Tracing genetic history of modern humans using X-chromosome lineages.

Genetic variability of the compound interrupted microsatellite DXS1238, in intron 44 of the dystrophin gene, provides evidence for a complex structure of the ancestral population that led to the emergence of modern humans. We sequenced DXS1238 in 600 X-chromosomes from all over the world. Forty four percent of African-specific chromosomes belong to the ancestral lineage that did not participate in the out-of-Africa expansion and subsequent colonization of other continents. Based on the coalescence analysis these lineages separated from those that contributed to the out-of-Africa expansion 366 +/- 136 thousands years ago (Kya). Independently, the analysis of the variance in the repeat length and of the decay of the ancestral alleles of the two DXS1238 repeats, GT and GA, dates this separation at more than 200 Kya. This suggests a complex demographic history and genetic structure of the African melting pot that led to the emergence of modern humans and their out-of-Africa migration. The subsequent subdivisions of human populations among different continents appear to be preceded by even more structured population history within Africa itself, which resulted from a restricted gene flow between lineages allowing for genetic differences to accumulate. If the transition to modern humans occurred during that time, it necessarily follows that genes associated with this transformation spread between subpopulations via gene flow. Otherwise, in spite of subsequent anatomical variation, Homo sapiens as a species could have emerged in Africa already between 300 and 200 Kya, i.e. before the mitochondrial DNA and well before the Y-chromosome most recent common ancestors.