Human Evolution

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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.

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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.

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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.

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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.

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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.

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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.

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