Neanderthals, our closest relatives, inhabited parts of Europe and Asia until they became extinct about 30,000 years ago. Recent genetic studies have shed light on the connection between modern humans and Neanderthals, revealing that interbreeding between the two species occurred around 47,000 years ago. However, one question remains unanswered: what happened to the Neanderthal Y chromosome?
The Homo sapiens genome contains traces of Neanderthal DNA, except for the Y sex chromosome responsible for male development. There are several theories as to why the Neanderthal Y chromosome is missing. It could have been lost by chance, due to mating patterns or inferior function. However, a century-old theory about the health of interspecies hybrids may hold the answer.
Neanderthals and modern humans diverged between 550,000 and 765,000 years ago in Africa. Neanderthals migrated to Europe while our ancestors remained in Africa. The two species did not meet again until modern humans migrated to Europe and Asia between 40,000 and 50,000 years ago.
Scientists have sequenced the full male and female Neanderthal genomes using DNA from well-preserved bones and teeth. The Neanderthal genome is similar to ours, consisting of 20,000 genes bundled into 23 chromosomes. Like us, they had two copies of 22 chromosomes and a pair of sex chromosomes. Females had two X chromosomes, while males had one X and one Y.
Sequencing the Neanderthal Y chromosome is challenging due to its repetitive DNA. However, the sequenced portion contains similar genes to the modern human Y chromosome. The SRY gene, responsible for male development in all apes, likely played the same role in Neanderthals.
By examining DNA sequences, scientists have found that modern humans of European descent have about 2% Neanderthal DNA. Asians and Indians have even more, while Africans have none. This Neanderthal DNA was acquired during a 7,000-year period around 47,000 years ago when modern humans migrated to Europe. At least half of the Neanderthal genome can be reconstructed from fragments found in contemporary human genomes.
However, no modern humans carry any part of the Neanderthal Y chromosome. The reasons for its absence are still unclear. It could have been lost over generations if there were no male offspring to pass it on. Alternatively, it is possible that only modern human men mated with Neanderthal women, resulting in offspring with the modern human Y chromosome. However, this theory conflicts with the absence of Neanderthal mitochondrial DNA in modern humans.
Another possibility is that the Neanderthal Y chromosome was not as functional as its modern human counterpart. Due to their small population size, harmful mutations may have accumulated in Neanderthals. Y chromosomes with advantageous genes tend to replace others in a population, which may have contributed to the loss of the Neanderthal Y chromosome.
Haldane’s rule, which states that infertile or unhealthy hybrids are typically of the sex with unlike sex chromosomes, may also explain the absence of the Neanderthal Y chromosome. It is possible that the Neanderthal Y chromosome did not work well with genes from modern human chromosomes due to small genetic differences. This could explain why male hybrids between species often face infertility or health issues.
In conclusion, the fate of the Neanderthal Y chromosome remains a mystery. It may have been lost by chance or due to mating patterns or inferior function. The possibility that it played a role in reproductive barriers between Neanderthals and humans raises questions about how the two species became separate in the first place.