Over the past few years one of the major finds of ancient DNA is that human genetic patterns as a function of time often exhibit discontinuity. In plain language, the people who live in a given location are often unlikely to have descendants at that location 10,000 years down the line. This has resulted in an update to long held null and consensus models of of modern human dispersal across the world. To sum it up, that family of models tended to be predicated on a sequence of unidirectional migrations out of Africa in a step-wise fashion. This resulted in the stylized fact that genetic diversity decreased as a function of distance, with groups like Native Americans and Oceanians the most “steps” from Africans. Though all non-African populations are separated by the same number of generations from Africans, one result that would be implied and was seen in the data is that genetic distance from Africans of these populations was often higher than Eurasians, likely a function of their elevated drift (more drift means more divergence from ancestral shared allele frequencies). Once these regions were settled at a given time in the past genetic diversity so partitioned would slowly equilibrate through gene flow between adjacent demes.
Though this model captures some element of the truth, the reality of sharp local discontinuities in a given region is strongly indicative of the fact that there was no stable state achieved once the initial founders arrived. Geographical reality seems to dictate the sort of pattern of settlement outlined by the model of serial bottleneck Out-of-Africa model, but it seems likely that future population arrivals could be drawn from both closer to, and further out, from Africa. Second, it turns that most of the world’s populations are the product of relatively recent admixtures between very different ancestral lineages. Instead of overlaying a phylogenetic tree over a spatial landscape, one has to conceive of it as a reticulate network. This revised model is outlined in Joe Pickrell and David Reich’s Towards a new history and geography of human genes informed by ancient DNA. In place of diffusion and continuous genetic exchange between adjacent demes, the empirical data seems to point to a non-trivial proportion of “pulse admixtures.” That is, people who were very genetically different arrived, and mixed in with the local population, in a very short period. Sort of what happened in the New World with the arrival of Europeans.
But that’s all talk. A new paper in Molecular Biology & Evolution formally models these processes, Long distance dispersal shaped patterns of human genetic diversity in Eurasia (open access):
…However, it is likely that the Last Glacial Maximum (LGM) affected the demography and the range of many species, including our own. Moreover, long-distance dispersal (LDD) may have been an important component of human migrations, allowing fast colonization of new territories and preserving high levels of genetic diversity. Here, we use a high-quality microsatellite dataset genotyped in 22 populations to estimate the posterior probabilities of several scenarios for the settlement of the Old World by modern humans. We considered models ranging from a simple spatial expansion to others including LDD and a LGM-induced range contraction, as well as Neolithic demographic expansions. We find that scenarios with LDD are much better supported by data than models without LDD. Nevertheless, we show evidence that LDD events to empty habitats were strongly prevented during the settlement of Eurasia. This unexpected absence of LDD ahead of the colonization wave front could have been caused by an Allee effect, either due to intrinsic causes such as an inbreeding depression built during the expansion, or to extrinsic causes such as direct competition with archaic humans. Overall, our results suggest only a relatively limited effect of the LGM-contraction on current patterns of human diversity. This is in clear contrast with the major role of LDD migrations, which have potentially contributed to the intermingled genetic structure of Eurasian populations.
One of the things the authors found is that low population pairwise genetic distances across a wide range of human populations in Eurasia is probably due to LDD events homogenizing the landscape. Continuous gene flow between demes after the initial settlement Out-of-Africa would not have resulted in these patterns. Second, it seems reading the paper that the weak effect of the LGM population reductions on genetic diversity are partly a function of this mixing across long distances. Finally, it is notable in within Eurasia at least (they suggest that the Americans and Oceania may not fit this pattern) a sort of diffusion/wave of advance model does hold for the initial arrival of modern humans in Eurasia. They posit that this might be because archaic populations prevented long distance movements, or, that population fitness became too low when the bands were too small, the reference to the allee effect. Additionally, they also note that the evidence in Europe suggests both replacement with minimal admixture, and then later admixture with the local substrate.
But the details are less important than the big picture. The authors note that there are aspects of the data (dozens of microsatellites) that leave something to be desired, but this is a first pass. At the top of this post you see Peter Turchin’s Ultrasociety. Though the authors don’t get into much specificity in the discussion, I think the solution to what’s going on, and how LDD seems prevalent when you have a populated landscape, is that cultural complexity resulted in sharply increased returns to the victors in inter-group competition. Though some of the dynamics date back to the Pleistocene, the re-patterning of the world with “LDD”, what I call “leapfrogging”, is probably most salient for Eurasia during the Holocene. And, as the story about the Yakutian horses implies, this is also relevant to many domestic lineages.