From the composition of just two human genomes, geneticists have computed the size of the human population 1.2 million years ago from which everyone in the world is descended.

They put the number at 18,500 people, but this refers only to breeding individuals, the “effective” population. The actual population would have been about three times as large, or 55,500.

Comparable estimates for other primates then are 21,000 for chimpanzees and 25,000 for gorillas. In biological terms, it seems, humans were not a very successful species, and the strategy of investing in larger brains than those of their fellow apes had not yet produced any big payoff. Human population numbers did not reach high levels until after the advent of agriculture.

Geneticists have long known that the ancestors of modern humans numbered as few as 10,000 at some time in the last 100,000 years. The critically low number suggested that some catastrophe, like disease or climate change induced by a volcano, had brought humans close to the brink of extinction.