Today, there are more than 8 billion human beings on the planet. We dominate Earth’s landscapes, and our activities are driving large numbers of other species to extinction. Had a researcher looked at the world sometime between 800,000 and 900,000 years ago, however, the picture would have been quite different. Hu et al. used a newly developed coalescent model to predict past human population sizes from more than 3000 present-day human genomes (see the Perspective by Ashton and Stringer). The model detected a reduction in the population size of our ancestors from about 100,000 to about 1000 individuals, which persisted for about 100,000 years. The decline appears to have coincided with both major climate change and subsequent speciation events.
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