Research of modern African populations, including studies of their mitochondrial DNAa genetic marker passed down from mothers to daughterspoint to a major population expansion about 80,000 years ago.
(See an overview of human genetics.)
Peter Forster, a University of Cambridge geneticist who was not involved with Mellars' study, says a "remarkable expansion" of distinctive DNA lineages took place between 80,000 and 60,000 years ago.
Mellars says such DNA evidence points to an expansion centered initially in one small region of Africa, probably in eastern or southern Africa.
Meanwhile, archaeological evidence shows signs of major cultural advances during this same key period, he adds.
Important clues come from finds at sites such as Blombos Cave in South Africa.
Tools and decorative objects discovered there suggest an advanced culture that was closer to those of the first modern humans in Europe and western Asia than to earlier African groups.
(Read "Oldest Jewelry? 'Beads' Discovered in African Cave" [April 2004].)
Some researchers say this emergence of modern human culture was driven by a rapid advance in human cognitive ability, due to mutations in the brain.
But sharp shifts in the environment may also have been responsible, Mellars says, causing some populations to adapt quickly in response to changing conditions in order to survive.
The most obvious candidate, Mellars says, was a period of rapid climate change between about 80,000 and 70,000 years ago that saw some regions affected by increasingly wetter, and then drier, conditions.
Evidence for such marked fluctuations has been detected in climate records from ancient seabed sediments and polar ice cores. The records suggest that sub-Saharan Africa underwent a period when rainfall varied by up to 50 percent annually.
Previous studies have indicated that such conditions would have had a dramatic impact on populations in more arid areas of Africa, such as those living around the Kalahari Desert and the Sahara.
"It would, in short, be possible to see changes in human technology, subsistence, settlement patterns, and associated patterns of communication as a fairly direct response to the new environmental challenges that emerged at this time," Mellars writes in the new study.
A further potentially significant factor, Mellars adds, could have been the climatic and environmental effects of the "supereruption" of Mount Toba in what is now Indonesia some 73,000 years ago.
"People reckon that this could have caused a six-year volcanic winter, which would have obscured the sun," he said.
These major environmental changes happened at "precisely the time when the archaeological evidence indicates that technological and other behavioral changes were occurring most rapidly," he added.
Changes to the environment and food availability in Africa might also have forced people to travel farther, bringing them into contact with other groups.
"You might trade or exchange with other groups in case of short periods of starvation," Mellars said. He adds that the archaeological evidence points to increased mobility among some humans.
"You start getting a lot of imported, high-quality raw materials," he said. Such interactions could in turn have led populations to develop more complex forms of communication, including more advanced language.
Yet some researchers say there may be other ways to explain the long lag between the emergence of the first genetically modern humans and their expansion out of Africa.
Forster, the University of Cambridge geneticist, says that cognitive ability may not have been as important as swimming ability, for instance.
"If a change like swimming ability occurred at a certain point in time and, say, happened 60,000 years ago, people wouldn't have been afraid to go out on a raft to explore," he said.
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