Redrawing the Family Tree

By MICHAEL D. LEMONICK

Now that's a big monkey, thought Paleontologist Alan Walker as he plucked the skull fragment from a gully west of Kenya's Lake Turkana. But that was no monkey. The bone belonged to a 2.5 million-year-old ape-man called Australopithecus boisei. The discovery surprised Walker, since he and most anthropologists believed the boisei species had evolved 2.2 million years ago. "This is probably more significant than almost anything we've had for a good number of years," says Anthropologist Richard Leakey, one of Walker's coauthors of a report about the fossil in last week's issue of Nature. Leakey's excitement is understandable: the find casts doubt on a widely held belief that the human lineage arose from the earliest known species of Australopithecus. It also upsets the accepted view of australopithecine evolution.

Most scientists agree that the small-brained australopithecines were the first manlike creatures to walk upright, 3.5 million or more years ago, and that their evolution ran parallel to that of humanity's direct ancestors. The dispute arises over details. Some researchers, including Anthropologist Donald Johanson, director of the Berkeley-based Institute of Human Origins, think that a single species, Australopithecus afarensis, which includes the celebrated 3 million-year-old skeleton called Lucy, was the common ancestor of all later australopithecines, as well as man. The two branches, they say, split about 3 million years ago, with the Australopithecus line dying out 1 million years ago. Leakey, on the other hand, believes the common ancestor is an older species, which is yet to be found.

Last summer Walker, a professor at the Johns Hopkins University medical school, was looking for baboon fossils, when he spotted the skull fragment. By studying volcanic ash and other bones nearby, his colleagues determined the skull's age. Its pedigree was trickier. It has the structure of a late australopithecine: wide palate, huge rear molars, enormous cheekbones and a pronounced crest of bone running along the top of the skull. But other features -- a for- ward-thrusting muzzle, an orangutan-size brain and an apelike jaw structure -- are primitive. Leakey believes this mosaic suggests, as he has argued for years, that Johanson is wrong and that his reconstruction of afarensis is actually based on two different species. And, Leakey says, the new fossil, labeled WT 17000, resembles one of them.

The fossil record had also apparently shown that the australopithecines evolved in an orderly way: first came afarensis, followed by africanus, then robustus and boisei. But the age and form of WT 17000 convinced Leakey and Walker that the lineage was not simple after all. Boisei did not descend from robustus, and probably not even from afarensis.

"This skull is the most exciting find since Lucy," says Eric Delson, an anthropologist at the City University of New York's Lehman College. "Relationships among australopithecines will need to be somewhat revised." That will not surprise anthropologists. Although the current diagram of humanity's family tree is based on thousands of specimens, most of them are frustratingly incomplete. Walker's fossil may force a revision in the textbooks, but it is not likely to be the last one.

With reporting by Maryanne Vollers/Nairobi