornaments. Pieces of ocher, a soft red iron mineral, were found at Blombos Cave in South Africa and have been dated to between 75,000 and 70,000 years ago. The objects were scraped and ground to create a flat surface and then etched with complex geometric lines as apparent decoration or symbolic meaning. Some researchers have interpreted grooves made on a large rock in a cave in Botswana as possible artistic or ritual acts that also might date to around 70,000 years ago. Part of the natural rock formation is said to resemble the head of a python. The proposed date for the markings and the artifacts found in the cave, as well as their possible significance, will require further study. Only a few other possible art objects are currently known from between 200,000 and 50,000 years ago. These items come from western Europe and are usually attributed to Neandertals. They include two objects that may have been simple pendants--a tooth and a bone with bored holes--and several grooved or polished fragments of tooth and bone. Sites dating from at least 400,000 years ago contain fragments of red and black pigment. Humans might have used these pigments to decorate bodies or perishable items, such as wooden tools or clothing of animal hides, but this evidence would not have survived to today. Solid evidence of the sophisticated use of pigments for symbolic purposes--such as in religious rituals--comes only from after 40,000 years ago. From early in this period, researchers have found carefully made types of crayons used in painting and evidence that humans burned pigments to create a range of colors. People began to create and use advanced types of symbolic objects after about 50,000 years ago. The archaeological record shows a tremendous blossoming of art between 32,000 and 15,000 years ago. During this period much of the art appears to have been used in rituals--possibly ceremonies to ask spirit beings for a successful hunt. People also adorned themselves with intricate jewelry of ivory, bone, and stone. They carved beautiful figurines representing animals and human forms. Many carvings, sculptures, and paintings depict stylized images of the female body. Some scientists think such female figurines represent fertility. Early wall paintings made sophisticated use of texture and color. The area of what is now southern France contains many famous sites of such paintings. These include the caves of Chauvet, which contain art over 32,000 years old, and Lascaux, in which paintings date from as much as 18,000 years ago. In some cases, artists painted on walls that can be reached only with special effort, such as by crawling. The act of getting to these paintings gives them a sense of mystery and ritual, as it must have to the people who originally viewed them, and archaeologists refer to some of the most extraordinary painted chambers as sanctuaries. Yet no one knows for sure what meanings these early paintings and engravings had for the people who made them. See also Paleolithic Art. E3 Religion A cave site near Atapuerca in Spain dated to around 400,000 years ago may contain the earliest evidence of human religion or ritual. The site includes a pit called Sima de los Huesos ("Pit of Bones"), which holds thousands of human bones belonging to about 30 individuals. Humans apparently did not live in the cave so the bodies must have been brought to the pit and deliberately thrown in. A single, carefully worked symmetrical hand ax was found with the bones. The remains have been attributed to Homo heidelbergensis, the human species common in Europe at the time and the possible ancestor of Neandertals. Graves from Europe and western Asia indicate that the Neandertals were the first humans to bury their dead, at least on occasion. Some sites contain very shallow graves, which group or family members may have dug simply to remove corpses from sight. In other cases it appears that groups may have observed rituals of grieving for the dead or communicating with spirits. Some researchers have claimed that grave goods, such as meaty animal bones or flowers, had been placed with buried bodies, suggesting that some Neandertal groups might have believed in an afterlife. In a large proportion of Neandertal burials, the corpse had its legs and arms drawn in close to its chest, which could indicate a ritual burial position. Other researchers have challenged these interpretations, however. They suggest that perhaps the Neandertals had practical rather than religious reasons for positioning dead bodies. For instance, a body manipulated into a fetal position would need only a small hole for burial, making the job of digging a grave easier. In addition, the animal bones and flower pollen near corpses could have been deposited by accident or without religious intention. Many scientists once thought that fossilized bones of cave bears (a now-extinct species of large bear) found in Neandertal caves indicated that these people had what has been referred to as a cave bear cult, in which they worshiped the bears as powerful spirits. However, after careful study researchers concluded that the cave bears probably died while hibernating and that Neandertals did not collect their bones or worship them. Considering current evidence, the case for religion among Neandertals remains controversial. The earliest evidence for religion or ritual in modern humans may come from Ethiopia, where paleoanthropologists found three skulls dated to about 160,000 years ago. The skull bones of two adults and a child show evidence of repeated handling and polishing, along with apparent decoration with scratch marks. The skulls were not found with other bones from the bodies, suggesting early modern Homo sapiens possibly carried around the detached skulls for ritual purposes. See also Religion: Rituals and Symbols. F Domestication, Agriculture, and the Rise of Civilizations One of the most important developments in human cultural behavior occurred when people began to domesticate (control the breeding of) plants and animals. Domestication and the advent of agriculture led to the development of dozens of staple crops (foods that form the basis of an entire diet) in temperate and tropical regions around the world. Almost the entire population of the world today depends on just four of these major crops: wheat, rice, corn, and potatoes. See also Crop Farming. F1 Human Manipulation of the Environment