Ancient 'Volcanic Winter' Tied To Rapid Genetic
Divergence In Humans
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.
This story has been adapted from a news release issued by
University Of Illinois At Urbana-Champaign. About This
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