Maanasa Raghavan, Pontus Skoglund et al.
There has been a lot of discussion on this recently announced paper. First, I will say that I think the title is a little unfortunate and I am surprised that Nature let this one get by them. We don't really know how many migrations into the Americans there were. We also don't have a good understanding of back migrations from Beringia into Eurasia. Many hunter gatherers were migratory and highly mobile groups, so I really disagree with the "dual ancestry of Native Americans" phrase in the title.
Beringia was a huge area, now submerged, 20,000 thousand years ago. It would have supported animal and human diversity. The paper I posted a few days ago on The Black Dog at the River of Tears myth well illustrates that migrations in Siberia, Beringia and into the Americans were comprised of people of different backgrounds and cultures.
That being said, I think the result is interesting in that it shows that people carrying both Y-chromosome R and mtDNA U were present in Beringia during the last Ice Age. It's likely that the boy from which the main sample was taken, had freckles.
The link with this sample and paleo Amerindian populations is also interesting, but it should be said that many more ancient DNA samples of Ameridian, Beringian, and Eurasian populations would be needed to understand the timing and dynamic of these population movements.
One further effect of having only one reliable ancient DNA sample is that the sample in this paper is likely subject to genotyping error. That's according to Joe Pickrell who developed the software package Treemix, as shown in this Twitter exchange:
The comment refers to the long arm of the Siberian sample in the Treemix diagram in the paper.
When I hear geneticists who are expert in the field discussing the source of error in a paper, and they are clearly unsure of the source of error, it's a concern that the conclusions of the paper title are overstated.
There is probably additional error because the sample is old.
I don't have much to say except that I wish the genetics community would do a more considered job conveying the limitations of these results to the curious public.