Lake Malawi, for instance, contained just one-twentieth as much water in 135,000 BCE as it does today. The tougher environment must have changed the rules for staying alive, which may explain why mutations favoring braininess began flourishing. It may also explain why we have found so few sites from this period; most protohumans probably died out. Some archaeologists and geneticists in fact estimate that around 100,000 BCE there were barely twenty thousand Homo sapiens left alive.
