The researchers have some theories. If early humans had left Africa much earlier, they would have faced stiff competition from other early human species; the north was a stronghold of the Neanderthals, and much of eastern Asia was probably populated by another extinct human lineage, the Denisovans. The models also suggest that dry spells often followed favorable windows, which could have isolated populations that undertook an exodus. But the authors also note that even if times were good and wet, people may not have taken advantage of these periods to migrate.
The model had to make several assumptions, including that the southern strait would always have been passable by humans, and that those humans may have had the boat technology to make the crossing. The model breaks down the geography of the region into a grid with a latitude and longitude resolution of half a degree, or about 30 miles. This approach inevitably ignores the mosaic of vegetation and topography that exists on the ground.
dr. Tierney, the paleoclimatologist, said the new paper’s climate models were too simple to predict what climate change was like hundreds of thousands of years ago. She also questioned some of the model’s rules, such as that people can only migrate with a minimum of rainfall. “I think it makes sense to make that assumption,” said Dr. tierney. “On the other hand, the Nile River is always there. They can leave almost any time that way.”
Similarly, Emily Beverly, an earth scientist at the University of Houston who was not involved in the study, said the authors did not consider the existence of freshwater sources that could have served as a drinking water source for migrating humans during dry spells.
On the other hand, Dr. Potts, the paleoanthropologist, notes that the minimum rainfall in the model would have been “far too low” to allow hunter-gatherers to successfully spread out of Africa. dr. Potts pointed to previous research that suggested that early humans could only spread on the continent if the average average rainfall was more than 3.9 inches per year, and usually spread when there was at least 10 inches of rain. The more interesting research question, in the eyes of Dr. Potts, is what distribution paths would have been available in these windows of more abundant rainfall.
Perhaps the biggest question is still unanswered. “Increasing evidence suggests we’ve done this multiple times,” said Dr. Beverly. “The question that always haunts me is: why?”
Abdullah Alsharekh, an archaeologist at King Saud University in Riyadh, Saudi Arabia, who was not involved in the study, said he appreciated the paper’s research of the prehistoric Arabian climate. “The last few decades have shown that many of our questions about models outside of Africa can be significantly improved by more on-the-spot research in Arabia,” wrote Dr. Alsharekh in an email. “What lies beneath those sandy deserts?”