A cache of stone tools found on the east coast of the Arabian Peninsula has reopened the critical question of when and how modern humans escaped from their ancestral homeland in eastern Africa.
The present view, based on both archaeological and genetic evidence, holds that modern humans, although they first emerged in Africa some 200,000 years ago, were hemmed in by deserts and other human species like Neanderthals and did not escape to the rest of the world until some 50,000 years ago.
An archaeological team led by Hans-Peter Uerpmann of the University of Tübingen in Germany now reports the discovery of stone tools 127,000 years old from a site called Jebel Faya in what is now the United Arab Emirates, just south of the entrance to the Persian Gulf. If the new tools were made by modern humans, as the researchers assert, then modern humans got out of Africa much earlier than believed.
The finding, reported in Thursday’s issue of Science, points to the importance of Arabia in understanding the human story. “This is a huge milestone, but unfortunately it raises more questions than it answers,” said Jeffrey Rose, an archaeologist at the University of Birmingham in England.
The major question is whether the people who reached Jebel Faya, if they were indeed modern humans, traveled farther and spread throughout the rest of the world, or whether they died out and got no farther.
The most comprehensive genetic data so far available, based on material called mitochondrial DNA, indicates that all modern humans outside Africa are descended from a single, small population that left Africa less than 60,000 years ago.
Dr. Uerpmann said that the genetic data was unreliable and that, in any case, mitochondrial DNA is a tiny fraction of the whole human genome.
Another question raised by the new finding is whether some social or cultural advance, possibly an evolutionary one, was required for modern humans to escape from Africa.
In Dr. Uerpmann’s view, the Jebel Faya tools are similar to ones found in Africa, showing that no cultural advance was required for the escape, just an improvement in climate that for a short time converted the Arabian Desert into a grassland that hunter-gatherers could cross.
This idea is at odds with a proposal advanced by Richard Klein, a paleoanthropologist at Stanford University, that the emergence of some social or behavioral advantage — like the perfection of the faculty for language — was required for modern humans to overcome the surrounding human groups. Some kind of barrier had to be surmounted, it seems, or modern humans could have walked out of Africa 200,000 years ago.
Dr. Klein said that the Uerpmann team’s case for an earlier out-of-Africa expansion was “provocative, but in the absence of human remains, it’s not compelling.”
The stone tools of this era are all much alike, and it is hard to tell whether early modern humans or Neanderthals made them. At the sites of Skhul and Qafzeh in what is now Israel, early modern humans were present around 100,000 years ago and Neanderthals at 60,000 years, but archaeologists cannot distinguish their stone tools, Dr. Klein said.
A warmer and wetter climate around this time let modern humans get as far as Israel but apparently no farther, and the new findings from Jebel Faya could represent a second limited excursion. But in this case, it is Africa that is expanding, or at least the African ecological zone, and not modern humans, Dr. Klein said. “The key issue is whether this is an early out-of-Africa movement, but if so, it was far more limited than the modern human expansion to Eurasia roughly 45,000 years ago,” he said.
Christopher Stringer, a paleoanthropologist at the Natural History Museum in London, said that Arabia had long been a black hole in terms of early human migrations and that the new discovery was an impressive first start. He, like Dr. Klein, said it was hard to say who made the tools without having any fossil bones from the same site. But the tools are “suggestive” of having been made by people who came out of Africa, Dr. Stringer said.
Stone tools hewn by unknown makers 75,000 years ago have been found in central India, so possibly the people at Jebel Faya did get farther east rather than dying out, Dr. Stringer said. Adding to the complexity, the stone tools at Jebel Faya do not resemble those made by early modern humans at the Skhul and Qafzeh sites.
The new finding has stirred discussion of whether modern humans could have interbred with Neanderthals, an assertion made last year by Svante Paabo of the Max Planck Institute in Leipzig, Germany, and David Reich of Harvard Medical School after statistical analysis of the recently decoded Neanderthal genome. They calculated that between 1 and 4 percent of the genomes of Europeans and Asians, but not Africans, are derived from Neanderthals.
This puzzling result, if true, means that the interbreeding may have occurred just as modern humans were moving out of Africa — in other words, after the migrating group had split from the ancestral human population in Africa, but before the ancestors of Europeans and Asians had separated.
Although fossils of both modern humans and Neanderthals have been found at the Skhul and Qafzeh sites, it is not clear if both were there at the same time, or if modern humans were there in warm periods and the cold-adapted Neanderthals took over when freezing weather returned.
If the tools found at Jebel Faya were made by modern humans, the finding would increase the geographical area over which possible interbreeding with Neanderthals might have taken place before the separation of Europeans and Asians.
Jebel Faya is near the Persian Gulf, which is now a shallow sea. But before 8,000 years ago, when the sea level was about 330 feet lower than today, the gulf area was a low-lying plain, with the Euphrates running through it. The region would have been an oasis that served as a refuge during dry periods. Neanderthal sites are known from central Iraq, so perhaps Neanderthals came down the river to the gulf oasis before it was inundated, making it “an interesting candidate for the place of hybridization,” Dr. Rose said.