Kerio River

to a changed diet—possibly much harder food—even though its jaws and some skull features were still very apelike. We also know that anamensis had only a tiny external ear canal. In this regard, it is more like chimpanzees and unlike all later hominids, including humans, which have large external ear canals. (The size of the external canal is unrelated to the size of the fleshy ear.)

The most informative bone of all the ones we have uncovered from this new hominid is the nearly complete tibia—the larger of the two bones in the lower leg. The tibia is revealing because of its important role in weight bearing: the tibia of a biped is distinctly different from the tibia of an animal that walks on all four legs. In size and practically all details of the knee and ankle joints, the tibia found at Kanapoi closely resembles the one from the fully bipedal afarensis found at Hadar, even though the latter specimen is almost a million years younger.

Fossils of other animals collected at Kanapoi point to a somewhat different paleoecological scenario from the setting across the lake at Allia Bay. The channels of the river that laid down the sediments at Kanapoi were probably lined with narrow stretches of forest that grew close to the riverbanks in otherwise open country. Researchers have recovered the remains of the same spiral-horned antelope found at Allia Bay that very likely lived in dense thickets. But open-country antelopes and hartebeest appear to have lived at Kanapoi as well, suggesting that more open savanna prevailed away from the rivers. These results offer equivocal evidence regarding the preferred habitat of anamensis: we know that bushland was present at both sites that have yielded fossils of the species, but there are clear signs of more diverse habitats at Kanapoi.

An Even Older Hominid?

AT ABOUT THE SAME TIME that we were finding new hominids at Allia Bay and Kanapoi, a team led by our colleague Tim D. White of the University of California at Berkeley discovered fossil hom-inids in Ethiopia that are even older than anamensis. In 1992 and 1993 White led

TURKANA BASIN was home to anamensis roughly four million years ago. Around 3.9 million years ago a river sprawled across the basin [left). The fossil site Allia Bay sat within the strip of forest [green] that lined this river. Some 4.2 million years ago a large lake filled the basin [right); a second site, Kanapoi, was located on a river delta that fed into the lake.

an expedition to the Middle Awash area of Ethiopia, where his team uncovered hominid fossils at a site known as Ara-mis. The group's finds include isolated teeth, a piece of a baby's mandible (the lower jaw), fragments from an adult's skull and some arm bones, all of which have been dated to around 4.4 million years ago. In 1994, together with his colleagues Berhane Asfaw of the Paleoanthropology Laboratory in Addis Ababa and Gen Suwa of the University of Tokyo, White gave these fossils a new name: Australopithecus ramidus. In 1995 the group renamed the fossils, moving them to a new genus, Ardipithecus. Earlier fossils of this genus have now been found dating back to 5.8 million years ago. Other fossils buried near the hominids, such as seeds and the bones of forest monkeys and antelopes, strongly imply that these hominids, too, lived in a closed-canopy woodland.

This new species represents the most primitive hominid known—a link between the African apes and Australopithecus. Many of the Ardipithecus ram-idus fossils display similarities to the anatomy of the modern African great apes, such as thin dental enamel and strongly built arm bones. In other features, though—such as the opening at the base of the skull, technically known as the foramen magnum, through which the spinal cord connects to the brain— the fossils resemble later hominids.

Describing early hominids as either primitive or more advanced is a complex issue. Scientists now have almost decisive molecular evidence that humans and chimpanzees once had a common ancestor and that this lineage had previously split from gorillas. This is why we often use the two living species of chimpanzee (Pan troglodytes and P. panis-cus) to illustrate ancestral traits. But we must remember that since their last common ancestor with humans, chimpanzees have had exactly the same amount of time to evolve as humans have. Determining which features were present in the last common ancestor of humans and chimpanzees is not easy.

But Ardipithecus, with its numerous chimplike features, appears to have tak en the human fossil record back close to the time of the chimp-human split. More recently, White and his group have found parts of a single Ardipithecus skeleton in the Middle Awash region. As White and his team extract these exciting new fossils from the enclosing stone, reconstruct them and prepare them for study, the pa-leoanthropological community eagerly anticipates the publication of the group's analysis of these astonishing finds.

But even pending White's results, new fossil discoveries are offering other surprises. A team led by Michel Brunet of the University of Poitiers has found fragments of Australopithecus fossils in Chad. Surprisingly, these fossils were recovered far from either eastern or southern Africa, the only areas where Australopithecus had appeared. The Chad sites lie 2,500 kilometers west of the western part of the Rift Valley, thus extending the range of Australopithecus well into the center of Africa.

These fossils debunk a hypothesis about human evolution postulated by Dutch primatologist Adriaan Kortlandt and expounded in Scientific American by Yves Coppens of the College of France [see "East Side Story: The Origin of Humankind," May 1994]. This idea was that the formation of Africa's Rift Valley subdivided a single ancient species, isolating the ancestors of hominids on the east side from the ancestors of modern apes on the west side.

Brunet's latest discovery, an important cranium older than six million years, is also from Chad and shows that early hominids were probably present across much of the continent. This cranium, which the team called Sahelanthropus tchadensis, together with fragmentary jaws and limb bones from about six million years ago in Kenya [see "An Ancestor to Call Our Own," on page 4], are even older than the Ardipithecus fossils.

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