"By JOHN NOBLE WILFORD
Published: February 15, 2005
The scientists did not get around to the nitty-gritty question until the fourth hour of a two-and-a-half-day symposium on Neanderthals, held recently at New York University.
A strong consensus was emerging, they agreed, that the now-extinct Neanderthals were a distinct evolutionary entity from modern humans, presumably a different species. They were archaic members of the human family, robust with heavy brow ridges and forward-projecting faces, who lived in Europe and western Asia from at least 250,000 years ago until they vanished from the fossil record about 28,000 years ago.
Neanderthals may have seen their first modern Homo sapiens some 100,000 years ago in what is now Israel. The two people almost certainly came in contact in Europe in the last centuries before the dwindling Neanderthal population was replaced forever by the intruding modern humans.
Taking his turn at the symposium lectern, Dr. James C. M. Ahern, a paleoanthropologist at the University of Wyoming, acknowledged: 'Neanderthals are different. The degree of difference is relatively vast, but that is not the most interesting question out there.'
The question was, he continued, 'Did Neanderthals and modern humans do it?'
There it was, out in the open again, the question that has persisted since the first fossils of these people were discovered in the Neander Valley of Germany in 1856. Could the two people with a shared distant ancestry and family resemblance have interbred? Is there any evidence that Europeans today carry some Neanderthal genes?
For the international gathering of scientists, the issue exposed the uncertainty over the definition of species. Its conventional meaning is a group of interbreeding creatures that are reproductively isolated from others. Hybridization of species is rare in mammals. One common example is the mating of an ass and a mare, producing the sterile mule.
The conferees debated, but never resolved, the possibility that Neanderthals could have been an evolutionary and anatomical species, distinct from Homo sapiens, but not strictly an isolated biological species. That is, the two species may have been enough alike to mate and produce fertile offspring.
Again, Dr. Ahern encapsulated the issue, "How much difference is too much" for viable interbreeding to occur?
Dr. Chris Stringer, a paleontologist at the Natural History Museum in London, noted that some species apparently less close than Neanderthals and modern humans can interbreed and produce hybrids. Dr. Stringer is a leading proponent of the theory that modern Homo sapiens emerged in Africa as early as 150,000 years ago and then spread to Asia and Europe, replacing the remnants of archaic humans they encountered there.
Dr. Erik Trinkaus, a Neanderthal expert at Washington University in St. Louis, who was not at the meeting, contends that the 24,500-year-old skeleton of a young boy found in Portugal appeared to be a Neanderthal-Homo sapiens hybrid. The interpretation has so far been viewed with skepticism.
Dr. Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said that he and colleagues had looked for answers in the patterns of genetic variation in contemporary human populations and the analysis of ancient DNA from fossils of Neanderthals and early modern humans. Neither approach, he said, provided any indication of interbreeding between the two species.
"That does not rule out some genetic contribution" from Neanderthals to Europeans' ancestry, Dr. Stoneking said.
Dr. David Serre of McGill University in Montreal described the analysis of the maternally inherited mitochondrial DNA found in 24 Neanderthals and 40 early modern human remains. The results seemed to exclude any significant contribution of Neanderthal genes to Homo sapiens, perhaps less than 1 percent. Therefore, he concluded, they were "two distinct biological species."
Dr. Katerina Harvati, also of the Planck Institute in Leipzig, recently conducted research applying a "quantitative method" to determine the degree of anatomical difference that justifies classifying specimens as different species. She and colleagues examined the variation of specific parts of the craniums and faces of modern humans and Neanderthals as well as 12 existing species of nonhuman primates. The two living species of chimpanzees, for example, appeared to be more closely related to each other than Neanderthals are to humans.
Dr. Harvati and Dr. Terry Harrison, a paleontologist at N.Y.U., organized the symposium, "Neanderthals Revisited: New Approaches and Perspectives."
More than species differences may have kept Neanderthals and humans sexually apart, if indeed that was the case. Their opportunities may have been limited.
Dr. Ahern said in an interview that it was "surprising how little overlap there was" between the two species in Europe." It had been thought that modern humans from Africa began arriving in Europe about 40,000 years ago and so could have competed with and mingled with the local population for at least 12,000 years. But the dating of fossil and archaeological evidence is now being revised, leaving much less time when the two species could have had close contact.
"It's a real scientific problem," said Dr. Randall White, an archaeologist specializing in European ice age culture at N.Y.U. "How to interpret the overlap of Neanderthals and modern humans, their interactions and cultural exchanges, the causes of Neanderthal extinction, all depends on what are the real dates of their possible contact."
Some of the most solid evidence for overlap, the researchers said, does not appear until toward the end of the Neanderthals' known existence, when their populations were probably sparse.
Dr. Stringer said some explanations for Neanderthal extinction were being re-examined. Perhaps the technological superiority of modern humans was "not as clear-cut as some of us thought," he said. Perhaps Neanderthals, though adapted to a cold climate, could not survive the rapid and repeated changes of cold and warm periods of that time.
"It was not bad genes but bad luck for the Neanderthals," Dr. Stringer said. "Modern humans may have had no direct effect on Neanderthal extinction. They actually walked into empty spaces where Neanderthals had already disappeared."
Dr. Ian Tattersall of the American Museum of Natural History was not entirely joking when he suggested that few genes were exchanged because "no self-respecting Neanderthal female would fancy a Homo sapiens male."
In making a case for the distinct differences between the two species, Dr. Tattersall showed slides of upright skeletons of the two. But skeletons are unrevealing of Paleolithic desire."