FAMILY TREE of the hominid Australopithecus (red) includes a number of species that lived between roughly 4 million and 1.25 million years (Myr) ago. Just over 2 Myr ago a new genus, Homo (which includes our own species, H. sapiens), evolved from one of the species of Australopithecus.

joint structures typical of tree climbers. Some scientists argue that such characteristics indicate that these hominids must have spent at least some time in the trees. But others view these features as simply evolutionary baggage, left over from arboreal ancestors. Underlying this discussion is the question of where Australopithecus lived—in forests or on the open savanna.

By the beginning of the 1990s, researchers knew a fair amount about the various species of Australopithecus and how each had adapted to its environmental niche. A description of any one of the species would mention that the creatures were bipedal and that they had ape-size brains and large, thickly enameled teeth in strong jaws, with nonprojecting canines. Males were typically larger than females, and individuals grew and matured rapidly. But the origins of Australopithecus were only hinted at, because the gap between the earliest well-known species in the group (afarensis, from about 3.6 million years ago) and the postulated time of the last common ancestor of chimpanzees and humans (about six million years ago, according to molecular evidence) was still very great. Fossil hunters had unearthed only a few older fragments of bone, tooth and jaw from the intervening 1.5 million years to indicate the anatomy and course of evolution of the earliest hominids.

Filling the Gap discoveries in Kenya over the past several years have filled in some of the missing interval between 3.5 million and 5 million years ago. Beginning in

MEAVE LEAKEY and ALAN WALKER, together with Leakey's husband, Richard, have collaborated for many years on the discovery and analysis of early hominid fossils from Kenya. Meave Leakey is a researcher and former head of the division of paleontology at the National Museums of Kenya in Nairobi. Walker is Evan Pugh Professor of Anthropology and Biology at Pennsylvania State University. He is a MacArthur Fellow and a member of the American Academy of Arts and Sciences.

1982, expeditions run by the National Museums of Kenya to the Lake Turkana basin in northern Kenya began finding hominid fossils nearly four million years old. But because these fossils were mainly isolated teeth—no jawbones or skulls were preserved—very little could be said about them except that they resembled the remains of afarensis from Laetoli. But our excavations at an unusual site, just inland from Allia Bay on the east side of Lake Turkana [see maps on page 18], yielded more complete fossils.

The site at Allia Bay is a bone bed, where millions of fragments of weathered tooth and bone from a wide variety of animals, including hominids, spill out of the hillside. Exposed at the top of the hill lies a layer of hardened volcanic ash called the Moiti Tuff, which has been dated radiometrically to just over 3.9 million years old. The fossil fragments lie several meters below the tuff, indicating that the remains are older than the tuff. We do not yet understand fully why so many fossils are concentrated in this spot, but we can be certain that they were deposited by the precursor of the present-day Omo River.

Today the Omo drains the Ethiopian highlands located to the north, emptying into Lake Turkana, which has no outlet. But this has not always been so. Our colleagues Frank Brown of the University of Utah and Craig Feibel of Rutgers University have shown that the ancient Omo

River dominated the Turkana area for much of the Pliocene (roughly 5.3 to 1.8 million years ago) and the early Pleistocene (1.8 to 0.7 million years ago). Only infrequently was a lake present in the area at all. Instead, for most of the past four million years, an extensive river system flowed across the broad floodplain, proceeding to the Indian Ocean without dumping its sediments into a lake.

The Allia Bay fossils are located in one of the channels of this ancient river system. Most of the fossils collected from Allia Bay are rolled and weathered bones and teeth of aquatic animals— fish, crocodiles, hippopotamuses and the like—that were damaged during transport down the river from some distance away. But some of the fossils are much better preserved; these come from the animals that lived on or near the river-banks. Among these creatures are several different species of leaf-eating monkeys, related to modern colobus monkeys, as well as antelopes whose living relatives favor closely wooded areas. Reasonably well preserved hominid fossils can also be found here, suggesting that, at least occasionally, early homi-nids inhabited a riparian habitat.

Where do these Australopithecus fossils fit in the evolutionary history of hominids? The jaws and teeth from Allia Bay, as well as a nearly complete radius (the outside bone of the forearm) from the nearby sediments of Sibilot just

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