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Scientists are still trying to pin down the early history of humans. The first modern humans evolved in Africa about 200,000 years ago. When and how they eventually left Africa and fanned out across the globe is still up for debate.

Some newly-discovered stone tools suggest that our ancestors might have left Africa much earlier and by a different route than previously thought.

NPR's Nell Greenfieldboyce reports.

NELL GREENFIELDBOYCE: The tools are primitive hand axes and scrapers made of stone. They were dug up next to a rocky desert mountain in the United Arab Emirates, by the Persian Gulf. And they're ancient, 100,000 to 125,000 years old. What's more, they look very similar to the tools made by the early humans living in East Africa around that time.

The startling implication is that these humans may have left Africa by heading right across the Arabian Peninsula.

Dr. SIMON ARMITAGE (Geologist, Royal Holloway, University of London): I think the interpretation of the site is probably going to be fairly contentious.

GREENFIELDBOYCE: Simon Armitage works at the University of London and was part of the research team.

Dr. ARMITAGE: What it does is push back, by quite a lot, the timeframe in which we think anatomically modern humans - so you and me - migrated out of Africa where we'd evolved and subsequently began to populate the rest of the world.

GREENFIELDBOYCE: Scientists have generally thought that modern humans departed from Africa around 60,000 years ago, by moving northward along the Nile, and then the coast of the Mediterranean Sea.

But Armitage and his colleagues now think our ancestors may have set out tens of thousands of years earlier, when unusual climate conditions would have opened up a different route. Back then, the strait at the southern end of the Red Sea would have been drier and more narrow than it is today. Humans could have made an easy crossing to a place that would have looked very different from today's deserts.

Dr. ARMITAGE: The southern end of the Arabian Peninsula was actually quite wet, so it was a kind of savannah-type landscape. And that's reasonably conducive to human habitation.

GREENFIELDBOYCE: From there, humans could have moved on to Asia and beyond.

Researchers say the new report in the journal Science is provocative, but cautioned against making too many assumptions based on just one archaeological find.

Alison Brooks is an archaeologist at George Washington University.

Professor ALISON BROOKS (Anthropology Department, The George Washington University): I think that the comparisons to Africa from a few stone tools are always basically just a hypothesis. And we need more work and more stone tools to really cement that connection.

GREENFIELDBOYCE: Brooks points out that no fossilized bones were found to show who really made these tools. The discovery site is outside the range of the Neanderthals. But there's no way to know what the tool makers looked like, and whether they really were our ancestors, who had left Africa, or some other group that independently invented the same techniques.

Prof. BROOKS: Certainly, it's a very intriguing find, and it should hopefully spur research in all kinds of places and directions that haven't been undertaken before.

GREENFIELDBOYCE: Researchers say that's already started to happen.

Tony Marks is an archaeologist with Southern Methodist University in Dallas, who analyzed the tools. He says in the past, archaeologists believed that the windswept deserts of this region weren't such a great place to find artifacts buried in a way that preserved their history.

Dr. TONY MARKS (Anthropology Department, Southern Methodist University): So the fact that this material was really in the ground, undisturbed, was really exceptional.

GREENFIELDBOYCE: He predicts that as more expeditions get underway, they'll make more discoveries like this one.

Nell Greenfieldboyce, NPR News.

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