GENETIC-STRUCTURE OF THE ANCESTRAL POPULATION OF MODERN HUMANS

Neutral DNA polymorphisms from an 8-kb segment of the dystrophin gene, previously ascertained in a worldwide sample (n = 250 chromosomes), w ere used to characterize the population ancestral to the present-day h uman groups. The ancestral state of each polymorphic site was determin ed by comparing human variants with their orthologous sites in the gre at apes. The ''age before fixation'' of the underlying mutations was e stimated from the frequencies of the new alleles and analyzed in the c ontext of these polymorphisms' distribution among 13 populations from Africa, Europe, Asia, New Guinea, and the Americas (n = 860 chromosome s in total). Seventeen polymorphisms older tan 100,000-200,000 years, which contributed similar to 90% to the overall nucleotide diversity, were common to all human groups. Polymorphisms endemic to human groups or continentally restricted were younger than 100,000-200,000 years. Africans (six populations) with 13 such sites stood out from the rest of the world (seven populations), where only 2 population-specific var iants were observed. The similarity of the frequencies of the old poly morphisms in Africans and non-Africans suggested a similar profile of genetic variability in the population before the modern human's diverg ence. This ancestral population was characterized by an effective size of about 10,000 as estimated from the nucleotide diversity; this size may describe the number of breeding individuals over a long time duri ng the Middle Pleistocene or reflect a speciation bottleneck from an i nitially larger population at the end of this period.