A bit belatedly due to my relocation to Boston, I’d like to contribute a few observations regarding the phylogenetic position of the mtDNA sequence uncovered from a 4,000 year-old lump of hair in Greenland. Kambiz has already reviewed this Science article, hence the readers should have a good idea of what it took to obtain this valuable piece of ancient DNA.
Although Gilbert et al. (2008) claim that the ancestry of this sequence is unique and goes directly back to Siberian populations, I don’t think this statement is borne out by facts. First, they overlook the fact that the clade D2 is divided between Na-Dene (D2a), Aleuts (D2b) and Eskimos (D2c). The fact that D2a is found in Na-Dene populations all the way down to the Apaches (see Derbeneva et al. 2002. “Analysis of mitochondrial DNA diversity in the Aleuts of the Commander islands and its implications for the genetic history of Beringia.” Am J Hum Gen 71: 415-421) is something the article completely left out.
Second, their claim that “D2 derives from an Asian-specific Hg D4e and not from any of the 5 Native American founding haplogroups (Hgs A2, B2, C1, D1 and X)” is based on confusion. On the one hand, D2 was reported among such Mexican groups as the Tarahumara and Nahua, with no recent relationship to Beringia (Penaloza-Espinosa et al. 2007. “Characterization of mtDNA Haplogroups in 14 Mexican Indigenous Populations.” Hum Bio 79 (3): 313-320). On the other hand, the confusion regarding the distribution and phylogeny of D lineages in the Americas has recently been reinforced by Tamm et al. (Beringian Standstill and Spread of Native American Founders. PLoS One 9 (2007): 2), who label the oldest known D sequence from the On Your Knees Cave on Prince of Wales Island, Alaska (10,300 YBP), as D4h3 and claim that it’s identical to a range of Native American groups distributed all the way from Alaska to Tierra del Fuego (Chumash, Tarahumara, Mapuche, etc.).
Gilbert et al. (2008) openly consider D1 a subset of D4. However what the On Your Knees Cave actually demonstrated is the fact the assignment of some sequences to either Asian-specific D4 or to Amerindian-specific D1 can be problematic, for these two subclades may stand in a sister relationship to each other (Kemp et al. 2007. “Genetic Analysis of Early Holocene Skeletal Remains from Alaska and Its Implications for the Settlement of the Americas.” Am J Phys Anth 132: 617). That’s how one can explain the apparent confusion created by the Tamm paper and the Gilbert paper, in which a D sequence found all over America is considered to be Asian-specific, and the relatively autonomous D1 clade is declared a subclade of D4.
An analogy with another ancient DNA study of the sequences from several individuals at China Lake, BC, which recorded a “modal” M lineage in prehistoric American Indians (Malhi et al. 2007. “Mitochondrial haplogroup M discovered in prehistoric North Americans.” Journal of Archaeological Science 34, 4: 642-648), may point to the same situation with the On Your Knees sequence. It’s one of the “modal” D sequences that predates the fission into D1 and D4 lineages in Beringia. Consequently, what Gilbert et al (2008) actually found was a sequence that doesn’t fall outside of American Indian and Eskimo-Aleut mtDNA variation (all the D branches from D1 through D4 may have representatives in the Americas), but constitutes an interesting survival of an ancient Beringian population with links to modern Aleuts and Na-Dene.
Although Beringia appears to be a late Pleistocene/early Holocene refugium and a genetic melting pot, it’s still unclear whether there was an identifiable migration from Northeast Asia to America. It’s more likely that Beringian hunter-gatherers trickled into interior Siberia (the so-called “Paleoasiatic” peoples such as the Chukchi and the Koryaks, but also the Kets, are the modern examples of this back-migration) as well as along the coast into Greenland. The fact that Gilbert et al. reported only one single sequence makes any further generalizations about the genetic layers in Greenland premature, for any random act of gene flow, transcultural adoption, or slave trade could account for its presence in the territories historically occupied by the Eskimos.