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CARTA: The Origin of Us – Richard “Ed” Green: Interbreeding with Archaic Humans

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 titanic1    244





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).

Read more... http://humanorigins.si.edu/evidence/genetics/ancient-dna-and-neanderthals/interbreeding

Sex with Neanderthals had its ups and its downs. Cross-breeding may have given modern humans genes useful for coping with climates colder than Africa's, but the hybrid offspring probably suffered from significant fertility problems.

Those conclusions come from two papers published today in Science1 and Nature2, which identify the slices of the genome that contemporary humans inherited from Neanderthals, the stocky hunter-gatherers that went extinct around 30,000 years ago.

Homo sapiens and Neanderthals share a common ancestor that probably lived in Africa more than half a million years ago. The ancestors of Neanderthals were the first to move to Europe and Asia while the modern-human lineage stayed in Africa. But after modern humans began to leave Africa less than 100,000 years ago, they interbred with the Neanderthals who had settled on a range stretching from Western Europe to Siberia.

“These were bits of the genomes that had not seen each other for half a million years,” says David Reich, a population geneticist at Harvard Medical School in Boston, Massachusetts, who led the Nature study along with colleague Sriram Sankararaman. “That’s something that doesn’t happen in human populations today.”

The magic number
Genome sequences harvested from Neanderthal bones have previously confirmed that the two groups mated, and that about 2% of the genomes of people who descend from Europeans, Asians and other non-Africans is Neanderthal3, 4. The Neanderthal contributions are peppered across the genome, and different people have different Neanderthal genes.

Research has indicated that some of these genes are involved in functions such as battling infections5, 6 and coping with ultraviolet radiation7. But the latest studies are the first to identify a large proportion of the genome segments that humans inherited from Neanderthals.


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