Skull Puzzle Helps Scientists Trail Mankind's Early Origins

FOR fossil hunter Donald Johanson and his colleagues, matching up a collection of skull fragments was like trying to put Humpty Dumpty together again. But unlike the king's horses and men, they were able to do the job. Now they can look back across 3 million years of evolution and see what they believe is the face of an ancient relative.

They have reconstructed the first nearly complete skull of Australopithecus afarensis - a small-brained, upright-walking hominid that is the earliest known ancestor of our own human species.

What's remarkable about this 3 million-year-old fossil is not that it is so old but that it's so young. It is 200,000 years younger than the famous Lucy partial skeleton found in 1974, which first brought the species to public attention, and a million years younger than the oldest specimen. Yet it looks like those ancestors.

``One of the astonishing aspects is that we seem to have established stasis [stability of form] in a species for a million years,'' Dr. Johanson said in a telephone interview.

Yet he noted that, within a few hundred thousand years after the date of the skull, there was rapid evolution into several different hominid species, one of which led to the human evolutionary line. He added that the ``next excitement'' will come with discovery of hominid fossils in the 2.5 million-year-old time frame when this adaptive radiation of species took place.

Johanson, president of the Institute of Human Origins in Berkeley, Calif.; his colleague, William Kimbel; and Israeli paleoanthropologist Yoel Rak of Tel Aviv University described the skull reconstruction in yesterday's issue of the journal Nature. Dr. Rak and local assistants found the skull's fragments in the Hadar area of Ethiopia's Afar region. It's one of 53 hominid fossils that investigators from the Berkeley institute and the Ethiopian Center for Research and Conservation of Cultural Heritage have found in the area during the past four years.

Asked how investigators know the fragments make up a single skull, Dr. Kimbel said that ``it's a straightforward process.'' He explained that the fragments were found in a cluster and that they fit together well to form 70 to 75 percent of a complete skull.

``It's as if you were putting together a broken plate,'' he said.