The researchers have some theories. If early humans could have moved out of Africa much earlier, they would have faced stiff competition from other early human species; the north was a Neanderthal stronghold, and much of East Asia was likely populated by another extinct human lineage, the Denisovans. The models also suggest that dry periods often followed the favorable windows, which could have isolated any populations undertaking an exodus. But the authors also note that even if times were good and wet, humans may not have taken advantage of these periods to migrate out.
The model had to make several assumptions, including that the southern strait would always have been crossable by humans and that those people might have had the boat technology to make the crossing. The model breaks down the geography of the region to a grid with a resolution with half a degree latitude and longitude, or around 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 rules of the model, such as humans only being able to migrate alongside a minimum level of rainfall. “I guess it makes sense to make that assumption,” Dr. Tierney said. “On the other hand, the Nile River is always there. They could move out that way almost any time.”
Similarly, Emily Beverly, an earth scientist at the University of Houston who was not involved with the research, said the authors did not consider the existence of freshwater springs that could have served as a source of potable water for migrating humans during dry periods.
On the other hand, Dr. Potts, the paleoanthropologist, noted that the minimum level of rainfall in the model would have been “far too low” to allow hunter-gatherers to successfully disperse out of Africa. Dr. Potts pointed to previous research suggesting that early humans could only have dispersed in the continent when the mean average rainfall was more than 3.9 inches per year, and typically dispersed when there was at least 10 inches of rain. The more interesting research question, in Dr. Potts’ eyes, is what dispersal paths would have been available in these windows of more abundant rainfall.
Perhaps the largest question still remains unanswered. “More and more evidence suggests we did this multiple times,” Dr. Beverly said. “The question I’m always left with is, Why?”
Abdullah Alsharekh, an archaeologist at King Saud University in Riyadh, Saudi Arabia, who was not involved with the research, said he appreciated the paper’s examination of the prehistoric Arabian climate. “The last couple of decades have shown that many of our questions about out-of-Africa models can be greatly enhanced by more on-the-ground research in Arabia,” Dr. Alsharekh wrote in an email. “What lies beneath those sandy deserts?”