SCIENCE -- January 27, 2011 at 3:41 PM ET
Tools Hold Clues to Early Human Migration Out of Africa
A large sampling of ancient stone tools unearthed from a once-fertile area in the Persian Gulf Basin may indicate that early humans migrated from Africa much earlier than previously thought.
The artifacts, dug up from the Jebel Faya archeological site and dated to 120,000 years ago, were created with several techniques, some of which resembled a sort of sculpting. But one specific technique used, called "bifacial reduction," which involves removing material from two rock faces, was abundant in Africa, but has never before been found in the Arabian Peninsula, said Anthony Marks, an archaeologist with Southern Methodist University who analyzed the tools. The study was published online today in the journal, Science.
"When I looked at this material, none of it looked Near Eastern," he said. "The more I looked at the material, the more it became clear that it had a connection with the kind of tools of East Africa."
Tools like this were believed to be used to kill and butcher animals, scrape fat from hides, make clothes from animal skin and create other, possibly wooden, tools. Scientists dated the artifacts with a technique called optically stimulated luminescence data. "In essence, this is a technique which allows us to determine the time which has elapsed since sand grains were last exposed to sunlight," said Simon Armitage, a professor of geography with Royal Holloway, University of London, and co-author on the study.
Additional evidence supports the theory that these tools belonged to a group of people that migrated from Africa to Arabia, scientists say. The standard model, which relies on data from mitochondrial DNA, says the first humans left Africa about 60,000 years ago, traveled to Arabia and eventually, about 50,000 years ago, arrived in Australia.
But Marks says he's skeptical that the migration occurred so quickly. "If that's true, my God, they never had time to sit down," he said. "10,000 years is nothing in archaeology, it's the blink of an eye. The earlier movement out of Africa makes a lot more sense to me."
Plus, 120,000 years ago, the area, which is now arid, had been transformed by monsoon rains into lush grasslands with lakes and acacia trees. And for a brief window of time, sea levels were low and the Red Sea was relatively narrow and easy to cross.
"Up until now, we thought cultural developments were leading to the opportunity for people to move out of Africa," said Hans-Peter Uerpmann, of the University of Tübingen in Germany. "Now we see, I think, that it was the environment that was the key to this, and that the change from a glacial period into an interglacial period opened the other possibility to leave Africa through the southern corridor."
There are many reasons why early modern humans might have migrated, such as environmental problems or neighboring tribes that drove them out, but Marks thinks it was probably a pull rather than a push, during this climactic window of opportunity. The desert would have been a fierce obstacle for their predecessors; not for them.
"In my mind, what likely was involved was range expansion," he said. "The weather's getting better everywhere and they can see across the Bab al-Mandab...Low sea level and increased rainfall allowed them to populate Arabia. There was no purposeful movement, but rather, a series of events that led to expansion."
The paper has raised questions. Harold Dibble, a professor of anthropology at University of Pennsylvania, has worked in East Africa, North Africa and the Nile Valley on early modern humans. "Boy, I tell you, I'm not convinced at all," he says. "What the argument is lacking is can you really associate these with modern humans. And as far as I can see, there's nothing in the assemblage that screams modernity." The artifacts don't show anything specifically characteristic to East Africa or modern humans, as opposed to other human relatives, such as Neanderthals, he adds.
Having a skeleton would help, Marks says, but the desert is too dry for bone preservation. "The odds of us finding a skeleton, it just won't happen...I bet in a year or two we'll have other sites that show African material in Southern Arabia during this last interglacial period, and there won't be a fuss. But the first thing you find is always, "Oh my God, is it real, does it mean anything?"