23 February 2010

Quest For Ancient Genetic History Reviewed

What have we learned about the pre-history of the human race from genetics?

A special issue of Current Biology linked at Dienekes' Anthropology blog, summarized the findings to date in this rapidly advancing field.

UPDATE: A number of interesting points are made in the articles.

* Climate is the major driving force in human pre-history and ancient history.

* Modern humans appear to have made their way to the Near East from Africa in the period from 130,000 to 75,000 years ago, with documentation of that presence at least as far back as 100,000 years ago in the Levant. But, coincident with climate change, this population may have gone extinct or retreated to Africa and been replaced by Neanderthals from 75,000 to 45,000 years ago, when desert conditions no longer blocked modern human access to the Levant. Southern Coastal migration leaving Africa via the Southern end of the Red Sea to India, Southeast Asia and Australia may have commenced 70,000 to 60,000 years ago while deserts blocked access to the Levant.

* While modern humans entered Europe 45,000 years ago or so, there was a retreat from Northern Europe and a decline in the sophistication of tool making for several thousand years leading up to the Last Glacial Maxmium around 20,000 years ago. The recolonization of Europe by modern humans from Southern European areas (e.g. Iberia and Italy and the Balkans) started around 11,500 years ago, probably about a thousand years before migration into Europe associated with the Neolithic revolution (i.e. the domestication of plants and animals for use in agriculture).

* Khoesan (East African and Southern African hunter-gatherer ancestors) and Pygmy (tropical African) populations in Africa appear from genetic evidence to have diverged from each other around 35,000 years ago, and proto-population appears from genetic evidence to have diverged from other African populations (from whom Eurasian and American indigeneous people trace their roots) around 70,000 years ago. The Eastern and Western Pygmy populations, in turn, appear to have been divided around 18,000 years ago. This suggests that the extinct Pygmy language, which probably went extinct during the early Bantu expansion in Africa (ca. 5,000 years ago) or earlier, may have been a Khoesan-like click language.

* The Nilo-Saharan languages probably have their roots in Sudan more than 8,000 years ago and expanded both East and West from there. Some speakers of Chadic languages appear to be genetically closely related to Nilo-Saharan speakers in East Africa and probably adopted a Chadic language as a population without much genetic change around 8,000 years ago.

* The linguistic and genetic evidence show remarkably little commonality between pre-Austronesean expansion New Guinea (i.e. pre-3,400 years ago) and Australia, despite the fact that they were geographically linked by land until 8,000 years ago and despite a link by an island chain that was navigated and supported a thin level of trade for at least several thousand years prior to European contact. This points to either deep isolation of the regions from very early after the 50,000 years ago settlement of Australia, or a separate migration from the West (i.e. from Indonesia) rather than the South (i.e. from Australia).

* Northeast Siberia, from which all indigeneous human populations of the Americas derive, was populated prior to the Last Global Maximum around 20,000 years ago. This population retreated or went extinct at this peak of the last ice age in Northeast Siberia and was then repopulated as climate warmed again. It isn't entirely clear to what extent it is the pre-ice age, or post-ice age population of Northeast Siberia that provided the ancestral population for the Americas. Genetic evidence tends to favors a split between Northeastern Siberian and American gene pools not later than 17,000 years ago, but it is possible that there was a Beringian refugia population (i.e. in Alaska and the exposed land bridge across the Bering strait) derived from the pre-ice age population of Northeast Siberia that was the core population for both and that Northeastern Siberia was recolonized in whole or in part from the East rather than from the South. This would delay the split from a population genetics perspective.

* There is a genetic evidence of a split between Eastern North American and Pacific Coastal genetics in the Americas. The Eastern group lacks the X halotype, while the Pacific group has it. The Clovis culture archeology seems to be a better fit with the Eastern branch, implying a colonization of North America via a Northern route and Atlantic North American migration pattern that turned inward. There are no Clovis sites in South America and they are more common in the Eastern United States than the West. The oldest archeological sites in the Americas are found near the Pacific Coast of South America (as far back as 14,000 years) and signficantly pre-date the oldest Clovis sites (13,000 years old). This genetic and archeological evidence supports a two wave theory of migration into the Americas, with a first wave along the Pacific coast a thousand years earlier or so, than the wave of migration indicated by the Clovis culture.

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