CHAMPAIGN, Ill. -- A new hypothesis about recent human evolution suggests that a horrific "volcanic winter" 71,000 years ago, followed by the coldest 1,000 years of the last Ice Age, brought widespread famine and death to modern human populations around the world. The abrupt "bottleneck," or decrease, in our ancestors' populations, in turn, brought about the rapid "differentiation" - or genetic divergence - of the surviving populations.
The hypothesis, offered by anthropologist Stanley Ambrose of the University of Illinois, proposes that a volcanic winter reduced populations to "levels low enough for evolutionary changes, which occur much faster in small populations, to produce rapid population differentiation," Ambrose said. If, as he believes, the eruption of Mount Toba in Sumatra caused the bottleneck, "then modern human races may have diverged abruptly, only 70,000 years ago," Ambrose wrote in the June issue of the Journal of Human Evolution.
Geneticists long have argued that the human species passed through a recent bottleneck, but they never offered explanations for the population crash or recovery, nor considered its consequences for modern human diversification. Ambrose's model, which he calls the Weak Garden of Eden/Volcanic Winter model, is an offshoot - with significant additions - of the Weak GOE model proposed by Henry Harpending and others. The Weak GOE model proposes an African origin for modern humans about 130,000 years ago, and credits the invention and spread of advanced stone tool technology, 40,000 to 50,000 years ago, for population growth after the bottleneck. Ambrose argues that volcanic winter resulting from the super-eruption of Toba "caused the bottleneck, and that populations may have expanded in response to climatic warming 10,000 years before the advent of modern technology."
Ambrose has linked geneticists' research to that of volcanologists Michael Rampino, Stephen Self, Greg Zielinski and colleagues, which shows the super-eruption of Toba caused a volcanic winter that lasted six years and significantly altered global climate for the next 1,000 years. Those six years of "relentless volcanic winter" led to substantial lowering of global temperatures, drought and famine, and to a global human population crash during which, if geneticists are correct, no more than 15,000 to 40,000 people survived.
"The standard view of human evolution has been that modern populations evolved from an ancient African ancestor. We assumed that they differentiated gradually because we assumed ancestral populations were large and stable," Ambrose said. But, he noted, genetic research now demonstrates that changes in population size were sometimes dramatic. The new model resolves the paradox of the recent African origin model: If we are all so recently "out of Africa," why don't we all look like Africans?
"When our African recent ancestors passed through the prism of Toba's volcanic winter, a rainbow of differences appeared," Ambrose said.
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