Migration is part of the great evolutionary story of our species – humans first evolved in Africa several million years ago, before leaving those homelands and spreading across the world in a series of epic journeys.
But there has long been debate about exactly when humans left Africa and the routes that were taken. This is partly because the evidence, including fossils and artefacts found outside Africa, is scarce.
“We have to work with very fragmentary bits of information,” says Andrea Manica, a professor of evolutionary ecology at the University of Cambridge.
“We often lack archaeological sites in the interesting places – the coastlines that might have acted as corridors are now underwater, and, in general, the areas that might have been important don’t have very favourable climates to preserve ancient remains.”
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Manica has co-authored a new study in Nature Communications that uses a different approach to narrow down early human migration. It models the climate over the past 300,000 years to pinpoint when conditions might have made it easier for humans to move.
“To move out of Africa, you need to cross large areas that, at times, are very inhospitable,” he explains. “The main barrier for the exit are very dry regions in North Africa and the Arabian Peninsula, so we looked specifically at variation in precipitation.”
By combining past climatic conditions with the distributions of contemporary hunter-gatherers, the team determined that humans can survive in areas that receive more than 90 millimetres of rain per year. This provided a threshold, above which were lusher, wetter conditions that the researchers suggested created ‘corridors’ along which humans could travel out of Africa.
So, when did those conditions occur?
“We found that there were a number of time windows that were suitable (for) human migration out of Africa well before the major exit that happened approximately 60,000 to 50,000 years ago – from which all contemporary humans descend, based on genetics,” explains Manica.
They looked at the conditions along two potential routes from Africa into Eurasia: one along the Nile (northern route), and the other along the narrow Bab-el-Mandeb Strait (southern route).
They found several windows where a wet corridor opened up along the northern route, including between 246,000 and 200,000 years ago, a number of times between 130,000 and 90,000 years ago, and around 78,000 to 67,000 years ago.
“After that, this route likely remained closed until the wet Holocene,” the team writes in the paper.
Whether humans seized these opportunities remains to be seen.
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On the other hand, the team says, “provided that maritime travel was in principle possible, climatic conditions would have made southern exits feasible for a substantial proportion of the last 300,000 years.”
The largest climate window occurred between 65,000 and 30,000 years ago along this southern route, which coincides with previous genetic and archaeological research showing that this was the most successful human migration out of Africa.
“Our work provides a catalogue of favourable periods when exits might have happened, which helps assessing how plausible some of the other lines of evidence for earlier exits are,” says Manica.
Michelle Langley, an archaeologist from the Australian Research Centre for Human Evolution at Griffith University, comments that “new analyses which narrow down the possibilities are very useful for working out where we should focus our investigations”.
“The several early dates (windows of opportunity) that the authors suggest are interesting to consider in light of what we know and emerging evidence from a number of different sites,” adds Langley, who was not involved in the research.
The big question now, he says, is why the exit around 65,000 years ago was so successful, leading to the full colonisation of the Eurasian continent and then the world.
“Something must have happened, because there were plenty of previous opportunities,” Manica says. “Changes in competition with other hominins (Neanderthals and Denisovans were on their way out at that time) might have been the key, but we really don’t know at the moment.”