A study authored by researchers from Stanford University, UC Berkeley and the Hebrew University of Jerusalem shows that the spread of infectious diseases may help explain the relationship between humans and Neanderthals.

Many theories have been proposed for the extinction of Neanderthals, including the cognitive superiority of humans, human technological advances and differences in adaptation to climates, according to Gili Greenbaum, a postdoctoral researcher at Stanford University and the first author of the study.

Greenbaum said, however, that theories less frequently consider the “small contact zone” that precedes the replacement of Neanderthals by humans.

“No good theory has been suggested for explaining both a persistence of a stable narrow contact zone and a rapid expansion that follows,” Greenbaum said in an email.

This “contact zone” between humans and Neanderthals existed in the Levant region of the Middle East for tens of thousands of years, before the Neanderthal species died out and humans spread from Africa into Eurasia. The study proposes that this period of coexistence can be explained by the different pathogens and infectious diseases common in human and Neanderthal populations.

The study used mathematical models to investigate the way diseases acted on populations under different conditions, according to Oren Kolodny, a study researcher and postdoctoral student at Stanford.

“I was surprised by the extent to which these different models and variants aligned in their qualitative predictions,” Kolodny said in an email.

While humans evolved in more diverse climates in Africa, Neanderthals evolved in the more temperate and “less diverse” climates of Europe, according to Wayne Getz, a UC Berkeley professor and co-author of the study. This difference in climates could have resulted in humans having adapted to a more complex and lethal “pathogen package” than that of the Neanderthals.

“Neanderthals had less time to adapt to (their) new environment and co-evolve with parasites. The human genome might have been better adapted to their environment,” Getz said.

He added that these adaptations may have been advantageous to humans for fighting diseases when the two species came into contact.

When the researchers modeled this interaction between one population with a more lethal pathogen package and another with a less lethal one, they found that the populations could “coexist in equilibrium for a while followed by a collapse,” according to Getz.

Getz likened the scenario described in the study to the arrival of Europeans in the Americas during the 15th and 16th centuries. At this point, disease spread rapidly among Native Americans because of a foreign, and more potent, pathogen package, he added.

The study makes several predictions about human and Neanderthal population changes over time, Greenbaum said.

“These predictions could be investigated with archaeological studies, and perhaps also with genetic studies if we recover appropriate ancient genetic samples in the Levant,” he said in the email.