Disease and introgression explain the long-lasting contact zone of Modern Humans and Neanderthals and its eventual destabilization

Neanderthals and modern humans both occupied the Levant for tens of thousands of years prior to modern humans spread into the rest of Eurasia and their replacement of the Neanderthals. That the inter-species boundary remained geographically localized for so long is a puzzle, particularly in light of the rapidity of its subsequent movement. We propose that disease dynamics can explain the localization and persistence of the inter-species boundary; in this view, each species carried pathogens to which it was largely immune and tolerant, but that could spread to the other, vulnerable, species, inducing a significant disease burden. Epidemics and endemic diseases along the interspecies boundary would have mitigated against bands of one species migrating into regions dominated by the other species. Together with decreased population densities and limited inter-group interactions due to disease burden, this mechanism could have resulted in a fixed and narrow contact-zone. We further propose, and support with results from dynamical systems models, that genetic introgression, including transmission of alleles related to the immune system, would have gradually allowed the two species to overcome this barrier to pervasive inter-species interaction, leading to the eventual release of the inter-species boundary from its geographic localization. Asymmetries between the two species in the initial size of their associated \"pathogen package\" could have created feedback loops that influenced the rates at which immunity to and tolerance of the novel pathogens were acquired. These asymmetries could have allowed modern humans to overcome the disease burden earlier than Neanderthals, giving them a significant advantage in their subsequent spread into Eurasia, particularly upon interaction with Neanderthal populations that had previously been far from the original contact zone in the Levant.