DNA Tests Cast Doubt on Link Between Neanderthals and Modern Man

By NICHOLAS WADE

Did modern humans wipe out the Neanderthal people who inhabited Europe until 28,000 years ago, or did the two populations merge through interbreeding? New DNA evidence, extracted from the ribs of a Neanderthal infant, one of the last of its kind, supports the thesis that these hardy, beetle-browed people left little or no genetic legacy in today's populations.

Even though Neanderthals perished long ago, the surprising retrieval of intact DNA, the second such sample to be recovered, has set biologists speculating that with further finds the genetics of this extinct human species could become quite well understood.

The two DNA retrievals, both suggesting that Neanderthals were a separate human species, were separated in time by a startlingly contradictory finding made last June. After studying the remains of a thick-set boy recovered from a cliff-side grave in Portugal, considered a final hold-out of the Neanderthals, paleoanthropologists said that the human child had strong Neanderthal features, and that this "refuted" the idea that modern humans had exterminated the Neanderthals without interbreeding.

Neanderthals and their forebears occupied modern Europe from around 300,000 years ago. They were adapted to the cold conditions of the ice age and had stocky bodies, thick bones and enormous strength. Though their stone tools seem similar to those of modern humans who started to enter Europe from Asia around 35,000 years ago, they ceased to flourish and abruptly disappeared throughout their home range around 28,000 years ago, leaving no clues in the archaeological record as to the reason for their extinction.

Neanderthal DNA was first isolated three years ago, from the original bones first found in the Feldhofer Cave in the Neander Valley near Düsseldorf in 1856.

The finding was startling because no human DNA of such antiquity -- at least 30,000 years old -- had been recovered and because it showed a pattern of DNA that was quite different from that of modern humans.

Though the Feldhofer DNA was extracted with elaborate precautions, the finding was greeted with some reservation because it was a single result.

Confirmation has now come from a second Neanderthal.

The remains were recovered by a Russian expedition from the Moscow Institute of Archaeology to the Mezmaiskaya Cave in the Caucasus, to the northeast of the Black Sea. They belonged to a Neanderthal infant less than 2 months old, too young for the sex to be determined from the bones. The bones were dated by the carbon isotope method to 29,000 years ago, making the infant among the last generations of the Neanderthals.

A sample of the infant's ribs was made available by the Russian researchers to Dr. William Goodwin of the Human Identification Center at the University of Glasgow. Dr. Goodwin works on paternity cases and plane-crash victim identification, and studies ancient DNA as a sideline.

Dr. Goodwin and his Russian and Swedish colleagues report in this week's issue of Nature that the DNA sequence from the Mezmaiskaya Cave is 3.5 percent different from that of the Feldhofer Cave Neanderthal, suggesting a considerable genetic diversity within the Neanderthal population.

But the two Neanderthal DNA sequences are very different from those of modern humans, Dr. Goodwin and his colleagues say.

Based on the rate at which DNA changes over time in living organisms, Dr. Goodwin calculated that the two Neanderthals last shared a common ancestor at least 150,000 years ago, a date that matches the first fully Neanderthal remains, and that the Neanderthal and modern human lineages split some 600,000 years ago.

Two paleoanthropologists who favor the Neanderthal-human assimilation theory, Dr. Fred Smith of Northern Illinois University and Dr. Erik Trinkaus of Washington University in St. Louis, said they did not dispute the new DNA analysis but noted that it did not completely rule out the possibility of some interbreeding. Dr. Smith said the new DNA data was "incredibly important and significant" and "certainly strengthens the fact that there is quite a gap between Neanderthals and recent humans in terms of mitochondrial DNA."

Mitochondrial DNA, inherited from the egg cell alone and thus through the maternal line, is far more plentiful and likely to survive than the DNA of the nucleus; both Neanderthal samples were of the mitochondrial type.

But Dr. Smith and Dr. Trinkaus, who are experts on the Neanderthals, believe that there was some interbreeding on the evidence of the Portuguese boy with Neanderthal affinities.

Other anthropologists think the boy was just a "chunky" human lad who in any case lived far too many generations after the last Neanderthal had died for any evident influences to be expected.

"It's got one feature that is arguably Neanderthal -- the shortness of length between the knee and ankle -- and even that is not striking," said Dr. Richard Klein, an archaeologist at Stanford University.

Dr. Smith and Dr. Trinkaus say that even though Neanderthal DNA differs from that of modern people, it might be more similar to that of their human contemporaries, the Cro-Magnons.

Curiously, no DNA has yet been recovered from very ancient Homo sapiens fossils.

Dr. Klein agreed that new efforts should be made to retrieve Cro-Magnon DNA, though he said he expected it would prove similar to that of modern humans, sewing up the case that the Neanderthals were replaced.

The factors that allow DNA to be preserved for thousands of years are not well understood. "Even with two bodies in the same grave, the level of preservation can vary considerably," Dr. Goodwin said.

He thinks that something about the limestone cave may have favored the durability of the Caucasus Neanderthal DNA.

If the reasons for preservation were better understood, DNA experts would know which precious museum specimens were worth sampling and which to leave alone.

Source: http://www.nytimes.com/library/national/science/032900sci-anthro-neanderthal.html