A favorable climate
If early modern humans were on the verge of leaving Africa today, the Arabian Peninsula would not likely be the most enticing route. The current global climate is currently vastly different than it was 200,000 to 130,000 years ago. As a global ice age locked up much of the Earth's water in frigid poles, sea levels receded, narrowing straights practically down to large lakes. By the end of the glacial period, water levels in the Red Sea would have been more than 100 meters lower than they are currently.
As the glaciers began to thaw, weather patterns would have shifted, bringing the Indian monsoon system to drench Arabia with rains, bringing with them greenery and freshwater lakes and rivers. This transitional time, Adrian Parker, also of Royal Holloway and a member of the research group, explained on the Wednesday conference call, was the chance climatic opening during which it made sense for H. sapiens to endeavor crossing the then-small sea.
"This led to a brief window of time when sea levels were still low, and Arabia experienced a wetter climate," Parker said.
After the makers of the C group of tools arrived in Arabia, continued glacial melting would have raised sea levels again, making more trans–Red Sea trips less likely. But, Uerpmann noted, "another window opened shortly after this event, increasing monsoon during the full interglacial enabled hunter–gatherers to cross the desert separating south[ern] Arabia from southeast[ern] Arabia. A drop in sea level during the subsequent cooler period provided an opportunity to reach parts of Mesopotamia, extending much farther into what is now the Persian Gulf and the Iranian coast—and the Indian subcontinent could be reached [from there]."
The climatic conditions so favorable to human expansion did not last indefinitely. The Jebel Faya archaeological site has yielded evidence from the Neolithic, Bronze and Iron ages as well, but the trail of artifacts has a gaping hole between 38,000 and 10,000 years ago when the area would have once again become desertlike.
The Paleolithic period climatic cycles' influence on early human dispersal likely have many more discoveries to divulge. The lowering and rising of seas and the shifting of precipitation patterns "happened many times during the Quaternary [period], and this leaves a lot of possibilities for human migrations," Uerpmann said. "And keeping this in mind might change our view completely."