March 22, 2001

Skull May Alter Experts' View of Human Descent's Branches

By JOHN NOBLE WILFORD

The New York Times

Paleontologists in Africa have found a 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 that the fossil species Australopithecus afarensis, which lived from about four million to three 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 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 molars were strikingly different from those of the contemporary afarensis, or Lucy, species.

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

"Kenyanthropus shows persuasively that at least two lineages existed as far back as 3.5 million years," Dr. Leakey said in a statement issued by the National Museums of Kenya in Nairobi, where she is the principal paleoanthropologist. "The early stages of human evolution are more complex than we previously thought."

In a telephone interview, Dr. Leakey said the diversity in fossil hominids should not be surprising, because the ancestry of mammals is usually marked by many different branches. When the early hominids split off from ancestors of the ape and started walking on two legs, they would have been capable of moving into new habitats and developing into new species.

Dr. Ian Tattersall, an evolutionary biologist at the American Museum of Natural History in New York, said the discovery was "very important because it finally recognized the diversity among fossil hominids."

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

Ever 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.

In the absence of any other hominid fossils between about 3.8 million and 3 million years ago, it seemed to be the only tentative conclusion scientists could draw.

Dr. Frank Brown, a University of Utah geologist who was a member of Dr. Leakey's team, said the place of afarensis and the new fossil species in human ancestry would be debated in the coming years. Dr. Leakey's journal article does not take a position on the issue.

"Anthropologists will have to decide which of these forms of early human actually lies in our ancestral tree," Dr. Brown said. "It cannot be both."

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."

Or as Dr. Tim D. White of the University of California at Berkeley said in an interview, "The arms will be waving faster than helicopter blades."

Dr. White is a paleoanthropologist working in Ethiopia who was earlier associated with Dr. Johanson in research on the Lucy specimen. He reserved judgment on the new discovery, saying that whether the "new fossils expand an envelope beyond a single lineage or fit within the old envelope of afarensis" was still an open question.

Meave Leakey is the wife of Richard Leakey, himself a paleontologist and the 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 carrying on the family's fossil-hunting tradition.

If the issue becomes an Australopithecus afarensis vs. Kenyanthropus platyops debate, it could reopen old wounds from previous conflicts between the Leakey family and Dr. Johanson, which became bitter after the Lucy discovery. Among other issues, members of the Leakey family initially questioned the place of afarensis in early human evolution.

In a foretaste of arguments to come, Dr. 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.

Dr. Leakey acknowledged that "the genus designation is going to be what people question most."

Defending her decision, she said that the fossils definitely did not resemble the genus Homo, which evolved much later, and that the teeth were too small and the face too distinctive to belong to a member of the Paranthropus genus. And she said she resisted placing the species within the Australopithecus genus simply because they were contemporaries.

"Australopithecus has become too much of a dumping place," Dr. Leakey complained.

In particular, she said in the team's report that the fossils' "unique pattern of facial and dental morphology" probably reflected the fact that the species occupied a new habitat and ate different foods than the afarensis.

From about 2.5 million years onward, until the extinction of Neanderthals about 28,000 years ago, there were always two or more species of hominid in existence.

For two decades, afarensis stood alone as the earliest hominid species. Then Dr. White in 1994 discovered fossils in Ethiopia that are thought to be 4.4 million years old and have been named Ardipithecus ramidus; details of the fossils remain scant. A year later, Dr. Leakey identified the earliest known australopithecine, the four-million-year-old anamensis.

The new discovery, Dr. Leakey said, "just shows that the hominid diversity that was apparent from 2.5 million years on is now extended much earlier."