by Erin Wayman Thursday, January 5, 2012
How and when did modern humans leave Africa and colonize the rest of the world? Many archaeologists would probably tell you that about 60,000 years ago, Homo sapiens walked up through Egypt, crossed the Sinai Peninsula into the Levant region of the Middle East and then continued on to Eurasia.
But maybe not.
Researchers announced today in Science the discovery of 125,000-year-old stone tools found in the United Arab Emirates. The tools — some of the earliest evidence of Homo sapiens outside of Africa — point to an earlier exodus. And instead of the northward route to Eurasia via the Levant, humans may have taken a southern route, departing from the Horn of Africa (probably in the region of modern Eritrea and Djibouti), crossing the Red Sea when sea levels were low and then migrating up through the Arabian Peninsula.
I first reported on the possibility of a southern route in January 2008. Back then, a few archaeologists had discovered stone tools in southern Arabia that resembled Middle Stone Age tools found in Africa. That led researchers like Jeffrey Rose, now of the University of Birmingham in England, to conclude that people from Africa traveled across the Red Sea and brought those tools into Arabia. The problem with that study was that no one knew when the tools were made. Most of the artifacts have been found on the surface of the desert. It’s hard to find geologic sites with stratigraphic layers that can be dated in the Arabian Peninsula, Rose told me in 2008.
That’s where the authors of the new study — Simon Armitage of Royal Holloway, University of London and his colleagues — have lucked out. They found a variety of stone tools, including hand axes and scrapers, buried in a rockshelter in Jebel Faya, a limestone mountain in the United Arab Emirates that’s about 55 kilometers inland from the Persian Gulf. They used optically stimulated luminescence — which measures how long it’s been since something was last exposed to light — to get a date for the oldest tools: about 125,000 years.
No human fossils were found, but the team concluded modern humans were the toolmakers based on similarities to tools made by humans in Africa. The tools were made using a combination of technologies that people of the Middle Stone Age in Africa used, including bifacial reduction, in which a rock is shaped into a tool by flaking it on two sides. The same combination of technologies was not present in tools made by other hominins living at the time, such as Neanderthals in Europe and North Asia, co-author Anthony Marks of Southern Methodist University in Texas said at a press conference yesterday when the team unveiled the findings.
The age of the tools indicates that the choice to move into Arabia was probably an opportunistic one. When modern humans arose approximately 200,000 years ago, Earth was in the midst of a glacial period, and the expansion of deserts in Africa and Arabia probably blocked people from leaving Africa, said co-author Adrian Parker of Oxford Brookes University in England. But about 130,000 years ago, an interglacial period began. The Indian Ocean monsoon moved north, bringing Arabia lots of rain. The added water transformed the peninsula’s deserts into savannahs incised by numerous rivers and lakes. At the onset of the interglacial, there would have been a brief window when sea levels were still low and the Red Sea was still narrow and more easily crossable than it is today (perhaps by rafts, one co-author suggested).
The scientists also speculate that Arabia would have then been a good launching pad for humans to move on to colonize the rest of the world. Humans reached India, Southeast Asia and Australia by 60,000 years ago.
But was this the migration of out Africa that really populated the rest of the world? Or were these people just members of a dead-end population that got stuck in the deserts of Arabia?
That’s still unclear, and the findings of this study will probably be controversial. First, there are no fossils at the site to definitively connect modern humans with the tools. And second, the researchers are suggesting humans might have started their global journey much earlier — as much as 50,000 years earlier — than current thinking, which is based largely on genetic evidence.
The question of how humans left Africa is far from resolved — but I suspect many more archaeologists will now be writing grant proposals to study human evolution on the Arabian Peninsula.
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