Long distance dispersal of modern humans outside of Africa

Long distance dispersal (LDD) has long known to be an artifact of human migrations out of Africa. However, the effects of LDD on modern human diversity, and models of LDD in human colonization are yet to be characterized. Using an ABC (Approximate Bayesian Computation) framework, Alves et al. (2015) estimate probabilities of four plausible scenarios of migration of anatomically modern humans – (1) simple range expansion out of East Africa, with a “stepping-stone” model of migration between adjacent populations, (2) model (1), with range contraction due to the Last Glacial Maximum (LGM) in Europe and Asia, (3) model (1) with LDD events to previously unoccupied demes, and (4) comprising all three events. Their simulations utilized diversity data from across 50 microsatellite loci, bootstrapped from 87 filtered loci from Tishkoff et al. (2009) and Pemberton et al. (2009). Their analyses clearly rejected models without LDD, with strongest support for model 4, also confirmed through accuracy and goodness of fit estimations. Interestingly, their findings reveal greater support for LDD into previously occupied demes, than for previously unoccupied demes in Eurasia. Estimates of demographic parameters (ancestral and current population sizes, migration rates, growth rates) under the model with LDD were in agreement with previous estimates.

Posterior probability density distributions under the four models of human expansion out of Africa simulated using 1000 bootstrap datasets. Image courtesy: Figure 2 of Alves et al. (2015) http://dx.doi.org/10.1093/molbev/msv332

Posterior probability density distributions under the four models of human expansion out of Africa simulated using 1000 bootstrap datasets. The full model (4) indicates the highest density. Image courtesy: Figure 2 of Alves et al. (2015) http://dx.doi.org/10.1093/molbev/msv332

Outstanding questions that Alves et al. (2015) discuss on the basis of these findings include the effects of archaic human introgression into modern humans outside of Africa on demographic expansion, ascertainment bias while utilizing SNP, and LD data for similar studies of demography, and Neolithic population growth in tandem with LDD events to offer insights into currently observed genetic diversity.

…LDD events from the core to the front might have quickly restored diversity and reshuffled the genetic diversity of populations in Eurasia. These LDD events might also explain why the gene pool of many human populations shows signals currently interpreted as admixture events between isolated populations (Moorjani, et al. 2013; Patterson, et al. 2012; Pickrell, et al. 2014) that could just represent normal patterns having been built since the exit of modern humans from Africa.

Reference:

Alves, Isabel, et al. “Long distance dispersal shaped patterns of human genetic diversity in Eurasia.” Molecular Biology and Evolution (2015): msv332. DOI: http://dx.doi.org/10.1093/molbev/msv332

Tishkoff, Sarah A., et al. “The genetic structure and history of Africans and African Americans.” Science 324.5930 (2009): 1035-1044. DOI: http://dx.doi.org/10.1126/science.1172257

Pemberton, Trevor J., et al. “Sequence determinants of human microsatellite variability.” BMC genomics 10.1 (2009): 612. DOI: http://dx.doi.org/10.1186/1471-2164-10-612

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About Arun Sethuraman

I am a computational biologist, and I build statistical models and tools for population genetics. I am particularly interested in studying the dynamics of structured populations, genetic admixture, and ancestral demography.
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