Multiple migrations to the New World look less likely

 "Mitochondrial Population Genomics Supports a Single Pre-Clovis Origin with a Coastal Route for the Peopling of the Americas", The American Journal of Human Genetics, Volume 82, Issue 3, 583-592, 28 February 2008.

http://www.ajhg.org/AJHG/fulltext/S0002-9297%2808%2900139-0

Abstract:

It is well accepted that the Americas were the last continents reached by modern humans, most likely through Beringia.  However, the precise time and mode of the colonization of the New World remain hotly disputed issues.  Native American populations exhibit almost exclusively five mitochondrial DNA (mtDNA) haplogroups (A-D and X).  Haplogroups A-D are also frequent in Asia, suggesting a northeastern Asian origin of these lineages.  However, the differential pattern of distribution and frequency of haplogroup X led some to suggest that it may represent an independent migration to the Americas.  Here we show, by using 86 complete mitochondrial genomes, that all Native American haplogroups, including haplogroup X, were part of a single founding population, thereby refuting multiple-migration models.  A detailed demographic history of the mtDNA sequences estimated with a Bayesian coalescent method indicates a complex model for the peopling of the Americas, in which the initial differentiation from Asian populations ended with a moderate bottleneck in Beringia during the last glacial maximum (LGM), around ~23,000 to ~19,000 years ago.  Toward the end of the LGM, a strong population expansion started ~18,000 and finished ~15,000 years ago.  These results support a pre-Clovis occupation of the New World, suggesting a rapid settlement of the continent along a Pacific coastal route.

Authors:

Nelson J.R. Fagundes, Ricardo Kanitz, Roberta Eckert, Ana C.S. Valls, Mauricio R. Bogo, Francisco M. Salzano, David Glenn Smith, Wilson A. Silva, Marco A. Zago, Andrea K. Ribeiro-dos-Santos, Sidney E.B. Santos, Maria Luiza Petzl-Erler andSandro L. Bonatto,

Excerpts:

The maternally inherited mitochondrial DNA (mtDNA) has been widely used to understand the peopling of the Americas. Since the first studies, it has been found that extant Native American populations exhibit almost exclusively five mtDNA haplogroups (A-D and X) classified in the autochthonous haplogroups A2, B2, C1, D1, and X2a.  Haplogroups A-D are found all over the New World and are frequent in Asia, supporting a northeastern Asian origin of these lineages.  This distribution, together with the similar coalescence time for these haplogroups, was used to suggest a single-migration model.  However, a different pattern of diversification and distribution of haplogroup B found in some studies led some authors to hypothesize that it could represent a later and separate migration from the joint arrival of haplogroups A, C, and D.  The history of haplogroup X is more elusive; it is presently found in the New World at a relatively low frequency and only in North America, it is rare in West Eurasians, and it is almost absent in Siberia.  In addition, some have claimed that Native American haplogroup X is less diverse and has a younger coalescence time than haplogroups AD.  These differential features have been cited to argue that haplogroup X represents an independent migration to the Americas from Asia or even Europe.  More specifically, it has been used to support a putative connection between the European Solutrean and the American Clovis lithic technologies.  This so called Solutrean hypothesis proposed the colonization of North America by Europeans through the North Atlantic, even though this interpretation is heavily debated.  All the five founding haplogroups have been shown to be present in Native Americas in pre-Columbian times.

Our results strongly support the hypothesis that haplogroup X, together with the other four main mtDNA haplogroups, was part of the gene pool of a single Native American founding population; therefore they do not support models that propose haplogroup-independent migrations, such as the migration from Europe posed by the Solutrean hypothesis.  We infer that haplogroup X experienced a more limited expansion in intensity than the former four haplogroups, and this is compatible with its current very limited distribution.

The fact that the five most common Native American mtDNA haplogroups display similar diversity patterns strongly indicates that they have not been much affected by natural selection.  Because human mtDNA does not recombine, directional selection upon a specific substitution would favor the haplotype in which this variant occurs, mimicking a demographic expansion.  It is very unlikely that in all haplogroups specific variants that would be favored by natural selection with similar intensity would have occurred by chance and at a similar time. Therefore, our results strongly indicate that the diversity pattern in Native American mtDNA results from a demographic expansion in the founding population in which all founding haplotypes were present.