Neanderthals and Early Humans May Not Have Mingled Much

By NICHOLAS WADE

http://www.nytimes.com/2011/05/10/science/10neanderthal.html

An improvement in the dating of fossils suggests that the Neanderthals, a heavily muscled, thick-boned human species adapted to living in ice age Europe, perished almost immediately on contact with the modern humans who started to enter Europe from the Near East about 44,000 years ago. Until now bones from several Neanderthal sites have been dated to as young as 29,000 years ago, suggesting there was extensive overlap between the two human species. This raised the question of whether there had been interbreeding between humans and Neanderthals, an issue that is still not resolved.

But researchers report that tests using an improved method of radiocarbon dating, based on a new way to exclude contaminants, show that most, and maybe all, Neanderthal bones in Europe are or will be found to be at least 39,000 years old. Thomas F. G. Higham, a specialist in radiocarbon dating at Oxford University, and Ron Pinhasi, an archaeologist at University College Cork in Ireland, have dated the bones of a Neanderthal child less than 2 years old whose remains were found in the Mezmaiskaya Cave in the northern Caucasus Mountains. A second Neanderthal baby, found in a lower layer in the cave, was previously dated back 29,000 years.

The first baby, since its bones were retrieved from a higher layer, must be even younger, but in fact it turns out to be 39,000 years old when an improved version of the radiocarbon dating technique is used, Dr. Higham and Dr. Pinhasi reported Monday in The Proceedings of the National Academy of Sciences.

Radiocarbon dating depends on measuring the radioactive isotope of carbon known as carbon 14, which is ingested during a person’s lifetime and steadily decays after death. Very little carbon 14 remains in specimens more than 30,000 years old, and even tiny amounts of contaminating carbon 14 can make a sample seem much younger than it is.

Dr. Higham has developed a method of ultrafiltration that removes contaminants and leaves whole molecules of collagen recovered from fossil bone.

Reviewing other Neanderthal dates ascertained with the new ultrafiltration method, Dr. Higham sees an emerging pattern that no European Neanderthal site can reliably be dated to less than 39,000 years ago. “It’s only with reliable techniques that we can interpret the archaeological past,” he said.

He is re-dating Neanderthal sites across Europe and so far sees no evidence for any extensive overlap between Neanderthals and modern humans. “There was a degree of contemporaneity, but it may not have been very long,” he said. A short period of contact would point to the extinction of the Neanderthals at the hands of modern humans.

“It’s very unlikely for Neanderthals to go extinct without some agency from modern humans,” Dr. Higham said.

Paul Mellars, an expert on Neanderthals at Cambridge University in England, said that the quality of the dates from Dr. Higham’s laboratory was superb and that samples of bone re-dated by the lab’s method were almost always found to be several thousand years older than previously measured. The picture supported by the new dates is that the interaction between modern humans and Neanderthals in Europe was brief in each region, lasting perhaps a few hundred years, Dr. Mellars said, until the modern humans overwhelmed their competitors through better technology and greater numbers.

Richard Klein, a paleoanthropologist at Stanford University, said Dr. Higham’s re-dating was “compelling” and fit with his own view that “modern humans were technologically and intellectually far superior to the Neanderthals.” This, he said, “would have allowed them to spread very rapidly and to precipitate the extinction of the Neanderthals almost immediately on contact.”

The new radiocarbon findings show little evidence that the two species peacefully coexisted within Europe. But geneticists who have decoded the Neanderthal genome reported last year that some 2.5 percent of the modern human genome is derived from Neanderthals. The interbreeding, they postulate, occurred not in Europe 40,000 years ago but in an earlier encounter 100,000 years ago. They believe that this encounter must have been in the Near East.

Modern humans and Neanderthals occupied the same sites in what is now Israel, but it is not clear that the populations overlapped. The Neanderthals seem to have occupied the sites during cold periods and the modern humans during spells of warmer weather.

The presence of modern humans in Israel 100,000 years ago was long assumed to have been a failed attempt to leave Africa, since there is no archaeological evidence of modern humans outside Africa until some 44,000 years ago. But geneticists led by Svante Paabo of the Max Planck Institute for Evolutionary Anthropology argue that this earlier attempt was in fact successful, and that modern humans commingled with Neanderthals in the Near East before going on to occupy Europe and Asia. This would explain, they say, why Neanderthal genes are found in Europeans and Asians but not in Africans.

Dr. Klein said interbreeding between the two species was perfectly possible in principle, “but it’s kind of anti-archaeological because there is no evidence that they overlapped” in the Near East.