The Smithsonian magazine has an interesting article on the origin of modern humans and the events leading to their rapid spread ‘out of Africa’:
Then, about 80,000 years ago, says Blombos archaeologist Henshilwood, modern humans entered a "dynamic period" of innovation. The evidence comes from such South African cave sites as Blombos, Klasies River, Diepkloof and Sibudu. In addition to the ocher carving, the Blombos Cave yielded perforated ornamental shell beads—among the world's first known jewelry. Pieces of inscribed ostrich eggshell turned up at Diepkloof. Hafted points at Sibudu and elsewhere hint that the moderns of southern Africa used throwing spears and arrows. Fine-grained stone needed for careful workmanship had been transported from up to 18 miles away, which suggests they had some sort of trade. Bones at several South African sites showed that humans were killing eland, springbok and even seals. At Klasies River, traces of burned vegetation suggest that the ancient hunter-gatherers may have figured out that by clearing land, they could encourage quicker growth of edible roots and tubers. The sophisticated bone tool and stoneworking technologies at these sites were all from roughly the same time period—between 75,000 and 55,000 years ago.
The above archaeological findings match African mtDNA data presented by Watson et al. (1997):
We find that most African mitochondrial sequences appear to be the result of demographic expansions that started ~60,000-80,000 years ago, the earliest of which led to the colonization of Eurasia. Only a minority (13%) of sequences fall outside these expansion clusters, echoing a time, before the expansions, when the human mitochondrial gene pool was possibly more diverse (in terms of mean sequence divergence) than it is today.
These population expansions appear to have been driven by cultural innovations that gave some African populations an edge over others and, eventually, over archaic humans in Europe and Asia. Unfortunately, instead of speaking of an expansion out of Africa, the Smithsonian article prefers the term ‘exodus’ — as if life had become so intolerable in Africa that humans were forced to pick up and leave. In fact, almost the opposite happened: modern humans did so well in Africa that they elbowed out their archaic rivals not only on their own continent but on others as well.
The Smithsonian article also speculates about the reasons behind their demographic success. These humans had become better at constructing mental models of the world around themselves. They could imagine the things they had to make and tinker with them in their minds. And they could share this mental tinkering with others through language.
This improvement in mental functioning may have been made possible initially by increased consumption of fatty acids in seafood. Then, as humans developed a new cultural environment and its attendant demands on brainpower, there would have been selection for genes to ‘hardwire’ this neurological change, i.e., the Baldwin effect.
Virtually all of these sites had piles of seashells. Together with the much older evidence from the cave at Pinnacle Point, the shells suggest that seafood may have served as a nutritional trigger at a crucial point in human history, providing the fatty acids that modern humans needed to fuel their outsize brains: "This is the evolutionary driving force," says University of Cape Town archaeologist John Parkington. "It is sucking people into being more cognitively aware, faster-wired, faster-brained, smarter." Stanford University paleoanthropologist Richard Klein has long argued that a genetic mutation at roughly this point in human history provoked a sudden increase in brainpower, perhaps linked to the onset of speech.
Gugliotta, G. (2008). The Great Human Migration. Why humans left their African homeland 80,000 years ago to colonize the world. Smithsonian magazine, July.
Watson, E., Forster, P., Richards, M., and Bandelt, H-J. (1997). Mitochondrial footprints of human expansions in Africa. American Journal of Human Genetics, 61, 691-704.