Tuesday, August 06, 2002

PUZZLING ANCIENT SKULL

Skulls Found in Africa and in Europe Challenge Theories of Human Origins
By JOHN NOBLE WILFORD

Two ancient skulls, one from central Africa and the other from the Black Sea republic of Georgia, have shaken the human family tree to its roots, sending scientists scrambling to see if their favorite theories are among the fallen fruit....

The African skull dates from nearly 7 million years ago, close to the fateful moment when the human and chimpanzee lineages went their separate ways. The 1.75-million-year-old Georgian skull could answer questions about the first human ancestors to leave Africa, and why they ventured forth....

About a million years older than any previously recognized hominid, Toumai lived close to the time that molecular biologists think was the earliest period in which the human lineage diverged from the chimpanzee branch....

The most puzzling aspect of the new skull is that it seems to belong to two widely separated evolutionary periods. Its size indicates that Toumai had a brain comparable to that of a modern chimp, about 320 to 380 cubic centimeters. Yet the face is short and relatively flat, compared with the protruding faces of chimps and other early hominids. Indeed, it is more humanlike than the "Lucy" species, Australopithecus afarensis, which lived more than 3.2 million years ago.

"A hominid of this age," Dr. Wood wrote in Nature, "should certainly not have the face of a hominid less than one-third of its geological age."

Scientists suggest several possible explanations. Toumai could somehow be an ancestor of modern humans, or of gorillas or chimps. It could be a common ancestor of humans and chimps, before the divergence.

"But why restrict yourself to thinking this fossil has to belong to a lineage that leads to something modern?" Dr. Wood asked. "It's perfectly possible this belongs to a branch that's neither chimp nor human, but has become extinct."...