Skull find challenges early human theories

3.5 million-year-old fossil represents new genus, paleontologist says


Paleontologists in Africa have found the 3.5 million-year-old skull from what they say is an entirely new branch of the early human family tree, a discovery that threatens to overturn the prevailing view that a single line of descent stretched through the early stages of human ancestry.

The discoverers and other scientists of human evolution say they are not necessarily surprised by the findings, but certainly confused. Now it seems the fossil species Australopithecus afarensis, which lived from about 4 million to 3 million years ago and is best known from the celebrated Lucy skeleton, was not alone on the African plain. Lucy may not even be a direct human ancestor, after all.

Indeed, the family tree, once drawn with a trunk straight and true, is beginning to look more like a bush, with a tangle of branches of uncertain relationship leading off in many directions.

The skull discovery was made in 1999 by a research team led by Dr. Meave G. Leakey excavating on the western side of Lake Turkana in northern Kenya. Only after careful analysis did the scientists conclude that the nearly complete skull and partial jaw represented a completely different genus and species. The flattened face and small molar teeth were strikingly different from those of the contemporary afarensis, or Lucy, species.

In a report in today's issue of the journal Nature, Leakey named the new member of the hominid family Kenyanthropus platyops, flat-faced man of Kenya. The dates for the fossils, ranging from 3.2 million to 3.5 million years old, were derived from volcanic ash at the site. The gender of the individual has not been determined.

"Kenyanthropus shows persuasively that at least two lineages existed as far back as 3.5 million years," Leakey said in a statement issued by the National Museums of Kenya in Nairobi, where she is the principal paleoanthropologist.

In a telephone interview, Leakey said the diversity in fossil hominids should not be surprising because the ancestry of mammals is usually marked by many branches.

Until recently, scientists have recognized only three groups of hominids. The genus Homo evolved more than 2 million years ago and led to modern humans. Paranthropus was a robust contemporary of Homo that became extinct about 1 million years ago. Both of these groups were presumed to have descended from an early species, Australopithecus.

Since its discovery in 1974 in Ethiopia by Dr. Donald Johanson, the australopithecine known as Lucy, or afarensis, has been generally regarded as the most likely common ancestor of all subsequent hominids, including humans.

Dr. Frank Brown, a University of Utah geologist who was a member of Leakey's team, said, "Anthropologists will have to decide which of these forms of early human actually lies in our ancestral tree. It cannot be both."

Meave Leakey is the wife of Richard Leakey, himself a paleontologist and son of Louis and Mary Leakey, who pioneered the search for early hominid fossils in Africa. One of Meave Leakey's co-authors is her daughter, Louise, who is completing doctoral studies at the University of London and will carry on the family's fossil-hunting tradition.

In a commentary in Nature, Dr. Daniel E. Lieberman of George Washington University wrote, "I suspect the chief role of K. platyops in the next few years will be to act as a sort of party spoiler, highlighting the confusion that confronts research into evolutionary relationships among hominids."

Lieberman said the newly discovered skull "almost certainly" represents a new species. None of its main characteristics is in itself new, he noted, but "the combination of features is not found in any other known species." But he said he was less sure that the fossils belonged to a new genus, a broader grouping.

Leakey acknowledged that "the genus designation is going to be what people question most." Defending her decision, she said the fossils definitely did not resemble the genus Homo, which evolved much later, and the teeth were too small and face much more distinctive to be a member of the Paranthropus genus. And she said she resisted placing the species within the Australopithecus genus simply because they were contemporaries.

Baltimore Sun Articles
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.