In hybrid women born from the union between sapiens and Neanderthals there could be a high risk of miscarriages due to a genetic discrepancy with the unborn child.
A small piece of the Neanderthal extinction could also lie in the risky pregnancies carried out by “hybrid” women, i.e. born from the love between men or women of our species and partners belonging to this archaic and close human group. In fact, in women with mixed genetic heritage the risk of genetic incompatibility with the unborn child was perhaps higher, which made pregnancies more complex and increased the probability of spontaneous abortions. This is the hypothesis raised in a study posted on biorXiv.
Crossroads and disappearance
Homo sapiens and Neanderthals met repeatedly, giving rise to a hybrid generation between 50,000 and 45,000 years ago, and Neanderthals finally became extinct around 41,000 years ago. Traces of Neanderthal DNA remain in the DNA of modern humans of non-African descent, and overall constitute 1-2 percent of the entire genome. However, they are not found in mitochondrial DNA, i.e. the one transmitted along the maternal axis.
What prevented the passage of Neanderthal genes from mothers to unborn children? The hypothesis of Patrick Eppenberger and colleagues from the Institute of Evolutionary Medicine of the University of Zurich (Switzerland) is that women born to partially sapiens and in part Neanderthals had higher risk pregnancies, due to a problem of incompatibility between their genes and those of the fetus.
A problem in the transport of oxygen
Scientists have identified this incompatibility in a gene important for the transport of oxygen in the blood, called PIEZO1 and present in different variants in the DNA of Homo sapiens and Neanderthals. After modeling the possible interactions between the two variants and simulating the effect of the Neanderthal variant on human red blood cells in the laboratory, the team concluded that the Neanderthal variant (V1) caused the red blood cells to bind to oxygen with greater intensity than the variant present in sapiens DNA (V2). V1 is dominant: those who inherit both V1 and V2 will have red blood cells with this high chemical affinity for oxygen.
A fetus born from the union between a sapiens and a Neanderthal, or vice versa, would have developed without problems. Difficulties would arise with the next generation: a hybrid mother with both versions of the PIEZO1 gene, carrying a fetus with two copies of V2 would end up with a greater affinity for oxygen in her red blood cells, and would deliver less blood to the unborn child through the placenta. A mechanism that would have led to a greater risk of abortions.
“Over millennia of coexistence, even low levels of this modern human genetic contribution in Neanderthal populations could have introduced a gradual reproductive disadvantage that increased over generations,” the authors write. The same phenomenon would not have been a problem for theHomo sapienswhich had a much more stable and extensive population, already advantaged from many points of view.
A reduced contribution
Some mutations in the PIEZO1 gene not derived from Neanderthal DNA could still have a similar effect today and explain some cases of early miscarriages. However, as fascinating as the theory is of a genetic mismatch creating ripple effects in an already fragile population, its contribution to Neanderthal decline is likely to have been small, a small part of a host of other genetic, social and ecological pressures.
