Sriram Sankararaman, Nick Patterson, Heng Li, Svante Pääbo, David Reich
PLOS Genetics, October 2012
This paper evaluates various models of divergence between Africans, Europeans and Neandertals. Hybrid model (e) of the paper, shown in Figure 2, below, is most closely akin to the human version of the "refugia and range shifts model" of the Marbled White butterfly. In one possible scenario of this model, the first branch occurs when Africans diverge from Neandertals during the Holstein interglacial (Timeline of glaciation). Then early Scladina-like Neandertals admix back with Africans, possibly in Iberia or Morocco, 150 kya when the sea level is low enough to make it more likely. As the sea level rises, the Scladina Neandertal-Africans on the European side are cut off in an increasingly icy domain. The Scladina Neandertal-Africans in Morocco thrive. Their descendants marry Africans who have little or no Scladina ancestry. At about 50 kya, the Scladina Neandertals who are cutoff in Western Europe are replaced by the Vindija like Neandertals. Shortly thereafter, Africans with perhaps a remnant of Scladina Neandertal admixture arrive once again in Europe. Occasionally, they have children with the Vindija Neandertals. All of the Neandertals, Vindija in Europe and Scladina in Africa, are eventually absorbed into the dominant population of homo sapiens from Africa . . .
Figure 2. Classes of demographic models relating Africans (Y), Europeans (E), and Neandertals (N). a) Recent gene flow but no ancient structure. RGF I has no bottleneck in E. RGF II has a bottleneck in E after gene flow while RGF VI has a bottleneck in E before the gene flow. RGF IV and V have constant population sizes of Ne = 5000 and Ne = 50000 respectively. b) Ancient structure but no recent gene flow. AS I has a constant population size while AS II has a recent bottleneck in E. c) Neither ancient structure nor recent gene flow. NGF I has a constant population size while NGF II has a recent bottleneck in E. d),e) Ancient structure+Recent gene flow. HM IV consists of continuous migration in the Y-E ancestor and the Y-E-N ancestor while HM I consists of continuous migration only in the Y-E ancestor. HM II consist of a single admixture event in the ancestor of E while HM III also models a small population size in one of the admixing populations.
Further comments (November 8th): In reality, there is very strong evidence based on Y-chromosome and mtDNA, that for many if not most populations, Hybrid model (e) is incomplete. There are subsequent events, such as the migration of the carriers of the E y-chromosome into Europe, some time since the last ice age and before the Holocene Climatic Optimum. Carriers of the R-V88 y-chromosome and H1 mitochondrial DNA make their appearance in North Africa over the same time frame. Therefore, a hybrid model for the interaction of populations between Europe and Africa up to the Holocene Climatic Optimum demands allowance for at least four major admixing events: (1) Scladina-like Neandertals with Africans approximately 150kya, (2) Vindija-like Neandertals with Europeans in the window suggested in the above paper, (3) Europeans with North Africans during the last ice age, and (4) North Africans with Europeans during the last ice age. Further complicating the picture are the layers due to recent demographic events of the Holocene Climatic Optimum and the last several thousand years. This already complex model doesn't even begin to touch upon the migrations to Asia, Southwest Asia and the Americas. Drift, diffusion, isolation, migration and selection are always at work. It is therefore not surprising that finding the genetic signature to prove this complex model is very difficult.