Human Evolution

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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.

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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.

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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.

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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.

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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.

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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.

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