Fig. 1.

Schematic representation of interaction between modern humans and Neanderthals. The geographic range of modern humans appears in orange, that of Neanderthals is in blue, and the contact zone in the Levant is in purple. a Neanderthals and Moderns were separated for several hundred thousand years, with Moderns co-evolving with tropical pathogens in Africa and Neanderthals co-evolving with temperate pathogens in Eurasia. b As early as 180–120 kya, Moderns began migrating out of Africa into the Levant^4–6. Their range remained restricted to this region for tens of thousands of years, during which they interacted intermittently with Neanderthals^4,7–9. We propose that during this period, each species was exposed to novel pathogen packages carried by the other species and experienced disease burden. Note that the contact zone depicted here may have been larger, possibly including regions in the Arabian Peninsula³⁶. c Around 45–50 kya, the inter-species dynamics destabilized and Moderns began expanding further into Eurasia. Within several thousand years, Moderns replaced Neanderthals throughout Eurasia^4,5,7,13,14