Diseases influenced the extinction of Neanderthals more than previously thought.

They may even be the main reason why we are the only human group left on the planet.

Are Neanderthals and humans the same species?
Are Neanderthals and humans the same species?

How long did Neanderthals and humans coexist?

In a new study published in the journal Nature Communications, Gili Greenbaum, postdoctoral researcher in biology at Stanford University, and his colleagues propose that complex patterns of disease transmission can explain not only how modern humans were able to eliminate Neanderthals in Europe and Asia in just a few thousand years, but also, and perhaps more disconcertingly, why the end did not come sooner.

Archaeological evidence suggests that the initial encounter between Eurasian Neanderthals and a new emerging human species recently diverted from Africa, our ancestors, occurred more than 130,000 years ago in the eastern Mediterranean in a region known as the Levant.

However, it would be tens of thousands of years before the Neanderthals began to disappear and modern humans expanded beyond the Levant. Why did it take so long?

Using mathematical models of disease transmission and gene flow, Greenbaum and an international team of collaborators demonstrated how the unique diseases harbored by Neanderthals and modern humans could have created an invisible disease barrier that discouraged incursions into enemy territory.

Within this narrow contact zone, which focused on the Levant where first contact took place, Neanderthals and modern humans coexisted in an uncomfortable equilibrium that lasted for tens of millennia.

Diseases influenced the extinction of Neanderthals
Diseases influenced the extinction of Neanderthals

Are Neanderthals and humans the same species?

Ironically, what may have broken the stalemate and finally allowed our ancestors to overwhelm the Neanderthals was the union of our two species through crossbreeding. Hybrid humans born of these unions may have carried genes related to the immune systems of both species, which would have spread slowly through modern human and Neanderthal populations.

As these protective genes spread, the burden of disease or the consequences of infection within the two groups gradually rise. Eventually, a turning point was reached when modern humans acquired enough immunity to venture beyond the Levant and into the Neanderthal territory with few health consequences.

At this point, other advantages that modern humans may have had over Neanderthals, such as more deadly weapons or more sophisticated social structures, may have become more important. "Once a certain threshold is crossed, the burden of disease no longer plays a role, and other factors can come into play," Greenbaum said in a statement.