There was a nice article in the WSJ yesterday, which briefly summarizes the current state of understanding of Out-of-Africa migrations:
Did Early Humans Ride the Waves to Australia?
"Everybody is African in origin. Barring a smattering of genes from Neanderthals and other archaic Asian forms, all our ancestors lived in the continent of Africa until 150,000 years ago. Some time after that, say the genes, one group of Africans somehow became so good at exploiting their environment that they (we!) expanded across all of Africa and began to spill out of the continent into Asia and Europe, invading new ecological niches and driving their competitors extinct."
"There is plenty of dispute about what gave these people such an advantage—language, some other form of mental ingenuity, or the collective knowledge that comes from exchange and specialization—but there is also disagreement about when the exodus began. For a long time, scientists had assumed a gradual expansion of African people through Sinai into both Europe and Asia. Then, bizarrely, it became clear from both genetics and archaeology that Europe was peopled later (after 40,000 years ago) than Australia (before 50,000 years ago)."