Growing up in Israel, Gili Greenbaum would tour local caves once inhabited by Neanderthals and wonder with others why our distant cousins abruptly disappeared about 40,000 years ago. Now a Stanford scientist, Greenbaum believes there is an answer.
In a new study published in the journal by Nature Communications Greenbaum and his colleagues suggest that sophisticated patterns of disease transmission may explain not only how modern humans have managed to kill Neanderthals in Europe and Asia, but only a few for thousands of years, but also, perhaps more puzzling, why the end didn't come sooner.
"Our studies show that diseases may have played a more important role in the disappearance of Neanderthals than previously thought. They can even be the main reason why modern humans are the only human group left on the planet, "said Greenbaum, who was the first author of the research and doctorate at Stanford's Biology Division.
Archaeological evidence indicates that the initial encounter between the Eurasian non-Neanderthals and a new new human species that recently disappeared outside Africa ̵
Contributors demonstrated how unique ailments inflicted on Neanderthals and modern humans could create an invisible disease barrier that would discourage enemy territory attacks. In this narrow contact area, which was centered in Lebanon, where the first contact took place, Neanderthals and modern humans co-exist in an imbalanced equilibrium that has lasted for tens of thousands of years.
Ironically, what could have broken the impasse and ultimately allowed our ancestors to replace Neanderthals was to bring our two species closer by crossing them. The hybrid humans born of these unions may have carried immune-related genes of both species that would slowly spread through modern human and Neanderthal populations.
As these protective genes spread, the severity of the disease or the effects of infection in both groups gradually increased. Eventually it reached a tipping point when modern humans gained enough immunity to enter beyond the Levant and deeper into the Neanderthal Territory with little health implications.
At this point, other advantages that modern humans may have over Neanderthals – such as lethal weapons or more sophisticated social structures – could gain more importance. "Once the threshold is crossed, the severity of the disease no longer plays a role and other factors can begin to emerge," Greenbaum said.
To find out why modern humans are replacing Neanderthals, and vice versa, researchers are modeling what would happen if a set of tropical diseases that our ancestors were dead or more numerous than those transmitted by Neanderthals .
"The hypothesis is that the severity of the disease of the tropics was greater than that of the disease in the temperate regions. The asymmetry in the severity of the disease in the contact zone may be favorable to modern humans who have arrived there from the tropics." , says study co-author Noah Rosenberg, a professor at Stanford Population Genetics and Society at the School of Humanities and Sciences.
According to the models, even small differences in the severity of the disease between the two groups would initially increase over time, ultimately favoring our ancestors. "It may be that at a time when modern humans are almost completely exempt from the extra burden of Neanderthal disease, Neanderthals are still very vulnerable to modern human disease," Greenbaum said. "Moreover, as modern humans expand deeper into Eurasia, they would encounter Neanderthal populations that did not receive any protective immune genes through hybridization."
Researchers note that the scenario they propose is similar to that which occurred when Europeans arrived in the 15th and 16th centuries and depopulated indigenous populations with their more potent diseases.
If this new theory about the death of Neanderthals is correct, then supporting evidence can be found in the archaeological record. "For example, we predict that the Neanderthal and modern human density of the population in Lebanon will be lower than in the past when compared to other regions," said Greenbaum.
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Gili Greenbaum et al. Disease transmission and introgression can explain the long-term contact zone of modern humans and Neanderthals, Nature Communications (2019). DOI: 10.1038 / s41467-019-12862-7
Scientists link the disappearance of Neanderthals with human diseases (2019, November 7)
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