An article published in the journal “Science” reports the results of the sequencing of the Y chromosome, the one that determines the male sex among humans, of three Neanderthals and two Denisovans. A team of researchers led by Martin Petr and Janet Kelso of the Max Planck Institute for Evolutionary Anthropology performed high-quality sequencing using a very advanced approach to obtain the genetic sequences of the Y chromosomes of a total of 5 hominins. This made it possible to compare them with each other and with those of modern humans to obtain new information on the relationships among the various species. An interesting conclusion is that the Neanderthal Y chromosome was received from Homo sapiens with whom they must have interbred much earlier than we thought, no less than 100,000 years ago and possibly even 370,000 years ago.
The first genetic sequencing of a Neanderthal dates back to 1997, and it was only a fragment of mitochondrial DNA. In just over twenty years, genetic techniques made leaps forward also leading to the development of paleogenetics, which applies these techniques to extinct species. The Max Planck Institute for Evolutionary Anthropology is at the forefront of that field and continues to improve the results of sequencing genomes belonging to now extinct hominins.
Among the problems faced by paleogeneticists is the lack of availability of Neanderthal and Denisovan men among the individuals with DNA still sufficiently intact for quality sequencing. One cause is contamination by various microbes which is normal in millennia-old bones. A new approach made it possible to further improve the sequencing results and finally have the DNA of ancient humans and in particular the Y chromosome. These are the Neanderthals cataloged as Spy 94a (molar photo ©I. Crevecoeur), Mezmaiskaya 2, and El Sidrón 1253, and the Denisovans cataloged as Denisova 4, and Denisova 8.
Having obtained the Y chromosomes of these ancient men, the researchers compared them with each other and with the Y chromosome of modern humans. They found that Neanderthals and modern humans have a Y chromosome more similar to one another they are to Denisovan’s. Martin Petr stated it came as a surprise because Neanderthals and Denisovans are closely related to each other while modern humans are more distant relatives. Based on the mutations of the Y chromosome, the more recent common ancestors of Neanderthals and modern humans lived some 370,000 years ago, much more recently than previously thought.
Today we know that non-African modern humans inherited some Neanderthal genes, the result of interbreedings between 50,000 and 70,000 years ago, after Homo sapiens migrated from Africa. One topic under discussion concerns the genes Neanderthals may have inherited from Homo sapiens due to those interbreedings. The Y chromosome found in the Neanderthals examined indicates that there were interbreedings well before the known ones, as early as 370,000 years ago.
Genetic studies are increasingly discovering even very ancient migrations in human populations. It’s possible that some groups of early Homo sapiens or hominins closely related to Homo sapiens migrated from Africa and only the genetic traces discovered in the Neanderthals remain of them.
The replacement of the Y chromosome in a Neanderthal population is a remarkable event, but Janet Kelso is optimistic about the possibility of verifying it. To do this, she and her colleagues intend to try to obtain Y-chromosome sequences from more ancient Neanderthals who lived before the interbreeding that may have led to its replacement. The Neanderthals in this study lived towards the end of their species’ history while a population lived in Sima de los Huesos in Spain some 430,000 years ago and its men are expected to have a Y chromosome more similar to that of the Denisovans.
This study offered interesting insights into possible crossbreeding between hominins and laid the groundwork for further truly ambitious follow-up research. The increasingly perfected genetic techniques could make it possible to sequence at least in part the Y chromosome of really ancient Neanderthals. The result could confirm once again that human history is made up of migrations and interbreedings between populations that sometimes even belonged to different species.
