Less than 8,000 years ago, evidence shows modern people suddenly appeared en masse outside Africa, on the shores of the Persian Gulf.  An odd event, to be sure.  

Jeffrey Rose, writing in Current Anthroplogy, now says the reason is that the land that brought them there more gradually is now under the Gulf itself.

It makes sense as a hypothesis - you don't just go from sporadic hunting camps to dozens of archaeological sites without a trail, unless the trail is underwater.  Rose believes the that humans may have inhabited a fertile land mass where the Gulf now is for up to 100,000 years and it gradually became flooded by the Indian Ocean.

He examined historical sea level data and says such a Gulf basin would have been above water 75,000 years ago and, because it got water from the Tigris and Euphrates and likely underground springs, it would have been habitable.  How big was this sunken land mass, a so-called "Gulf Oasis"?  Around the size of Great Britain, he believes.

Really, we need to get back to comparing things to Wales.  Comparing things to Great Britain is too confusing.  

Given that the water levels likely varied quite a bit due to precipitation changes, Rose believes the dynamic demographics would account for the cultural evolution that seems to have occurred as well. 

Citation: Jeffrey I. Rose, 'New Light on Human Prehistory in the Arabo-Persian Gulf Oasis', Current Anthropology, 51:849–883, December 2010 DOI: 10.1086/657397