Tuesday, April 5, 2016

Peopling Of The Americas- Rapid Settlement And Substantial Lineage Extinction Upon European Contact

Here is the full paper : Ancient mitochondrial DNA provides high-resolution time scale of the peopling of the Americas

The exact timing, route, and process of the initial peopling of the Americas remains uncertain despite much research. Archaeological evidence indicates the presence of humans as far as southern Chile by 14.6 thousand years ago (ka), shortly after the Pleistocene ice sheets blocking access from eastern Beringia began to retreat. Genetic estimates of the timing and route of entry have been constrained by the lack of suitable calibration points and low genetic diversity of Native Americans. We sequenced 92 whole mitochondrial genomes from pre-Columbian South American skeletons dating from 8.6 to 0.5 ka, allowing a detailed, temporally calibrated reconstruction of the peopling of the Americas in a Bayesian coalescent analysis. The data suggest that a small population entered the Americas via a coastal route around 16.0 ka, following previous isolation in eastern Beringia for ~2.4 to 9 thousand years after separation from eastern Siberian populations. Following a rapid movement throughout the Americas, limited gene flow in South America resulted in a marked phylogeographic structure of populations, which persisted through time. All of the ancient mitochondrial lineages detected in this study were absent from modern data sets, suggesting a high extinction rate. To investigate this further, we applied a novel principal components multiple logistic regression test to Bayesian serial coalescent simulations. The analysis supported a scenario in which European colonization caused a substantial loss of pre-Columbian lineages.

Several points of interest come to mind.

a) Separation with Siberian common ancestors took place about 24 thousand years ago, followed by a long period of isolation in Beringia (see Fig.).

The founder lineages then settled the America's quite rapidly beginning around 16 thousand years. Sites in Chile show human habitation by 14 thousand years.

b) The much talked about Clovis culture were not the first Americans.

c) There was a demographic collapse among the Native American populations. Charles Mann's 1491 gives quite a sweeping account of both native and Spanish eye witness accounts of the wiping out of entire tribes and communities due to Old World diseases. This paper backs that up using genetics. A comparison of ancient DNA with present day Native American DNA shows that extensive extinction of mitochondrial lineages took place.  In plain language, entire tribes were wiped out to be replaced  much later by natives migrating from some other place, although the authors caution some of the discontinuity of lineages could be because  modern diversity may have been undersampled.  The data for ancient DNA comes principally from the western coast of South America which had large Native American population centers.   Lowland populations went extinct more. Highland populations were less affected. Perhaps population contact was more in the lowlands and diseases like malaria more prevalent in the warmer climes?

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