GENETIC ABSOLUTE DATING BASED ON MICROSATELLITES AND THE ORIGIN OF MODERN HUMANS

We introduce a new genetic distance for microsatellite loci, incorpora ting features of the stepwise mutation model, and test its performance on microsatellite polymorphisms in humans, chimpanzees, and gorillas. We find that it performs well in determining the relations among the primates, but less well than other distance measures (not based on the stepwise mutation model) in determining the relations among closely r elated human populations. However, the deepest split in the human phyl ogeny seems to be accurately reconstructed by the new distance and sep arates African and non-African populations. The new distance is indepe ndent of population size and therefore allows direct estimation of div ergence times if the mutation rate is known. Based on 30 microsatellit e polymorphisms and a recently reported average mutation rate of 5.6 x 10(-4) at 15 dinucleotide microsatellites, we estimate that the deepe st split in the human phylogeny occurred about 156,000 years ago. Unli ke most previous estimates, ours requires no external calibration of t he rate of molecular evolution. We can use such calibrations, however, to test our estimate.