The DNA of modern humans contains a record of the travels and encounters of our ancestors. The genotypes of people living today are the result of ancient human migrations, the continuous appearance of new mutations, selection by climate and infection for genetic alleles that conferred a survival advantage, and mating patterns determined by cultural norms. By sampling genotypes from people across the globe, geneticists have reconstructed the major features of our history: our ancient African origin, migrations out of Africa, movements and settlements throughout Eurasia and Oceania, and peopling of the Americas (1–5). As genomic technology has improved, these analyses of genotype have successively incorporated new markers: blood groups (2), protein polymorphisms (2), mitochondrial DNA sequences (1), Y chromosome haplotypes (3), and highly variable nuclear microsatellite markers (4, 5).