Concurrent but distinctive migrations into the Americas

As usual with papers such as this, it is important to note that their age estimates depend on (i) the assumption that coalescence ages of lineages correspond to migration events (in general, they do not), and (ii) the mutation rate used to estimate such ages, which may be time-dependent. In this paper:

We propose here for the first time a new mutation rate taking into account the previous estimates reported by Mishmar [35] for all coding-region base substitutions and by Kivisild [36] for only synonymous transitions. With three decimal digits used throughout, the rounded values were 5140 years per coding-region substitution and 6760 years per synonymous transition, respectively. The rho estimated (average distance of the haplotypes of a clade from the respective root) human coalescence times are then 202 kya according to Mishmar et al. [35] and 160 kya according to Kivisild et al.

The geographic patterns (Pacific coast vs. inland route) seem to be more robust than the conclusions about the supposed ages of the migrations and their temporal concurrency.

The ScienceNews reporting has some contrary views. Some related posts:

1. Language spread rates in the Americas
2. Earliest human occupation of southern South America
3. Three-stage colonization model for Americas reconsidered
4. Craniofacial shape variation and Native American origins

Current Biology doi:10.1016/j.cub.2008.11.058

Distinctive Paleo-Indian Migration Routes from Beringia Marked by Two Rare mtDNA Haplogroups

Ugo A. Perego et al.

Summary

Background

It is widely accepted that the ancestors of Native Americans arrived in the New World via Beringia approximately 10 to 30 thousand years ago (kya). However, the arrival time(s), number of expansion events, and migration routes into the Western Hemisphere remain controversial because linguistic, archaeological, and genetic evidence have not yet provided coherent answers. Notably, most of the genetic evidence has been acquired from the analysis of the common pan-American mitochondrial DNA (mtDNA) haplogroups. In this study, we have instead identified and analyzed mtDNAs belonging to two rare Native American haplogroups named D4h3 and X2a.

Results

Phylogeographic analyses at the highest level of molecular resolution (69 entire mitochondrial genomes) reveal that two almost concomitant paths of migration from Beringia led to the Paleo-Indian dispersal approximately 15–17 kya. Haplogroup D4h3 spread into the Americas along the Pacific coast, whereas X2a entered through the ice-free corridor between the Laurentide and Cordilleran ice sheets. The examination of an additional 276 entire mtDNA sequences provides similar entry times for all common Native American haplogroups, thus indicating at least a dual origin for Paleo-Indians.

Conclusions

A dual origin for the first Americans is a striking novelty from the genetic point of view, and it makes plausible a scenario positing that within a rather short period of time, there may have been several entries into the Americas from a dynamically changing Beringian source. Moreover, this implies that most probably more than one language family was carried along with the Paleo-Indians.

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