The human genome carries surprisingly little genetic diversity for a species of eight billion. New genomic research suggests one possible reason: hundreds of thousands of years ago, our ancestors may have dwindled to a population of only around 1,300 breeding individuals, leaving a genetic bottleneck whose effects can still be detected today.

Eight billion people can inherit a narrow genetic archivePopulation size and genetic diversity respond on different timescales. The 1,280 figure is not a literal head countThe key quantity is effective population size, usually written as Ne. One concluded that the signal was likely a statistical artifact, showing that ancient population structure could generate patterns resembling a sharp crash. What the genome can and cannot yet tell usThe human genome unquestionably records ancient changes in population size, migration and mixture. Bottlenecks associated with movements out of Africa are supported across many methods, and long-term effective population sizes have often been small relative to today’s census.