When you look at genes you find that people all over the world are amazingly similar. Some anthropologists believe that this genetic homogeneity is the result of a "population bottleneck"--that at some time in the past our ancestors went through an event that greatly reduced our numbers and thus our genetic variation.
Based on estimates of mutation rates, Penn State geneticist Henry Harpending says the bottleneck happened sometime after modern humans left Africa 100,000 years ago and before a population increase spurred by the advent of better stone tools around 50,000 years ago. Now archeologist Stan Ambrose of the University of Illinois has linked Harpending's theory with geologic evidence to explain what caused the bottleneck--a giant volcanic eruption.
From geologist Michael Rampino of New York University, Ambrose learned that 71,000 years ago Mount Toba in Sumatra blew 800 cubic kilometers of ash into the air--4,000 times as much as Mount St. Helens--the largest volcanic eruption in more than 400 million years. Toba buried most of India under ash and must have darkened skies over a third of the hemisphere for weeks.
Rampino believes that a six-year global volcanic winter ensued, caused by light-reflecting sulfur particles lingering in the atmosphere. Average summer temperatures dropped by 21 degrees at high latitudes, and 75 percent of the Northern Hemisphere's plants may have died. But the worst was yet to come. "Right at the end of those six years, temperatures bottomed out," says Ambrose. A thousand-year ice age began, he says, caused perhaps by an increasing amount of snow that failed to melt over the summer. This snow cover would have reflected more sunlight off Earth's surface, making the world still colder. The effect on humans, who had been enjoying a relatively warm period, must have been devastating. "After 60,000 years of basking," says Ambrose, "they were suddenly thrown into the freezer."
Perhaps only a few thousand people, living in isolated pockets in Africa, Europe, and Asia, survived. When the climate warmed again, about 70,000 years ago, these isolated groups began to grow. Ambrose and Harpending think today's races are but a small sample of the human diversity that once existed. "Imagine the volcanic winter as a dirty, fractured prism," says Ambrose. "It's absorbing some wavelengths, or some genes, and others get through. Since the prism is dirty, a lot of the diversity gets absorbed. No one comes out with all the original colors."