Archaic lineages in the history of modern humans

An important question in the ongoing debate on the origin of Home sapiens i s whether modern human populations issued from a single lineage or whether several, independently evolving lineages contributed to their genetic makeu p. We analyzed haplotypes composed of 35 polpmorphisms from a segment of th e dystrophin gene. We find that the bulk of a worldwide sample of 868 chrom osomes represents haplotypes shared by different continental groups. The re maining chromosomes carry haplotypes specific for the continents or for loc al populations. The haplotypes specific for non-Africans can be derived fro m the most frequent ones through simple recombination or a mutation. In con trast, chromosomes specific for sub-Saharan Africans represent a distinct g roup, as shown by principal component analysis, maximum likelihood tree, st ructural comparison, and summary statistics. We propose that African chromo somes descend from at least two lineages that have been evolving separately for a period of time. One of them underwent range expansion colonizing dif ferent continents, including Africa, where it mixed with another, local lin eage represented today by a large fraction of African-specific haplotypes. Genetic admixture involving archaic lineages appears therefore to have occu rred within Africa rather than outside this continent, explaining greater d iversity of sub-Saharan populations observed in a variety of genetic system s.