Gili Greenbaum grew up in Israel and passed through local caves where Neanderthals used to live. He wondered, among other things, why our distant cousins had suddenly disappeared about 40,000 years ago. As a Stanford scientist, Greenbaum believes 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 the modern human Neanderthals in Europe and Asia in just one Second could extinguish A few thousand years, but perhaps more mysteriously, why the end did not come earlier.
"Our research suggests that diseases played a more important role in the extinction of Neanderthals than previously thought, and could even be the main reason why modern humans today are the only human group on the planet," said Greenbaum the first author of the study and postdoctoral fellow in Stanford's Department of Biology.
The slow killing
Archaeological evidence suggests that the first encounter between Eurasian Neanderthals and a new human species that has recently strayed 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 by these associations could carry immunocompromised genes of both species that would have slowly spread to the modern human and Neanderthal populations.
With the spread of these protective genes, the burden of disease or the consequences of infection within the two groups increases gradually. Eventually, a turning point was reached when modern man gained sufficient immunity to venture beyond the Levant with little health consequences and deeper into the Neanderthal area.
At this point, other benefits that modern humans had over Neanderthals, such as more lethal weapons or more complex social structures, could have gained in importance. "Once a certain threshold is exceeded, the disease burden no longer plays a role and other factors can occur," Greenbaum said.
To understand why modern humans did not replace Neanderthals and humans, the researchers modeled what would happen if the range of tropical diseases hosted by our ancestors were more deadly or more numerous than that of Neanderthals.
"The hypothesis is that the disease burden of the tropics would be greater than that of the tropical disease burden in temperate regions." Asymmetry in disease burden in the contact zone may have favored modern humans who came there from the tropics, "said study co-author Noah Rosenberg Professor of Population Genetics and Society at the Stanford School of Humanities and Science.
According to the models, over time, small differences in disease burden between the two groups would increase, ultimately giving our ancestors the edge. "It could be that by the time modern man was almost completely freed from the added burden of Neanderthals, Neanderthals were still very vulnerable to modern human disease," Greenbaum said. "As modern man expanded deeper into Eurasia, he would also have encountered Neanderthals who did not receive protective immunogens through hybridization." America in the 15th and 16th centuries and decimated indigenous populations with their more severe 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.
How differences in the genetic "instructions for use" between humans and Neanderthals influenced properties
Gili Greenbaum et al. The transmission and invasion of diseases can explain the long-term contact zone of modern humans and Neanderthals, Nature Communications (2019). DOI: 10.1038 / s41467-019-12862-7
Scientists associate the extinction of Neanderthals with human disease (2019, 7 November)
retrieved on November 7, 2019
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