India, Sept. 11 -- Seventy-four thousand years ago, the sky went dark. On the island of Sumatra, the supervolcano Toba erupted with a force that was beyond anything humans had previously known. The Earth's atmosphere was filled with ash which blocked out the sun for years. Temperatures fell, and forests died. There had been no warning, and so there was no ability to plan for such a catastrophe. When animals disappeared and plants withered and died, food became harder to find. Genetic evidence suggests that human numbers fell to perhaps 3,000-10,000 breeding individuals.

Catastrophes such as this create an unusual kind of evolutionary pressure. Under ordinary conditions, many traits can co-exist because the environment can accommodate the...