Here's a nifty 3-minute Flash video of 160,000 years of human migration. It depicts the Toba catasprophe theory that circa 71,500 years ago, a supervolcano in Indonesia, at what's now called Lake Toba, erupted. The eruption's explosive energy was around a gigaton, 20 times the explosive energy of the biggest nuclear bomb detonation. India and Malaysia were blanketed with ash, in places meters thick.
Ash in the atmosphere created a volcanic winter, and in the ensuing planetary extinction event, the human race dwindled to as few as 10,000 adults.One hypothesis is that an effect of this was rapid human phenotypic differentation.
The Toba Catastrophe theory is fairly recent, but my web-searching hasn't revealed anything challenging it.
Have a nice day.