|Neanderthals lived alongside Homo sapiens in an area known as the Levant. Thousands of years passed and the Neanderthals began disappearing—all because of human diseases / Photo by: Procy via Shutterstock|
Neanderthals lived alongside Homo sapiens in an area known as the Levant. Thousands of years passed and the Neanderthals began disappearing—all because of human diseases.
In a study from Stanford University, scientists propose that deadly diseases carried by the modern human ancestors led to the extinction of Eurasian Neanderthals.
"Our research suggests that diseases may have played a more important role in the extinction of the Neanderthals than previously thought," said Gili Greenbaum, the first author of the study. "They may even be the main reason why modern humans are now the only human group left on the planet."
A Disease Barrier
Neanderthals are the modern human's closest ancient relatives before they mysteriously died out about 50,000 years ago. For hundreds of millennia, the sub-species lived with early humans in Africa before migrating to Europe some 500,000 years ago. Humans taking the same journey 100,000 years ago joined their ancient cousins and settled in the Levant.
The Levant is a historical geographical term referring to the area in the Eastern Mediterranean. During the tens of thousands of years living alongside each other, Neanderthals and Homo sapiens were divided by a "disease barrier" that prevented them from crossing over each other's territories.
That invisible disease barrier was later broken down due to interbreeding between the two species, the Daily Mail reports. It adds that the hybrid offsprings of this interbreeding may have carried immune-related genes from both the Neanderthals and Homo sapiens that slowly spread through the populations of both species.
The spread of these protective genes eased the burden of diseases in the two groups and eventually reached a tipping point; one where modern humans ruled over the Neanderthals. Homo sapiens were able to acquire enough immunity to allow them to venture beyond the Levant and into the Neanderthal's territory with lower risks to their health.
It was at this point that other advantages that modern humans may have had over their extinct ancestral cousin (i.e. deadlier weapons or more complex social structures) could have been used, according to the Daily Mail.
"Once a certain threshold is crossed, disease burden no longer plays a role, and other factors can kick in," Greenbaum said.
|For hundreds of millennia, the sub-species lived with early humans in Africa before migrating to Europe some 500,000 years ago. Humans taking the same journey 100,000 years ago joined their ancient cousins and settled in the Levant / Photo by: Angela Meier via Shutterstock|
Differences in Disease Burden
After coming up with a possible explanation for the Neanderthals' extinction, the researchers wanted to understand how modern humans survived. They did so by modeling the possible outcome if the suite of tropical diseases the Homo sapiens carried were deadlier or more numerous compared to those carried by the Neanderthals.
The hypothesis goes like this: The disease burden that originated from the tropics was greater than the disease burden in temperate regions, according to study co-author Noah Rosenberg.
"An asymmetry of disease burden in the contact zone might have favored modern humans, who arrived there from the tropics," said Rosenberg, who is also a professor of Population Genetics and Society at Stanford's School of Humanities and Sciences.
Based on their models, the researchers found that even small differences in disease burden between the two species at the outset were likely to grow over time and eventually led to giving Homo sapiens the upper hand.
Greenbaum explained that by the time that modern humans were nearly immune to the added risk of Neanderthal diseases, the extinct species may have remained vulnerable to diseases carried out by Homo sapiens.
"Moreover, as modern humans expanded deeper into Eurasia, they would have encountered Neanderthal populations that did not receive any protective immune genes via hybridization," the author added.
Not the First One
This scenario is similar to what happened when Europeans arrived in the Americas in the 15th and 16thh centuries—a point in history when Europeans eradicated indigenous populations with their more potent diseases, the researchers noted. They added that if their theory is correct, then the evidence to support it may be found in the archeological record.
Greenbaum and his colleagues predict that the densities of the Neanderthal and modern human populations in the Levant during the time period when they coexisted "will be lower relative to what they were before and relative to other regions."
The results of this study are similar to the findings of a 2016 study that concluded humans migrating out of Africa killed off Neanderthals with tropical diseases. Published in the American Journal Physical Anthropology, the research stated that exposure to new tropical pathogens may have been catastrophic for the ancient human species.
"As we now know that humans bred with Neanderthals, and we all carry 2% to 5% of Neanderthal DNA as a result, it makes sense to assume that, along with bodily fluids, humans and Neanderthals transferred diseases," said Charlotte Houldcroft, lead author of the 2016 study.
While infectious diseases may have played a major role in the extinction of Neanderthals, Houldcroft noted that other factors may have also contributed to the wipeout, CNN reports.
"I don't think any single factor was solely responsible, and we may never know which theory is correct, although we can continue to look for more evidence and try to test different theories."