Humerus

Primates such as chimpanzees that walk on their knuckles have a deep, oval hollow at the bottom of the humerus where the humerus and the ulna lock in place, making the elbow joint more stable

The human jaw widens at the back ofthe mouth

The human jaw widens at the back ofthe mouth

In the tibias of anamensis and humans, the top of the bone is wider because of the extra spongy bone tissue present, which serves as a shock absorber in bipedal creatures
Human and anamensis bones lack this feature, suggesting that, like humans, anamensis did not walk on its knuckles

FOSSILS from anamensis (center) share a number of features in common with both humans (right) and modern chimpanzees (left). Scientists use the similarities and differences among these species to determine their interrelationships and thereby piece together the course of hominid evolution since the lineages of chimpanzees and humans split some five or six million years ago.

to the north, show an interesting mixture of characteristics. Some of the traits are primitive ones—that is, they are ancestral features thought to be present before the split occurred between the chimpanzee and human lineages. Yet these bones also share characteristics seen in later hominids and are therefore said to have more advanced features. As our team continues to unearth more bones and teeth at Allia Bay, these new fossils add to our knowledge of the wide range of traits present in early hominids.

Across Lake Turkana, some 145 kilometers (about 90 miles) south of Allia Bay, lies the site of Kanapoi, where our story began. One of us (Leakey) has mounted expeditions from the National Museums of Kenya to explore the sediments located southwest of Lake Turka-na and to document the faunas present during the earliest stages of the basin's history. Kanapoi, virtually unexplored since Patterson's day, has proved to be one of the most rewarding sites in the Turkana region.

A series of deep erosion gullies, known as badlands, has exposed the sediments at Kanapoi. Fossil hunting is difficult here, though, because of a carapace of lava pebbles and gravel that makes it hard to spot small bones and teeth. Studies of the layers of sediment, also carried out by Feibel, reveal that the fossils here have been preserved by deposits from a river ancestral to the present-day Kerio River, which once flowed into the Tur-kana basin and emptied into an ancient lake that we call Lonyumun. This lake reached its maximum size about 4.1 million years ago and thereafter shrank as it filled with sediments.

Excavations at Kanapoi have primarily yielded the remains of carnivore meals, so the fossils are rather fragmentary. But workers at the site have also recovered two nearly complete lower jaws, one complete upper jaw and lower face, the upper and lower thirds of a tibia, bits of skull and several sets of isolated teeth. After careful study of the fossils from both Allia Bay and Kanapoi—including Patterson's fragment of an arm bone— we felt that in details of anatomy, these specimens were different enough from previously known hominids to warrant designating a new species. So in 1995, in collaboration with both Feibel and Ian McDougall of the Australian National University, we named this new species Australopithecus anamensis, drawing on the Turkana word for "lake" (anam) to refer to both the present and ancient lakes.

To establish the age of these fossils, we relied on the extensive efforts of Brown, Feibel and McDougall, who have been investigating the paleogeographic history of the entire lake basin. If their study of the basin's development is correct, the anamensis fossils should be between 4.2 and 3.9 million years old. Mc-Dougall has determined the age of the so-called Kanapoi Tuff—the layer of volcanic ash that covers most of the fossils at this site—to be just over four million years old. Now that he has successfully ascertained the age of the tuff, we are

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