Out-of-Africa hypothesis
Archeological evidence is generally considered to support the initial spread of humans within Africa from an East African origin during the first half of the last 100 000 years and the spread from the same origin to all the world in the last 50-60 000 years (50-60 kya).
Analyses of numerous classical markers under this assumption have estimated the dates of first occupation by anatomically modern humans of Asia, Europe and Oceania at 60?40 kya, in agreement with archeological and fossil data.
Dates for the first occupation of America are estimated at 15?35 kya.
Thus, genetically derived dates are consistent with evidence from physical anthropology, providing support for the use of population trees.
Recent analysis of DNA polymorphisms supports this timing of the earliest split between Africans and non-Africans.
Facts
Africa has played a central role in human evolutionary history. However, certain studies have suggested that not all contemporary human genetic diversity is of recent African origin.
Investigated 35 simple polymorphic sites and one T(n) microsatellite in an 8-kb segment of the dystrophin gene, Labuda et al. found 86 haplotypes in 1,343 chromosomes from around the world. Although a classical out-of-Africa topology was observed in trees based on the variant frequencies, the tree of haplotype sequences reveals three lineages accounting for present-day diversity. The proportion of new recombinants and the diversity of the T(n) microsatellite were used to estimate the age of haplotype lineages and the time of colonization events. (14513410)
- The lineage that underwent the great expansion originated in Africa prior to the Upper Paleolithic (27,000-56,000 years ago). (14513410)
- A second group, of structurally distinct haplotypes that occupy a central position on the tree, has never left Africa.
- The third lineage is represented by the haplotype that lies closest to the root, is virtually absent in Africa, and appears older than the recent out-of-Africa expansion. This lineage could have left Africa before the expansion (as early as 160,000 years ago) and admixed, outside of Africa, with the expanding lineage. (14513410)
- Contemporary human diversity, although dominated by the recently expanded African lineage, thus could represent a mosaic of different contributions. (14513410)
References
Cavalli-Sforza LL, Feldman MW. The application of molecular genetic approaches to the study of human evolution. Nat Genet. 2003 Mar;33 Suppl:266-75. PMID: 12610536
Cavalli-Sforza LL. The DNA revolution in population genetics. Trends Genet. 1998 Feb;14(2):60-5. PMID: 9520599