There have been, evidently, three times in human history when humanity was in danger of extinction. About 900,000 years ago the population was down to about 1,280 individuals and that number did not significantly increase for well over 100,000 years. We’ve known for a long time that hominids did not sweep across the globe in one steady progression. There were places where they existed for an extended period only to vanish and not reappear for many thousands of years.

A violent volcanic eruption called Toba on Sumatra, in Indonesia about 70,000 years ago blew about 650 miles of vaporized r...