Great attention and heated debate have been directed towards the causes of the Neanderthals’ rapid replacement across Europe by Modern humans between 50,000-40,000 years ago. Less attention has been directed towards an even more intriguing question: given that replacement across Europe could be so swift, what can explain the tens of thousands of years during which the two species’ front of interaction was geographically localized to the Levant, and the sudden breakdown of this front? We propose that disease dynamics can explain the persistence of the interspecies 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 inter-species 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 that genetic introgression, including transmission of alleles related to the immune system, would have gradually allowed one or both species to overcome this barrier to pervasive inter-species interaction, leading to the eventual release of the inter-species boundary from its geographic localization. I will discuss these dynamics in the context of eco-cultural interactions between the two species and the transition from the Middle to Upper Paleolithic which occurred during this time period in western Eurasia.

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