Early human ancestor unearthed in Ethiopia

Fossil bones and teeth spark new controversy over primitive hominids

March 05, 2004|By NEW YORK TIMES NEWS SERVICE

Another species has been added to the family tree of early human ancestors - and to controversies over how straight or tangled were the branches of that tree.

Long before Homo erectus, Australopithecus afarensis (Lucy, more than 3 million years ago) and several other distant kin, scientists are reporting today, there lived a primitive hominid species in what is now Ethiopia about 5.5 million to 5.8 million years ago.

That would make the newly recognized species one of the earliest-known human ancestors, perhaps one of the first to emerge after the chimpanzee and human lineages diverged from a common ancestor 6 million to 8 million years ago.

The timing of the fateful split has been determined by molecular biological research, and in recent years fossil hunters have found traces of what those earliest hominids, human ancestors and their close relatives, might have been like.

When the first fossil bones and teeth of this hominid were described three years ago, paleoanthropologists tentatively identified it as a more apelike subspecies that they named Ardipithecus ramidus kadabba. The original ramidus species had been found in 1994 in 4.4-million-year-old sediments, also in Ethiopia.

But with more discoveries and a closer study, especially of the teeth, the scientists decided that the kadabba fossils from five individuals were distinctive enough to qualify as a separate species: Ardipithecus kadabba. In that case, the scientists added, kadabba was not a subspecies but the likely direct ancestor of ramidus. But there were too few skeletal bones yet to learn much about the size and other aspects of kadabba.

The description and interpretation of the new hominid species appears in today's issue of the journal Science. The authors of the report are Dr. Yohannes Haile-Selassie of the Cleveland Museum of Natural History, Dr. Gen Suwa of the University of Tokyo and Dr. Tim D. White of the University of California, Berkeley.

The kadabba fossils were found in the Middle Awash valley about 180 miles northeast of Addis Ababa, the Ethiopian capital. These are arid badlands now, but in the time of the early hominids the land was wooded and more hospitable.

Haile-Selassie said the shapes and wear patterns of six teeth in particular were "significant in understanding how the dentition evolved from an apelike common ancestor into the earliest hominids." They were also critical, he said, in differentiating the earlier and later species of the genus Ardipithecus.

Other scientists familiar with the research, but not involved in it, said they agreed or at least were inclined to agree with the authors' designation of a separate species for the fossils. But they were not so sure about the authors' proposal that the fossils were so similar to those of two other recently discovered early species that all three species may have actually belonged to a single genus of closely related hominids.

In their report on kadabba, Haile-Selassie and his colleagues concluded: "Given the limited data currently available, it is possible that all of these remains represent specific or subspecific variation within a single genus."

But in an accompanying commentary in the journal, Dr. David R. Begun, a paleontologist at the University of Toronto, questioned this interpretation. He said it was unlikely that all three of the early hominids belonged to a single genus, noting instead that the three exhibited evidence of striking diversity.

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