Two polarising models, Recent African Origin (RAO) and Multiregional Evolution (MRE), have dominated modern human origins debates since the 1980s. A recent paper by Holliday, Gautney and Friedl published in Current Anthropology explores how the successful sequencing of the Neanderthal genome, indicating gene flow between Neanderthals and Modern Humans, influences the feasibility of these two extreme viewpoints.
RAO stipulates that modern humans were reproductively isolated from Neanderthals, in the sense that they could not produce fertile offspring. Conversely, significant gene flow between geographic variants is inherent to MRE, preventing populations to become reproductively isolated. Two intermediate models also exist: Replacement with Hybridization (RWH) and Assimilation (AM). RWH is close to the Out of Africa models, with only a small contribution of non-African local archaic humans to modern humans, while AM argues for a more substantial introgression. The genetic data clearly indicates that Neanderthals and moderns successfully interbred, disproving a strict RAO, but the authors caution to therefore see the other extreme, MRE, as the most likely model.
Subsequently, the paper explores Pleistocene human population size and density based on estimates of habitable land area and comparisons with mammalian carnivores. Although acknowledged that this is a very difficult exercise, a density of 0.15 humans per 100km² is proposed. This number is so low that the authors see it as an argument against global gene flow and MRE. Estimates then tackle issues of reproductive isolation and evolutionary time by exploring patterns for other mammals. It is concluded that 2 million years seem to be needed for diverged mammals to become reproductively isolated, a time scale far beyond splits within Homo. The authors rather explain the seemingly shared evolutionary trends across Homo as resulting from parallelophyly, relating to the general presence of very similar genotypes among closely related species.
It is concluded that with genetic evidence for the survival of Neanderthal DNA in living populations, and with the unlikelihood of consistent global gene flow, scenarios incorporating a degree of hybridization and assimilation are most likely, with current evidence making it difficult to favour one over the other. As typical to the Current Anthropology style, the paper then provides comments by 8 specialists in the field, providing further interesting opinions, ideas and suggestions. Striking is the dichotomy surrounding the importance of the 1-4% of Neanderthal DNA in present-day non-Africans, with some researchers stressing this number is too low to indicate assimilation, and others stressing we will never know the exact degree, timing and location of the interbreeding and assimilation provides the more complete working model.
Full reference: Trenton W. Holliday, Joanna R. Gautney, and Lukáš Friedl. 2014. Right for the Wrong Reasons: Reflections on Modern Human Origins in the Post-Neanderthal Genome Era. Current Anthropology, Vol. 55, No. 6, pp. 696-724. DOI: 10.1086/679068