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Getting here from there: Mitochondrial genome clarifies North American migration models

Posted September 26, 2013
It is generally agreed that the ancestors of modern Native Americans were Asian peoples who migrated to North America from Siberia and Beringia – a region proximate to the Bering Strait, Bering Sea and Chukchi Sea – over the so-called Bering Land Bridge, which was exposed, and therefore connected Asia with North America, at various times during the Pleistocene ice ages. At the same time, the question of whether this occurred through one or several streams of migration has long been a topic of considerable debate, with analyses based primarily on mitochondrial DNA (mtDNA) – which is directly inherited from in a maternal lineage – tending to coalesce around a model that hypothesized three distinct migratory streams (known as tripartite migration), which was originally proposed by combining anthropometric, genetic, and linguistic data. Recently, however, researchers at Università di Perugia and Università di Pavia, Italy (and a range of other institutions) evaluating these migratory models used mitochondrial genomes, or mitogenomes, to show that although the primary genetic signature was contributed by the first arrival, it was later modified not only my multiple addition migratory streams, but by local population dynamics as well. The scientists thus concluded that a standard three-wave model was too simplistic to account for the mitogenomic diversity revealed by their analysis.
Getting here from there: Mitochondrial genome clarifies North American migration models
Schematic phylogeny of complete mtDNA sequences belonging to haplogroups A2a and B2a. A maximum-likelihood (ML) time scale is shown. (Inset) A list of exact age values for each clade. Credit: Copyright © PNAS, doi:10.1073/pnas.0905753107

Researcher Alessandro Achilli discussed the paper that he, Prof. Antonio Torroni and their co-authors published in the Proceedings of the National Academy of Sciences(PNAS). “Over the previous decade there’s been a renewed interest in studying the origin of human populations, particularly indigenous groups of the New World,” Achilli tells “However,” he notes, “there remains a level of disagreement among scientists regarding the number of migratory events and entry routes that gave rise to Native American groups. The general consensus is that modern Native Americans trace their ancestry to a limited number of original founders whose gene pool ultimately derived from Asians that peopled northeast Siberia and parts of Beringia prior to the Last Glacial Maximum, or LGM. (The LGM is a period in the Earth’s climate history when ice sheets were at their maximum extension between 26,500 and 19,000–20,000 years ago.) The ancestral Beringian populations probably retreated into refugia (areas in which a population of organisms can survive through a period of unfavorable conditions) during the Ice Age, Achilli continues, where their genetic variation was reshaped not only because of drift, but also due to admixture with population groups newly arrived from regions located west of Beringia. “Therefore,” Achilli explains, “pre-LGM mitochondrial haplotypes” – mtDNA variants inherited together from a female parent – “of Asian ancestry were differently preserved, modified, and lost in Beringian enclaves – and the main traces within the New World could be found within the closest regions, such as North America.”


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