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The Relationship between Modern Humans and Neanderthals
The relationship between modern humans and archaic hominins, particularly Neanderthals, has been the subject of much debate. While the idea that modern humans originated in Africa and spread out to other parts of the world (Out of Africa) is widely accepted, several scenarios have been proposed to account for the replacement of archaic hominin populations. Under strict replacement, modern humans did not interbreed with the archaic populations as they expanded their geographic range. In less strict scenarios, admixture between the populations occurred, but in small amounts, with the bulk of modern human ancestry tied to Africa. The multiregional hypothesis holds that hominin populations in Eurasia and Africa were held together by gene flow. Fossil and genetic evidence supports an African origin for Homo sapiens.
Mitochondrial DNA shows differences between Neanderthals and modern humans. Neanderthal mtDNA also differed from that of anatomically modern Homo sapiens from the same time period. Proponents of multiregional and admixture models argue that these results are consistent with African origin for modern Homo sapiens, but do not explicitly rule out admixture between modern humans and archaic populations (Templeton 2007, Relethford 2008). Neanderthal genetic sequences introduced into the human genome may have been subsequently lost through genetic drift (Relethford 2001), while similarities between modern Europeans and Neanderthals, which would be expected if Neanderthals and modern humans interbred while in Europe, could have been lost due to gene flow between modern humans from different regions.
Various analyses have examined the amount of Neanderthal contribution to modern human mtDNA. One analysis was unable to find positive evidence for interbreeding, but could not rule out a small genetic contribution (Serre et al. 2004). Other researchers (Plagnol and Wall 2006, Wall et al. 2009) looked at the pattern of variation in modern human DNA to determine whether modern humans mixed with more ancient populations. Their recent models are consistent with between 1-4% archaic-modern admixture in European and American populations, and 1.5% admixture in East Asian populations. Nested clade phylogenetic analysis shows evidence of three expansions out of Africa at 1.9 Ma, 650,000 years, and 130,000 years, which is consistent with the admixture between ancient and modern populations rather than complete replacement (Templeton 2002, 2005, 2007). Other researchers contend that factors such as population structure within Africa could have preserved old haplotypes and produced the pattern found in the nested clade analysis (Satta and Takahata 2002).
Though it is difficult to prove or quantify admixture, small amounts of interbreeding were supported by a variety of analyses. However, the substantial differences between Neanderthal and modern human mtDNA is consistent with large-scale replacement and some amount of interbreeding between modern and archaic populations. Interbreeding between archaic and moderns may have involved different species of archaic hominins, including populations in Africa, Asia and Europe.
The draft sequence of the Neanderthal genome provides more evidence that interbreeding between Neanderthals and modern humans may have occurred. It showed more similarities between non-African modern humans and Neanderthals than between African modern humans and Neanderthals. This difference between regions is consistent with interbreeding between Neanderthals and the ancestors of Eurasian modern humans before they branched off into regional groups. Approximately 1 to 4% of non-African modern human DNA is shared with Neanderthals.