Genetic mutation in Africa

At http://phys.org/print267290115.html (and now at www.geneticarchaeology.com from today, the 21st Sept) … new research is suggesting genetic mutation explains how early humans were able to move from central Africa in what is known as the 'Great Expansion' event. I don't suppose many people outside anthropology circles were aware of this 'great expansion', which is dated to 85,000 years ago (for various interlocking and complex reasons involving Out of Africa, archaeology, genetics and geology etc). Neither are most people aware of a sudden increase in brain size at the same time, providing modern humans with their complexities and advanced functions (as it is seen in the eyes of anthropologists). We are talking here about Africa, and it is from Africa that modern humans are thought to have originated. However, in this case the research came about as a result of a higher frequency for certain illnesses in people of African descent living in modern North America, so it is primarily medical research and biochemistry, and nothing in particular to do with Out of Africa. The consensus appears to be that homo sapiens emerged in Africa approximately 180,000 years ago but they remained in one location, around a series of lakes in central Africa, for the next 100,000 years. This appears to be a strange concept as they are saying that none of these people looked on the other side of the hill to see if the grass was greener – but then the idea is central to what developed and moving people would spoil the theory. This is that Homo sapiens developed a taste for shell fish and fish from the lakes environment and over time so much fish in the diet led to the change in brain size etc. Around 85,000 years ago this smallish population, it is said, expanded into other regions of Africa, fairly rapidly, passing on their gene variants to other populations. So, what caused them to migrate – what was going on?