Almost all humans today of non-African heritage have some small percentage of Neanderthal DNA in them but the question is why don’t we have more? We know Homo sapiens (modern humans) interbred with Neanderthals so it’s logical to assume that we’d have more of their DNA in us but we don’t. Turns out it is all apparently because of the Y chromosome!
Researchers believe it was the Y chromosome that was key in keeping modern humans and Neanderthals apart often creating conditions that led to miscarriages. It’s assumed that most Neanderthals died out about 40K years ago. Some were assimilated via interbreeding with modern humans but others died of disease to which they had no natural immunity to. H. sapiens came out of Africa, perhaps, and into Eurasia and they brought their diseases with them. We’ve seen it time and time again with the Spanish and the Native Americans and all over the world as new populations move in, or conquer, and the native populations are almost wiped out by diseases they brought with them because they have no natural immunity to such diseases like smallpox, measles, etc.
The Neanderthal genome was successfully sequenced in 2010 and it contains a wealth of information not only about them but about us. The genomes showed that Neanderthals did in fact interbreed with H. sapiens and anywhere from 1.5% to 2.1% of DNA in non-Africans is of Neanderthal origin. But the last sequencing was the Neanderthal Y chromosome and in both species of homins it is the Y chromosome that determines if one is born of the male sex. Researchers have now completed the first in-depth study of the Neanderthal Y chromosome focusing on a Neanderthal male whose remains were found in El Sidron, Spain. This study confirms previous research that supports the theory that Neanderthal and H. sapiens Y chromosomes diverged about 600kya.
Further study and analysis has now revealed that genetic mutations may explain why the Neanderthal Y chromosome was mostly lost in H. sapiens. Three mutations have been found that can trigger immune responses from pregnant women that lead to miscarriages and two of them are unique to Neanderthals. Thus, there were genetic incompatibilities between the two homin species that appear to have been designed to keep us apart from the Neanderthals.
Immunity incompatibilities are important especially in population isolation lead researcher Fernando Mendez said. Further study and analysis of the Y chromosomes from more specimens of Neanderthal male remains is planned and further study may even confirm the theory that these gene mutations kept modern humans and Neanderthals apart in ways we do not know yet.
All this said I must still point out that there are some modern humans, Homo sapiens, that have some very distinct Neanderthal traits such as the large size and the distinctive bun at the base of the skull. How do we explain that? It would appear that some Neanderthals inbred with H. sapiens more than others and that the Y chromosome did NOT cause miscarriages but the birth of hybrid offspring was successful.