Well, it's widely accepted these days that modern humans originated in Africa and at first were restricted to that continent. Human groups that live by hunting and gathering need large territories to hunt in, and over time, as these groups have children and increase in numbers, they will occupy all the available land. Once that land fills up, people are going to try to leave, if they can, to reach richer pickings elsewhere. But our ancestors could not cross wide stretches of water; although they must have developed some sort of rafts to reach some of the places they did, truly seaworthy boats were still far in the future. So there were only three likely routes by which they could leave Africa: across the Straits of Gibraltar, across the Sinai, or across the Bab-el-Mandeb strait at the southern end of the Red Sea. The paper you quoted suggests that humans living in east Africa emigrated over that last route, based on genetic evidence.
That paper claims this would have happened around 60,000 years ago. Since this was during the last ice age, when lots of water was locked up in massive ice sheets covering the northern parts of both North America and Europe, the sea level was tens of meters lower than it is today. That would have made the strait narrower and the crossing easier. It would also have exposed large areas of land around the Arabian Peninsula which are now underwater, including the entire Persian Gulf. Furthermore, the climate was very different at that time. In particular, the paper mentions "the pluvial period 55-24 ka ago"; that's geologists' jargon for a period of a heavily rainy global climate ("pluvial" means "rainy") from 55,000 to 24,000 years ago. ("ka" is short for "kiloannum", meaning a thousand years.) So parts of the Arabian Peninsula would have been wetter and more hospitable than today. These would have made good territories for human hunter-gatherers, allowing them to occupy the region and spread further abroad.
By the way, how did you come across this paper? I'm just curious.
