Read more: 鈥�10 biggest puzzles of human evolution鈥�
OUR ancestors have achieved some epic migrations. Homo erectus made the first great trek out of Africa and into east Asia 1.8 million years ago. Around a million years later, the predecessors of Neanderthals turned up in Europe. And 125,000 years ago, Homo sapiens made an early foray into the Middle East. None of these populations lasted. But some 65,000 years ago, one group of modern humans left Africa and conquered the world 鈥� an extraordinary achievement for any species, let alone a puny, furless ape. What possessed them to spread so far and wide?
It may have begun with a big squeeze. All humans belong to one of four mitochondrial lineages (L0, L1, L2 and L3) corresponding to four ancestral mothers, but only L3 is found outside Africa. Quentin Atkinson at the University of Auckland, New Zealand, and colleagues have found that this lineage experienced a population explosion in the 10,000 years leading up to the exodus (). So overcrowding in the Horn of Africa may have pushed the group to cross the Red Sea and move along the southern coast of Asia.
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See graphic: 鈥淭o boldly go鈥�
That still leaves the question of why numbers increased. Atkinson notes that for 100,000 years the African climate had oscillated between drought and floods before becoming stable around 70,000 years ago. Perhaps the environmental instability had forced early humans to become more inventive, with adaptations that helped population expansion once conditions improved.
Paul Mellars at the University of Cambridge has argued that the explosion in numbers was driven by a major increase in the complexity of technological, economic, social and cognitive behaviour (). The ability to control fire came much earlier, as, probably, did language. But the period does see a blossoming of innovation such as the manufacture of complex tools, efficient exploitation of food sources, artistic expression and symbolic ornamentation. These cultural advances would have been crucial, says Mark Pagel at the University of Reading, UK. 鈥淣ot only can we walk, we can change the world when we get there.鈥� This flexibility would have propelled migrants ever onward, he notes, as populations quickly reached carrying capacity and individuals moved into new territory to avoid competition.
鈥淪ome of it would have been accidental,鈥� adds Chris Stringer of London鈥檚 Natural History Museum: the peopling of Australia may have come about when seafarers travelling between islands were blown further afield. Genetic mutations could also have made us more adventurous. For example, the so-called novelty-seeking gene, DRD4-7R, is more common in populations that migrated fastest and furthest from Africa (). 鈥淥f course there is the human spirit 鈥� to climb the unclimbed mountain,鈥� says Stringer.
