Gili Greenbaum grew up in Israel and passed through local caves that were once inhabited by Neanderthals. Greenbaum, a Stanford scientist, thinks he has an answer.
In a recent study published in the journal Nature Communications Greenbaum and his colleagues suggest that complex disease transmission patterns can not only explain how modern humans can be the Neanderthals in Europe and Asia in just a few thousand years, but also, perhaps more mysteriously, why the end did not come sooner.
"Our research suggests that diseases could have played a more important role in the disease than Neanderthal extinction as previously thought, and could even be the main reason why modern humans are today the only human group on the planet," said Greenbaum, who is the first author of the study and postdoctoral fellow at the Department of Biology at Stanford.
The slow killing
Archaeological evidence suggests that the first encounter between Eurasian Neanderthals and a novel human species that has recently moved from Africa ̵
But tens of thousands of years passed before Neanderthals disappeared and modern humans outgrew 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 barrier that deters hostile territory. Within this close contact zone, which was in the Levant where the first contact took place, Neanderthals and modern humans lived together in a restless equilibrium that took tens of thousands of years to displace Neanderthals, as our two species came together through intersection. The hybrid humans born of these associations could carry immunocompromised genes of both species, which would slowly spread in modern humans and Neanderthals.
With the spread of these protective genes, the burden of disease or the consequences of infection within the two groups increases gradually. A turning point was finally reached when modern man gained sufficient immunity to venture beyond the Levant with little health consequences and deeper into Neanderthal territory.
At this time, other benefits that modern humans had over Neanderthals, such as more deadly weapons or more complex social structures, could have gained more importance. "Once a certain threshold is exceeded, the burden of disease no longer matters and other factors can occur," said Greenbaum.
To understand why modern humans replaced Neanderthals, and not the other way around, the researchers modeled what would happen if the number of tropical diseases our ancestors were harboring were deadlier or more numerous than those of Neanderthals.
The hypothesis is that the disease burden of the tropics in temperate regions is greater than the disease burden in temperate regions. An asymmetry of disease burden in the contact zone may have favored modern humans who came there from the tropics, "said study co-author Noah Rosenberg, Stanford professor of population genetics and society at the School of Humanities and Sciences.
According to the models, at the beginning even small differences in the burden of disease between the two groups over time increase, which would ultimately give our ancestors the decisive advantage. "It could be that at the time when modern man was almost completely relieved of the added burden of Neanderthals "Neanderthals were still very vulnerable to modern human disease," said Greenbaum. "As modern humans expanded deeper into Eurasia, they would also have encountered Neanderthals who did not receive protective immunogens through hybridization." America in the 15th and 16th centuries Century and decimated indigenous people groups with their stronger diseases.
If this new theory of the downfall of Neanderthals is correct, then evidence can be found in the archaeological records. "For example, we predict that the population density of Neanderthals and modern humans in the Levant will be lower in the period they co-exist than before and compared to other regions," Greenbaum said.
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