Neanderthal Demise

A. The geographic range of modern humans (orange) and Neanderthals (gray) were separated for several hundred thousand years. B. As early as 180-120 thousand years ago (KYA), Modern humans began migrating out of Africa into the Levant (purple). C. Around 40-50 KYA, modern humans began expanding further into Eurasia and replacing Neanderthals throughout Eurasia.

Greenbaum et al., 2019

Neanderthals (Homo neanderthalensis) and modern humans (Homo sapiens) coexisted in the Levant region of the eastern Mediterranean for a period of tens of thousands of years—until about 40,000 years ago, much earlier than the spread of modern humans throughout the rest of Eurasia. Scientists have recently built a model to investigate whether disease transmission, combined with human-Neanderthal hybridization, could explain why this interspecies boundary persisted for so long and what led to Neanderthal decline.

Postdoctoral researcher Gili Greenbaum of Stanford University and five collaborators at universities in the United States, South Africa, and Israel used disease ecology models—developed for interspecies diseases such as bovine tuberculosis, which can be transmitted between badgers and cows—to structure the disease dynamics for modern humans and Neanderthals over ecological and evolutionary time. Their models assumed that the two species carried different “pathogen packages” to which each had acquired tolerance, since the two species came to the Levant from different geographic regions—modern humans from Africa and Neanderthals from Eurasia.

During initial contact, the model predicted a long, stable phase in which each species was vulnerable to the infectious diseases caused by the other’s pathogens. This kept population sizes small and constant, their distinct disease burdens preventing each species from expanding into the region of the other.

Then, destabilizing evolutionary forces broke the stalemate. “We know that these two species interacted not just physically but also genetically,” said Greenbaum. Both species swapped genes as they interbred, and the model suggested that both species gained immunity from the interactions.

But why did modern humans supersede Neanderthals and not the other way around? Modern humans started off with more diseases, owing to their evolution in the more biodiverse tropics. The models showed that this exposure would result in modern humans acquiring immunity to Neanderthals’ diseases in less time than it would take for Neanderthals to overcome modern humans’ diseases. Analogous to the disease transmission from European colonists that decimated indigenous North Americans, this inequality in immunity led to the unequal expansion of modern humans into Neanderthal regions. (Nature Communications)