Researchers working in Ethiopia say they have found the direct ancestors of humanity: fossil skull bones showing the last evolutionary step before anatomically modern people appeared about 115,000 years ago.
The fossil skulls so closely resemble those of people today that they represent a subspecies of Homo sapiens that lived 160,000 years ago, says Tim D. White, a professor of integrative biology at the University of California at Berkeley and one of the leaders of the international research team, which reported its findings on Wednesday in Nature.
"It's what you would expect as the evolutionary precursor of modern humans," says Mr. White of the fossil finds. "It's essentially a modern human face, but a little bit taller."
The fossils "are probably some of the most significant discoveries of early Homo sapiens so far, owing to their completeness and well-established antiquity of about 160,000 years," wrote Chris Stringer, a paleoanthropologist at the Natural History Museum, in London, in a commentary in the same issue of the journal.
Mr. White and his team say that the skulls, from two adults and one child, will end a argument about the relationship between modern people and ancient Neanderthals. For two decades, anthropologists have debated whether Neanderthals contributed to human ancestry or were an offshoot that left no heirs in the world today. "The idea that Neanderthals had anything to do with modern humans will be over now," says Mr. White.
The Ethiopian fossils bridge the gap between older human ancestors that lived in Africa hundreds of thousands of years ago and modern humans, whose fossils first appear in Israel about 115,000 years ago. The new discovery, he says, therefore adds further weight to the Out-of-Africa hypothesis, the idea that modern people evolved in Africa and migrated from there into Europe, where they replaced Neanderthals. Because the Ethiopian skulls look so much like people's today, the scientists say, they rule out the possibility that there was a Neanderthal stage in human evolution -- an argument bound not to sway researchers who see a clear link between Neanderthals and modern people.
The newfound fossils come from the Afar region in Ethiopia, where paleoanthropologists have also discovered older remains of humanity's ancient relatives, stretching back more than five million years. The team that made the recent finds includes researchers from the Rift Valley Research Service, in Addis Ababa; the University of Tokyo; Los Alamos National Laboratory; and Miami University, in Ohio.
The scientists uncovered three craniums -- skulls without lower jaws -- along with numerous stone tools and the butchered remains of many hippopotamuses, indicating the diet of the ancient humans. Their faces and the tops of their skulls resemble those of modern people in many ways, says Mr. White. But the Ethiopian craniums have a flatter midface and longer, more rugged braincases than those of people populating the earth now.
Aside from providing evolutionary clues about human origins, the partial skulls offer a glimpse into the culture of the Afar region during the last ice age. Cut marks and polished surfaces on the craniums reveal that the ancient people defleshed the skulls and handled them repeatedly for periods of perhaps months, indicating that the bones served some societal function, say the researchers. The people may have engaged in cannibalism, but they also were using the bones for something more complex. The same features do not appear on earlier fossils that show signs of cannibalism, says Mr. White's team.
The group made its discovery in 1997 and has spent the years since freeing the fossils from the surrounding rock, piecing together broken bone fragments, and interpreting the finds. The researchers also had difficulty assessing the age of the fossils. Using the decay of radioactive elements in volcanic rocks within and above the fossil layer, the team determined that the craniums were 154,000 to 160,000 years old.
That makes them a rarity in the world of paleoanthropology, where it has been notoriously difficult to find fossils of hominids -- the family that includes humans and near relatives -- from the period between 200,000 and 100,000 years ago. Scientists find the scarcity particularly vexing because they think that is precisely the span when modern people arose.
"It's tremendously exciting to have not one but three hominids in this time range," says Alison S. Brooks, a professor of anthropology at George Washington University. "It's the time range that's almost unrepresented in the fossil record in Africa."
Ms. Brooks raises questions, however, about designating the fossils as a separate subspecies. Although the fossil record is slim from that time, a few older and younger specimens have turned up in southern Ethiopia, Morocco, Sudan, and South Africa. "In order to make a solid case for a subspecies, it would have been good to compare the [newfound] remains with those," she says.
Mr. Stringer in London considers the newfound fossils "the oldest definite record of what we currently think of as modern H. sapiens," and not a member of a subspecies.
The report from Mr. White's team drew strong criticism from Milford H. Wolpoff, a professor of anthropology at the University of Michigan at Ann Arbor. Mr. Wolpoff has been among a minority of researchers who reject the Out-of-Africa hypothesis. They have proposed a multiregional model, in which African humans interbred with human populations that had appeared during the last ice age in several places in Europe and Asia. In that scenario, Neanderthals -- as the ancient residents of Europe -- would have contributed to the ancestry of modern Europeans.
Mr. Wolpoff, who was Mr. White's graduate adviser, says that his former student made a logical error in his claims about Neanderthals. "To say that these fossils show that Neanderthals have no descendants makes no sense to me," says Mr. Wolpoff.
That argument is a circular one, based on the assumption that evolution happened separately in Europe and North Africa, he says. "If Professor White had done this while he was here, he would have gotten a C on that."