Wang et al., publishing in PLoS Genetics, looked at the genetic diversity in Native American populations from Canada all the way down into South America. They wanted to see whether the genetic diversity observed in Native peoples correlated in any way with geography.
If the genetic diversity did not correlate with geography this would suggest that Native people colonized the New world from the Old world at different times in different waves of expansion. If, on the other hand, the genetic diversity correlated well with geography (on a North to South axis of decreasing diversity) this would suggest that a founder population crossed the Bering straits and populated the New World.
When they surveyed the genetics of Native peoples they found that the second situation was the case. Here is Figure 3 of the paper:
The top part shows the heterozygosity as a function of distance from Africa. Heterozygosity is the proportion of the population who possess to different alleles (versions of a gene) at a particular locus. If a population is relatively uniform and inbred, we would expect the heterozygosity to be low. If a population is not uniform or if it is old and had time to accumulate diversity from a founder population, we would expect heterozygosity to be high. The important thing to recognize is that this graph is shifted so that the distance is calculated from the top of the Bering strait. The correlation between lower diversity is maximal if you do this, strongly suggestly that the founder population originated there as opposed to other places.
The lower part depicts the relative heterozygosity of regions imposed on the map. Yellow equals low heterozygosity. Red equals high heterozygosity.
An important confound to rule out is whether the observed differences in heterozygosity are because some Native population are more assimilated into the majority culture than others. This could cause the illusion of differences in heterozygosity where there are really only differences in interbreeding. However, the authors address this confound by showing that if you throw out the more assimilated groups from the analysis the correlation still persists.
Here is their summary of the importance of this work:
The genomic continent-wide patterns observed here can be explained most parsimoniously by a single main colonization event, as proposed by some interpretations of archaeological, mitochondrial, and Y-chromosomal data. In this view, at each step in the migration, a subset of the population splitting off from a parental group moves deeper into the Americas, taking with it a subset of the genetic variation present in the parental population. This scenario would be expected to produce a set of low-diversity populations with distinctive patterns of variation at the far terminus of the migration, such as those we and others observe in the Ache and Surui populations. It can also explain the gradient of Siberian similarity, and the continent-wide distribution of D9S1120 allele 275. Alternatively, similar patterns could result from gene flow across the Bering Strait in the last few thousand years, together with continual interactions between neighbors on both sides of the Bering Strait. It is also possible to envision a series of prehistoric migrations, possibly from the same source population, with the more recent descendants gradually diffusing into pre-existing Native American populations. (Citations removed. Emphasis mine.)
I am just fascinated by the wonderful collusion between anthropology and genetics in recent years. For example, by showing that the gradient in genetic diversity is also West-East as well as North-South, the authors argue that the primary colonization route of Native peoples was through coastal expansion down the Pacific and then movement inland. That is just awesome. How would you even find evidence for that otherwise?
This like most things in science is not proof, but it is bringing evidence to bear on previously difficult questions of human origins.
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It looks to me like there are no samples from the USA. Is there some reason for this? It would seem to me to be a substantial omission.
Actually I think you are right, tenine. They didn't include US populations. I wonder why...
Maybe it was because US Native populations have been relocated or are much more assimilated than in other countries. I don't know enough about the history of Native cultures in the Americas besides the US to make a comparison.
I thought the linguistic evidence pointed to 3 separate migrations. Does this conflict with that, or is there enough wiggle room in the data?
Linguistic evidence supports at least three migrations, however Paleoindian migration data is still currently debated within anthropology. There used to be a hard and fast line, called the Clovis line, on the populating of the New World. Prior studies gave the earliest North American migration dates closer to 12K years ago and used Clovis points as lithic stone tool proof, thinking that there would have been no one else here earlier than those dates. More recent studies have used inland sites (those coastal sites are likely underwater) with stone tool evidence and good dates. These studies have found much more information from interior sites (Meadowcroft Rockshelter in PA, Topper site in S. Carolina) from pre-Clovis dates that indicate a peopling of the New World that likely earlier than the Bering Straight data through the center of Canada and down. Most anthropologists these days believe that coastal migration was quite likely and that people may have been in North and South America as long as 30K years ago.
As for not using US populations, I'd guess that the relationship between the Canadian First Nations and these scientists would be better rather than US Native Americans being more "assimilated". It may also be the North American populations were used for some reason (i.e. earliest linguistically, were willing to give genetic samples).
I'd like to know why they published where they published.
The linguistic evidence never suggested three migrations. It has suggested three different groupings. The link between a "group" and "migration" is a huge leap that early researchers made uncritically but that has not foundation. It requires the assumption that a) languages can not emerge, evolve, or break off in the New World at the "language group" (the term we are using here) level, and that people can only move in one direction (from OW to NW).