A warm spell during the Ice Age gave early humans a route out of Africa 20,000 years earlier than thought, say scientists who've uncovered a prehistoric tool kit in Arabia.
During this period of climate change, about 130,000 years ago, water travel would have been easier than in more typical Ice Age periods.
Seas in the region would still have been at relatively low, Ice Age levels, making for shorter crossings. On top of that, though, warmer, wetter weather would have created navigable lakes and rivers in what are now the deserts of the Arabian Peninsula, the study says.
Such a shift would have offered early modern humans—which arose in Africa about 200,000 years ago—a new route through the formerly parched northern deserts into the Middle East.
The new paper was spurred by the discovery of several 120,000-year-old tools at a desert archaeological site in the United Arab Emirates.
The presence of the tools—whose design is uniquely African, experts say—so early in the region suggests early humans marched out of Africa into the Arabian Peninsula directly from the Horn of Africa, roughly present-day Somalia (map). Previously, scientists had thought humans first left via the Nile Valley or the Far East.
"Up till now we thought of cultural developments leading to the opportunity of people to move out of Africa," said study co-author Hans-Peter Uerpmann, a retired archaeobiologist at the University of Tübingen in Germany. (See "Innovation Linked to Human Migration Out of Africa.")
"Now we see, I think, that it was the environment that was the key to this," Uerpmann said during a press briefing Wednesday.
The discovery "leaves a lot of possibilities for human migrations, and keeping this in mind, might change our view completely."
Climate Gave Humans "Brief Window" to Leave Africa
Between 2003 and 2010, a series of tools were discovered at the Jebel Faya site in the U.A.E., some of which—such as hand axes—that had a two-sided appearance previously seen only in early Africa.
Scientists used luminescence dating to determine the age of sand grains buried with the stone tools. This technique measures naturally occurring radiation stored in the sand.
For the climatic data, scientists studied the climate records of ancient lakes and rivers in cave stalagmites, as well as changes in the level of the Red Sea.
This warmer period 130,000 years or so ago caused more rainfall on the Arabian Peninsula, turning it into a series of lush rivers that humans might have boated or rafted.
During this period the southern Red Sea's levels dropped, offering a "brief window of time" for humans to easily cross the sea—which was then as little as 2.5 miles (4 kilometers) wide, according to Adrian Parker, a physical geographer from Oxford Brookes University in the United Kingdom.
Once humans entered the peninsula, they dispersed and likely reached the Jebel Faya site by about 125,000 years ago, according to the study, published tomorrow in the journal Science.
Middle East Migrants Not Direct Ancestors
Geneticist Spencer Wells called the discovery a "very interesting find," especially because the Arabian Peninsula is becoming a hot spot for archaeological finds—particularly underwater, since the Persian Gulf was a fertile river delta during early human migrations.
But he noted that the study doesn't "rewrite the book on what we know about human migratory history."
That's because tools dating to the same period have already been found in Israel, so it's "consistent with what we suspected" about an earlier wave of migration into the Middle East, said Wells, director of the National Geographic Society's Genographic Project. (The Society owns National Geographic News.)
Wells also noted there's no evidence yet that the migrants in the new paper were our ancestors—the group, and their genes, may have died out long ago (see a quick genetics overview).
According to Wells's DNA research, modern humans trace their lineage to Africa about 60,000 years ago. Though we would have looked the same, "these would have been cousins," he said.
Humans Driven Out of Africa by Curiosity?
Bence Viola, of the Max-Planck-Institute for Evolutionary Anthropology in Leipzig, Germany, agreed the finding was interesting but not that surprising, also citing the evidence of humans in Israel about 120,000 years ago.
Viola, who wasn't involved in the study, added that the migration route proposed in the paper makes sense on another level—the Arabian Peninsula would have been something early humans were used to.
"It's not like they needed to adapt to a completely different environment—it's an environment that they knew."
Why they made the trek is another question, since they wouldn't have been hurting for food or resources in their African homeland, Viola noted.
"Curiosity," he said, "is a pretty human desire."