There are many speculations about the extinction of the Neanderthals and the reasons for their disappearance 40,000 years ago. Recent research suggests that they lived side by side with modern humans (Homo sapiens) for 5000 years before their final extinction, and these two species of people interbred with each other quite often.
So, if Homo sapiens and Neanderthals could coexist peacefully for so long, why did they disappear and we stayed?
Getting along with modern humans could have decided the fate of Neanderthals, according to new research published in the journal Plos One.
The latest study, published in the journal Plos One, found signs of hemolytic disease of the fetus and newborn (HDFN) in the DNA of Neanderthals. And this blood disease that they got from breeding with modern humans led to the extinction of the Neanderthals.
Extinction of Neanderthals due to blood disease Homo Sapiens
This grim conclusion is the result of an important discovery by a group of genetic scientists from the French National Center for Scientific Research (CNRS) and the University of Aix-Marseille in France. These scientists analyzed the blood types of three Neanderthals whose DNA had been successfully extracted and sequenced from fossilized bone samples. These Neanderthals did not live together, but were born at different times and in different places.
Although they were not related to each other, scientists found that these people carried a genetic variant in their blood that made them highly vulnerable to a blood disorder known as hemolytic disease of the fetus and newborn, or HDNP. This condition develops in expectant mothers, and when it does, it can be fatal to the fetus or newborn.
HDFN can develop if the mother and father have incompatible blood types. When a woman with an Rh negative blood type (A-, B-, O-, or AB-) mates with a Rh-positive man, it can cause problems in conceiving a positive blood type child. The mother's immune system can perceive the fetal blood cells as foreign invaders and begin to attack them. As a result, the fetus or newborn may suffer from severe and life-threatening amentia (that is, not having enough red blood cells to supply oxygen to the body).
This does not happen every time a man and woman with both positive and negative blood groups mate. In modern humans, HDFN occurs in about three out of every 100,000 births. There are vaccinations that can help significantly reduce the risk to expectant mothers and their babies if the parents' blood types are known to be incompatible.
HDFN can develop if the mother and father have incompatible blood types. When a woman with an Rh negative blood type (A-, B-, O-, or AB-) mates with a Rh-positive man, it can cause problems in conceiving a positive blood type child. And that appears to have precipitated the extinction of the Neanderthals.
But, as the new study clearly shows, the risk of this dangerous condition in Neanderthal women was much higher than in people living on Earth today.
"The fact that these forms of genes were found in humans, separated by 4,000 km and 50,000 years, suggests that this genetic trait - and the risk [of fetal anemia - was fairly common among Neanderthals," explains lead author Stefan Mazier., a population geneticist from CNRS.
According to scientists, a Rh-negative mother of a Neanderthal would be more likely to have a strong HDFN reaction if she mated with a Rh-positive modern human rather than a Neanderthal man. It is this increased risk, plus the prevalence of genetic vulnerability in Neanderthals, that suggests that interbreeding between Neanderthals and Homo sapiens could have led to a dramatic increase in the incidence of HDFN.
One of the main problems with HDFN is that it is much more common in newborns after the first birth. The immune system reacts more strongly to the "invaders" it recognizes, so a Rh-negative Neanderthal woman trying to give birth to multiple Rh-positive children is unlikely to be able to do so.
It might not matter if Neanderthal populations remained large and interbreeding with modern humans was a relatively rare event. But if Neanderthal populations were declining for other reasons, their survival strategy could include more frequent mating with humans. Such a strategy could doom them rather than save them if childbirth became increasingly difficult due to the rapidly spreading HDFN.
"These elements could weaken the offspring to such an extent that it would lead to their death, especially when combined with competition with Homo sapiens for the same ecological niche," - the French scientists write in their article on Plos One.
The population collapse caused by such dynamics would have occurred gradually, which is how the Neanderthals appear to have disappeared.
Recent studies of ancient climate have shown that the Northern Hemisphere experienced two prolonged and cold droughts around the time the Neanderthal population began to decline. These cold droughts turned fertile woodlands and rich pastures into arid landscapes and intensified competition for sufficient food, where modern humans were better off.