One hurdle in deciphering human prehistory is the absence of evidence of a Neanderthal Y chromosome in the genetic record. Now, a new study, published [September 25] in the journal Science, suggests that the modern human Y chromosome completely replaced the Neanderthal Y chromosome when male Homo sapiens’ began mating with female Neanderthals at some point between 100,000 and 370,000 years ago, reports Ann Gibbons for Science magazine.
So far, scientists have only been able to sequence about a dozen Neanderthal specimens’ DNA—and most of the samples are from women. The male DNA from Neanderthals that does exist is damaged or contaminated, Science reports. Using a new breakthrough method of “binding” fragmentary chromosome sample, the team of evolutionary geneticists sequenced Y chromosome DNA from three Neanderthals and two Denisovans that lived in Eurasia.
The team theorizes that the human Y chromosome may have had a slight fitness advantage over their Neanderthal comrades. Because Neanderthals had a smaller population size than humans, they likely accumulated more deleterious—or harmful—mutations in their genome, especially on the sex chromosomes. Early modern humans, however, were more genetically diverse and likely had Y chromosomes that were free of undesirable mutations. These mutation-free chromosomes could have given humans a slight genetic advantage that was just enough to edge out the Neanderthal’s Y chromosome.