A new article in Horizon, Ice-age Europeans roamed in small bands of fewer than 30, on brink of extinction (via Eurogenes), basically gives away the game in the headline. But please keep in mind my earlier post, as low effective population numbers may not accurately convey the actual census size over long periods of time. These results are not particularly surprising, as the ancient genomes we have from hunter-gatherers tend to indicate a very high level of inbreeding in comparison to modern populations. The main difference here is that it seems that they have more and more ancient genomes sampled from diverse locations to add confidence to the original conjecture:

Prof. Pinhasi’s team has found that the genomes sequenced from hunter-gatherers from Hungary and Switzerland between 14 000 to 7 500 years ago are very close to specimens from Denmark or Sweden from the same period. These findings suggest that genetic diversity between inhabitants of most of western and central Europe after the ice age was very limited, indicating a major demographic bottleneck triggered by human isolation and extinction during the ice age.

The term “very close” is vague. I’m sure he has some quantitative measure in mind (e.g., identity by descent blocks). It is probably not coincidence that you see the same dynamic among Neandertals. Those from Europe are surprisingly similar to those from the Altai. Why the homogeneity? Probably on the huge broad northern expanse of hominin habitation metapopulation dynamics characterized by extinction and resettlement from survivor lineages was very common. There is circumstantial evidence from wolves that the same happened to them. Why? This might simply be a biogeographic tendency among Palearctic species during the Pleistocene. The Ice Age was tough, and the glaciers were capricious, and the warming could be ephemeral (see the Younger Dryas).

Of course I’m going to put European in quotes in the title. Europeans, like modern day Puerto Ricans, are trihybrid. They only emerged in their current form in the past ~5,000 years or so. The north-south gradient of increased heterozygosity in Southern Europe may then be a function not of serial founder effects from the expansion of the Pleistocene refugia, but the higher fraction of hunter-gatherer ancestry in Northern Europeans, who exhibited decrease genetic diversity due to the bottlenecks.