Neanderthal Extinction Tied to Disorder Caused by Mating with Humans

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Neanderthal Extinction Tied to Disorder Caused by Mating with Humans

There has been much speculation about Neanderthal extinction and why they disappeared 40,000 years ago. The latest research suggests they lived side-b

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There has been much speculation about Neanderthal extinction and why they disappeared 40,000 years ago. The latest research suggests they lived side-by-side with modern humans (Homo sapiens) for up to 5,000 years before their final disappearance, with the two species of humans interbreeding quite frequently.

So, if Homo sapiens and Neanderthals were able to co-exist peacefully for so long, why are they gone while we’re still here?

According to  a new study  published in the journal  Plos One , it was getting along so well with modern humans that might have sealed the Neanderthals’ fate.

The latest study published in the Plos One journal found evidence of hemolytic disease of the fetus and newborns (HDFN) in Neanderthal DNA. And this blood disorder, which they got from breeding with modern humans, led to Neanderthal extinction. (OpenStax College /  CC BY 3.0 )

Neanderthal Extinction From Homo Sapiens Blood Condition

This ironic thesis emerged from  an important discovery  made by a team of genetic scientists from the French National Centre for Scientific Research (CNRS) and Aix-Marseille University in France. These scientists analyzed the blood types of three  Neanderthals, whose DNA had been successfully recovered and sequenced from fossilized bone samples. These Neanderthals did not live together but were born at different times and at different places.

Despite not being related to each other, the scientist found that these individuals carried a genetic variant in their blood that would have made them highly vulnerable to a blood  disorder known as “hemolytic disease of the fetus and newborn,” or HDFN. This condition develops in expectant mothers, and when it does it can be fatal to a fetus or newborn infant.

HDFN can develop when a mother and a father have incompatible blood types. When a woman with a negative Rh blood type (A-, B-, O-, or AB-) mates with a man who has a positive Rh blood type, this can cause problems if a child with a positive blood type is conceived. The mother’s immune system may interpret the fetus’s blood cells as foreign invaders and start attacking them. This can leave a fetus or newborn suffering from a severe and life-threatening form of amenia (meaning there are not enough red blood cells to supply oxygen to the body).

This doesn’t happen every time a man and women with positive and negative blood types mate. In modern humans, HDFN occurs in approximately three out of every 100,000 births. Shots are available that can help dramatically reduce the risk for expectant mothers and their babies, when it is known that parental blood types are incompatible.

HDFN can develop when a mother and a father have incompatible blood types. When a woman with a negative Rh blood type (A-, B-, O-, or AB-) mates with a man who has a positive Rh blood type, this can cause problems if a child with a positive blood type is conceived. And this apparently accelerated Neanderthal extinction. ( valterz / Adobe Stock)

But as this new study makes clear,  Neanderthal women  were at a much higher risk for this dangerous condition than people living on the earth today.

“The fact that these forms of genes were detected in individuals separated by 4,000km and 50,000 years suggest that this genetic peculiarity — and the risk of [an] anemic fetus — would have been quite common amongst Neanderthals,” explained study lead author Stephane Mazieres , a population geneticist affiliated with the CNRS.

This genetic vulnerability did not enter the Neanderthal gene pool because they interbred with humans. But the scientists say a Rh-negative Neanderthal mother would be more likely to have a strong HDFN reaction if she mated with a Rh-positive modern human, rather than with a Neanderthal male. It is this magnification of risk, plus the commonness of the genetic vulnerability in Neanderthals, that suggests interbreeding between Neanderthals and Homo sapiens could have caused dramatic increases in the incidence of HDFN. 

One of the major problems with HDFN is that it is far more likely to occur in births after the first birth. Immune systems react more strongly to “invaders” they recognize, meaning a Rh-negative Neanderthal woman trying to give birth to multiple Rh-positive children would be unlikely to have much success.

This might not have mattered, if Neanderthal populations had remained high and interbreeding with modern humans was a relatively rare event. But if Neanderthal populations were on the decline for other reasons, their strategies for self-preservation might have included mating more often with humans. Such a strategy might have doomed them rather than saved them, if giving birth became increasingly hard because of rapidly-spreading HDFN. 

“These elements could have contributed to weakening the descendants to the point of leading to their demise, especially combined with the competition with  Homo sapiens  for the same ecological niche,” the French scientists wrote in their  Plos One  paper.

A population crash driven by this dynamic would have happened gradually, which is  how Neanderthals seemed to have disappeared .

Recent studies of the ancient climate have shown that the Northern Hemisphere experienced two long and cold dry spells around the time that Neanderthal numbers began to decline. These cold droughts transformed fertile woodlands and rich grasslands into arid landscapes and increased competition to obtain enough food, which modern humans were better at. ( allvision / Adobe Stock)

Unraveling the Complex Story of Neanderthal Extinction

Archaeologists, anthropologists, and evolutionary biologists have long wondered about the true causes of the Neanderthal extinction. Most believe a combination of factors likely contributed to their demise, shifting survival advantages away from Neanderthals and toward Homo sapiens.

Most scientists think the whole process started with  climate change . Recent studies of the ancient climate have shown that the Northern Hemisphere experienced two long and cold dry spells around the time that Neanderthal numbers began to decline. These each lasted several hundred years and occurred between 44,000 and 40,000 years ago.

These cold droughts transformed fertile woodlands and rich grasslands into arid steppes, creating conditions that made food and water scarce and put Neanderthal population groups under great stress.

Neanderthals hunting. Though they were good hunters when big game became less available as a result of climate change they increasingly had to compete with modern humans who were more flexible and better hunters under tough conditions. ( nicolasprimola / Adobe Stock)

As competition for hard-to-find resources increased, modern humans seemed to have gained a significant edge. This may have been due to certain intellectual or physical capacities they possessed that Neanderthals did not. It may also have been a matter of luck, as perhaps modern humans had claimed more areas that remained fertile than their Neanderthal cousins. 

It’s also possible that  vulnerability to disease  played a role in the final outcome. Neanderthals may have had less resistance to diseases  that started in modern humans  than was true in the reverse case. If Neanderthals were intentionally seeking out more contact with Homo sapiens as a way to gain access to increasingly scarce resources, these closer contacts could have caused deadly epidemics among Neanderthal populations (as  happened to Native Americans , after they came into contact with Europeans starting in the 15th century).

The discovery that Neanderthal women who mated with Homo sapiens men faced an elevated risk for pre- and post-birth complications adds a new element to this complicated and ultimately tragic equation. The incompatibility between modern humans and Neanderthals may have run too deep for the gap to be bridged, even if the two species tried to cooperate. In fact, their attempts at cooperation might have made things worse for Neanderthals, who left  traces of their DNA  in the human genome but could not survive the encounters that made that possible.

Top image: A Neanderthal male. Neanderthal extinction has long fascinated scientists and the latest study points to evidence of a blood disorder that Neanderthals got from Homo sapiens, plus climate change leading to increased competition between the two hominins. Source:  procy_ab / Adobe Stock

By Nathan Falde

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