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.
Did new technology, improved nutrition or some genetic mutation allow modern humans to explore the world? Possibly, but other scholars point to more mundane factors that may have contributed to the exodus from Africa. A recent DNA study suggests that massive droughts before the great migration split Africa's modern human population into small, isolated groups and may have even threatened their extinction. Only after the weather improved were the survivors able to reunite, multiply and, in the end, emigrate. Improvements in technology may have helped some of them set out for new territory. Or cold snaps may have lowered sea level and opened new land bridges.
Whatever the reason, the ancient Africans reached a watershed. They were ready to leave, and they did.
DNA evidence suggests the original exodus involved anywhere from 1,000 to 50,000 people. Scientists do not agree on the time of the departure—sometime more recently than 80,000 years ago—or the departure point, but most now appear to be leaning away from the Sinai, once the favored location, and toward a land bridge crossing what today is the Bab el Mandeb Strait separating Djibouti from the Arabian Peninsula at the southern end of the Red Sea. From there, the thinking goes, migrants could have followed a southern route eastward along the coast of the Indian Ocean. "It could have been almost accidental," Henshilwood says, a path of least resistance that did not require adaptations to different climates, topographies or diet. The migrants' path never veered far from the sea, departed from warm weather or failed to provide familiar food, such as shellfish and tropical fruit.
Tools found at Jwalapuram, a 74,000-year-old site in southern India, match those used in Africa from the same period. Anthropologist Michael Petraglia of the University of Cambridge, who led the dig, says that although no human fossils have been found to confirm the presence of modern humans at Jwalapuram, the tools suggest it is the earliest known settlement of modern humans outside of Africa except for the dead enders at Israel's Qafzeh and Skhul sites.
And that's about all the physical evidence there is for tracking the migrants' early progress across Asia. To the south, the fossil and archaeological record is clearer and shows that modern humans reached Australia and Papua New Guinea—then part of the same landmass—at least 45,000 years ago, and maybe much earlier.
But curiously, the early down under colonists apparently did not make sophisticated tools, relying instead on simple Neanderthal-style flaked stones and scrapers. They had few ornaments and little long-distance trade, and left scant evidence that they hunted large marsupial mammals in their new homeland. Of course, they may have used sophisticated wood or bamboo tools that have decayed. But University of Utah anthropologist James F. O'Connell offers another explanation: the early settlers did not bother with sophisticated technologies because they did not need them. That these people were "modern" and innovative is clear: getting to New Guinea-Australia from the mainland required at least one sea voyage of more than 45 miles, an astounding achievement. But once in place, the colonists faced few pressures to innovate or adapt new technologies. In particular, O'Connell notes, there were few people, no shortage of food and no need to compete with an indigenous population like Europe's Neanderthals.
Modern humans eventually made their first forays into Europe only about 40,000 years ago, presumably delayed by relatively cold and inhospitable weather and a less than welcoming Neanderthal population. The conquest of the continent—if that is what it was—is thought to have lasted about 15,000 years, as the last pockets of Neanderthals dwindled to extinction. The European penetration is widely regarded as the decisive event of the great migration, eliminating as it did our last rivals and enabling the moderns to survive there uncontested.