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.

B.1.

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.

B.2.

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.

C.1.

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.

C.2.

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.

C.3.

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.

C.4.

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.

C.5.

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 well-preserved 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.

C.6.

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.

C.7.

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.

C.8.

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.

D.1.

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.

D.2.

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.

D.3.

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.

D.4.

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 plant-eating 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.