A Shifting Climate Gave Humans Many Opportunities to Leave Africa

Until recently, scientists believed modern humans left Africa in one enormous exodus around 60,000 years ago. But a new climate model suggests that modern humans had several windows of opportunity to leave the continent far earlier.

The research, published Tuesday in the journal Nature Communications, reconstructed the climate of northeastern Africa over the last 300,000 years. The scientists identified when there would have been enough rainfall to allow a group of hunter-gatherers to survive the journey to the Arabian Peninsula.

Archaeological and genetic data still support the idea that all non-African people descended from a single migration that left the continent between 50,000 and 80,000 years ago. But the new paper bolsters the theory that Homo sapiens had multiple migrations out of Africa.

Even if various groups succeeded in leaving the continent, they may not each have played a large role in populating the world. An earlier constellation of fossils, some with contested dating, highlights some of Homo sapiens’s false starts: part of a middle finger from 85,000 years ago, found in Arabia; a human jawbone from at least 177,000 years ago, found in Israel; a skull from possibly 210,000 years ago, found in Greece.

It is inviting to extrapolate the timing and paths of these early journeys from these archaeological records. But the fossils offer “limited, rather gappy lines of evidence” of possible migrations, said Andrea Manica, an evolutionary ecologist at the University of Cambridge and an author on the new paper. Dr. Manica believes an ecological model could tackle the question from a new angle: first predict what would have been possible, then see if the fossils line up.

“It’s an intriguing question to ask whether there were environmental thresholds for those earlier dispersals, even though those dispersals may have been limited or short-lived,” said Rick Potts, a paleoanthropologist who directs the Human Origins Program at the Smithsonian National Museum of Natural History.

“The new paper grasps the important thing,” said Dr. Potts, who was not involved with the research. “There were multiple instances of our species’ dispersal beyond Africa prior to the main one.”

Jessica Tierney, a paleoclimatologist at the University of Arizona who also was not involved with the research, said she found the approach interesting but inconclusive. “Ultimately this is a model, not geology or archaeology,” Dr. Tierney said. “The mystery remains until you have better and more paleoenvironmental records.”

Dr. Manica and Robert Beyer, a researcher at the Potsdam Institute for Climate Impact Research in Germany, first devised their ecological approach in 2018. Scientists had already modeled the climate as far back as 125,000 years, but Dr. Manica and Dr. Beyer wanted to go back to the date of the earliest anatomically modern human fossils, which were found in Morocco and are estimated to be at least 300,000 years old.

“That’s the moment when you see our species actually existed,” Dr. Manica said. Mario Krapp, a research fellow at Antarctica New Zealand and an author on the paper, developed an emulator for the existing climate model to go back deeper into time.

To predict when Homo sapiens feasibly could have moved through northeastern Africa and the Arabian Peninsula, the researchers needed to find out the absolute minimal conditions in which humans could survive. “We wanted to build up this catalog of the good times and bad times,” Dr. Manica said.

They looked at distribution maps of present-day hunter-gatherers and found that human populations are generally not recorded in areas where precipitation falls below 3.5 inches of rain per year. Rainfall this trifling is not enough to sustain green patches of reeds, grasses and shrubs that feed the grazing animals that early humans may have depended on.

Once the researchers set the threshold of survivability at 3.5 inches, they overlaid their climate reconstructions to see when conditions might have been sweet enough to travel through two possible routes into Eurasia: the Sinai Peninsula up north and, further south, the Strait of Bab-el-Mandeb , which separates the Horn of Africa from contemporary Yemen.

Their model revealed a handful of historical windows during which there was enough rainfall and relatively low sea levels to sustain a human migration out of Africa. The Sinai land bridge was crossable several times, as early as 246,000 years ago, and the southern strait had even more favorable windows, including the period 65,000 years ago.

The sheer number of crossing opportunities surprised Dr. Manica, given the robust evidence suggesting that only the recent mass exodus had peopled the world with Homo sapiens. “I was hoping, maybe naïvely, that period would just be perfect, where everything was right,” Dr. Manica said. “But everything was right before as well. Several times, for a matter of fact.”

So the question still stands: If some Homo sapiens were able to colonize Eurasia far earlier, why were they not successful?

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?”

Dr. Manica has a similar hope, that future archaeological excavations and genetic investigations will shed more light on Homo sapiens’s staggered foray out of Africa: both the earlier, seemingly unsuccessful waves and the main migration that unleashed Homo sapiens to irrevocably alter the rest of the world.

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