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Peopling of the Americas as inferred through the analysis of mitochondrial DNA

Néstor O. Bianchi; Graciela Bailliet; Claudio M. Bravi

Instituto Multidisciplinario de Biología Celular (IMBICE), Calle 526 e/10 y 11, C.C. 403, 1900 La Plata, Argentina. Phone: (54 21) 25 3320/27 0652. Fax: (54 21) 25 3320. E-mail: bianchi@nbianc.ba.ar. Send correspondence to N.O.B.

ABSTRACT

MtDNA has been extensively employed to trace the origins and migration patterns of Amerindians. The advantage of mtDNA over nuclear DNA is that it accumulates base changes at an average rate 5-10 times faster than single-copy nuclear DNA does, which makes it suitable for the analysis of DNA differences among human populations. Moreover, its maternal inheritance and lack of recombination allow determination of phylogenetic divergence among lineages, without the ambiguities caused by the meiotic shuffling and mixing undergone by nuclear genes.
Thus far, the most useful mtDNA traits for Amerindian population studies have been the polymorphism of restriction sites, the variation in length of a region V non-coding segment containing a short repeat, and the base substitutions occurring in the D-loop region.
By analyzing mtDNA polymorphisms, several groups came to the conclusion that Amerindians could be clustered into four founder mitochondria] haplotypes. We reanalyzed the published data and studied a total of 673 Amerindians belonging to 23 different South American tribes. Our results showed that the four haplotypes proposed can be subdivided into several subsets to give rise to no less than 13 possible founder haplotypes. The number of founder Amerindian haplotypes is a problem at the center of an unsolved dispute. Some investigators support the idea that the colonization from Asia into the American continent was accompanied by a severe bottleneck that markedly restricted the number of maternal lineages entering the New World. Other groups hold an opposite view. Our findings of at least 13 founder haplotypes in Amerindians lend support to the latter position.
Estimations of the earliest date of colonization of the American continent are based on the construction of parsimonious trees. However, the trees so far reported have too many taxa and relatively too few phylogenetically informative sites to allow complete resolution of the phylogeny by parsimony. Additional uncertainties result from the inaccuracies in the estimated rate of mtDNA mutation used to calibrate the trees which varies from 2.2%/MYR (million years} to 33%/MYR according to different authors. Due to the above difficulties, the proposed dates of the New World colonization range between 14,000 and 29,000 years before present.
Studies of mtDNA have provided interesting, but preliminary, responses to some of the questions regarding the origin and evolution of Amerindians. Further clarification of these questions will require an understanding of the exact rates of mutation in different regions of the mitochondrial molecule, and to bear in mind that the answering of anthropological questions will always require a multidisciplinary approach in which the contribution of mtDNA analysis will be equivalent to the contribution provided by other methodologies.

Keywords: mitochondrial DNA.

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