The growth of farming and animal herding initiated one of the most remarkable changes ever in the relationship between humans and the natural environment. The change first began just 10,000 years ago in the Near East and has accelerated very rapidly since then. It also occurred independently in other places, including areas of Mexico, China, and South America. Since the first domestication of plants and animals, many species over large areas of the planet have come under human control. The overall number of plant and animal species has decreased, while the populations of a few species needed to support large human populations have grown immensely. In areas dominated by people, interactions among plants and animals usually fall under the control of a single species--Homo sapiens. By the time of the initial transition to plant and animal domestication, the cold, glacial landscapes of 18,000 years ago had long since given way to warmer and wetter environments. At first, people adapted to these changes by using a wider range of natural resources. Later they began to focus on a few of the most abundant and hardy types of plants and animals. The plants people began to use in large quantities included cereal grains, such as wheat in western Asia; wild varieties of rice in eastern Asia; and maize, of which corn is one variety, in what is now Mexico. Some of the animals people first began to herd included wild goats in western Asia, wild ancestors of chickens in eastern Asia, and llamas in South America. By carefully collecting plants and controlling wild herd animals, people encouraged the development of species with characteristics favorable for growing, herding, and eating. This process of selecting certain species and controlling their breeding eventually created new species of plants, such as oats, barley, and potatoes; and animals, including cattle, sheep, and pigs. From these domesticated plant and animal species, people obtained important products, such as flour, milk, and wool. F2 Effects of Food Production on Human Society By harvesting and herding domesticated species, people could store large quantities of plant foods, such as seeds and tubers, and have a ready supply of meat and milk. These readily available supplies gave people some long-term food security. In contrast, the foraging lifestyle of earlier human populations never provided them with a significant store of food. With increased food supplies, agricultural peoples could settle into villages and have more children. The new reliance on agriculture and change to settled village life also had some negative effects. As the average diet became more dependent on large quantities of one or a few staple crops, people became more susceptible to diseases brought on by a lack of certain nutrients. A settled lifestyle also increased contact among people and between people and their refuse and waste matter, both of which acted to increase the incidence and transmission of disease. People responded to the increasing population density--and a resulting overuse of farming and grazing lands--in several ways. Some people moved to settle entirely new regions. Others devised ways of producing food in larger quantities and more quickly. The simplest way was to expand onto new fields for planting and new pastures to support growing herds of livestock. Many populations also developed systems of irrigation and fertilization that allowed them to reuse cropland and to produce greater amounts of food on existing fields. F3 The Rise of Civilizations The rise of civilizations--the large and complex types of societies in which most people still live today--developed along with surplus food production. People of high status eventually used food surpluses as a way to pay for labor and to create alliances among groups, often against other groups. In this way, large villages could grow into city-states (urban centers that governed themselves) and eventually empires covering vast territories. With surplus food production, many people could work exclusively in political, religious, or military positions; or in artistic and various skilled vocations. Command of food surpluses also enabled rulers to control laborers, such as in slavery. All civilizations developed based on such hierarchical divisions of status and vocation. The earliest civilization arose over 7,000 years ago in Sumer in what is now Iraq. Sumer grew powerful and prosperous by 5,000 years ago, when it centered on the city-state of Ur. The region containing Sumer, known as Mesopotamia, was the same area in which people had first domesticated animals and plants. Other centers of early civilizations include the Nile Valley of Northeast Africa, the Indus Valley of South Asia, the Yellow River Valley of East Asia, the Oaxaca and Mexico valleys and the Yucatán region of Central America, and the Andean region of South America. See also Egypt: History; China: History; Aztec; Maya Civilization; and Inca Empire. All early civilizations had some common features. Some of these included a bureaucratic political body, a military, a body of religious leadership, large urban centers, monumental buildings and other works of architecture, networks of trade, and food surpluses created through extensive systems of farming. Many early civilizations also had systems of writing, numbers and mathematics, and astronomy (with calendars); road systems; a formalized body of law; and facilities for education and the punishment of crimes. See also Writing: History of Writing; Number Systems; Mathematics: History of Mathematics; History of Astronomy: Ancient Origins; and Calendar: Ancient Calendars. With the rise of civilizations, human evolution entered a phase vastly different from all that came before. Prior to this time, humans had lived in small, family-centered groups essentially exposed to and controlled by forces of nature. Several thousand years after the rise of the first civilizations, most people now live in societies of millions of unrelated people, all separated from the natural environment by houses, buildings, automobiles, and numerous other inventions and technologies. Culture will continue to evolve quickly and in unforeseen directions, and these changes will, in turn, influence the physical evolution of Homo sapiens and any other human species to come. Contributed By: Richard B. Potts Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.