Recent Common Ancestors in Structured Populations

Previous analyses have shown that the most recent common ancestor for a randomly mating population of size n would have lived approximately log 2 n generations ago; for example, a panmictic population of one million people would have a common ancestor about 20 generations in the past. Structured populations consisting of panmictic subpopulations connected by migration at frequencies as low as one migrant per generation also have remarkably recent common ancestors. Mathematical analysis of such a structured model reveals that the time to the most recent common ancestor is a typically modest graph-theoretic factor multiplied by the corresponding time for a panmictic population of the same overall size. Computer simulations of populations having more complex structure, including models not amenable to theoretical analysis, confirm and extend these results. Our models suggest that ancestors common to all living humans probably lived in the relatively recent past—perhaps just a few thousand years ago.