Matthias Meyer at work in the clean laboratory of the Max Planck Institute for Evolutionary Anthropology. Photo credits: MPI f. Evolutionary Anthropology
Neanderthals adopted the male sex chromosome from modern humans.
1997 the very first Neanderthal man DNA The sequence – only a small part of the mitochondrial genome – was determined by an individual who was discovered in the Neanderthal in 1856. Since then, improvements in molecular techniques have enabled scientists at the Max Planck Institute for Evolutionary Anthropology to determine high quality sequences of the autosomal genome of several Neanderthals and led to the discovery of an entirely new group of extinct humans, the Denisovans, the relative of Neanderthals in Asia were.
However, since all samples that are well preserved enough to provide sufficient amounts of DNA are from female individuals, extensive studies of the Y chromosomes of Neanderthals and Denisovans have not yet been possible. Unlike the rest of the autosomal genome, which is a rich tapestry of thousands of genealogies of an individual’s ancestors, Y chromosomes have a special type of inheritance – they are passed down exclusively from father to son. Y chromosomes as well as maternally inherited mitochondrial DNA were extremely valuable for the study of human history.
New method for identifying Y chromosome molecules
In this study, the researchers identified three male Neanderthals and two Denisovans who might be suitable for DNA analysis, and developed an approach to fish out human Y chromosome molecules from the large amounts of microbial DNA that typically contaminate old bones and teeth. This enabled them to reconstruct the Y chromosome sequences of these individuals, which would not have been possible with conventional approaches.
By comparing the archaic human Y chromosomes with each other and with the Y chromosomes of people living today, the team found that Neanderthals and modern human Y chromosomes are more similar than Denisovan Y chromosomes. “That was quite a surprise to us. We know from examining their autosomal DNA that Neanderthals and Denisovans were closely related and that the humans living today are their more distant evolutionary cousins. Before we looked at the data for the first time, we expected that your Y chromosomes would show a similar picture, ”says Martin Petr, the lead author of the study. The researchers also calculated that the youngest common ancestor of the Neanderthals and modern human Y chromosomes lived about 370,000 years ago, much younger than previously thought.
It is now common knowledge that due to the cross between Neanderthals and modern humans, about 50,000 to 70,000 years ago, shortly after modern humans emigrated from Africa and began to spread, all people of non-African descent have a small amount of Neanderthals. Carrying DNA around the world. However, whether Neanderthals could also carry modern human DNA was controversial. These Y chromosome sequences now provide new evidence that Neanderthals and early modern humans learned and exchanged genes before the great migration from Africa – possibly 370,000 years ago and certainly more than 100,000 years ago. This implies that some populations closely related to people of the early modern period must have already been to Eurasia by this time. Surprisingly, this cross resulted in the replacement of the original Neanderthal Y chromosomes with those of early modern humans, a pattern similar to that seen in an earlier study for Neanderthal mitochondrial DNA.
Upper molar of a male Neanderthal (Spy 94a) from Spy, Belgium. Photo credit: I. Crevecoeur
Selection on Y chromosomes from humans in the early modern period
At first, the complete replacement of both the Y chromosomes and the mtDNA of the early Neanderthals was puzzling, as it is unlikely that such replacement events would occur by chance alone. However, the researchers used computer simulations to show that the known small size of the Neanderthal populations could have led to an accumulation of harmful mutations in their Y chromosomes that would reduce their evolutionary fitness. This is pretty similar to situations where extremely small population sizes and inbreeding can sometimes increase the incidence of some diseases. “We speculate that given the important role played by the Y chromosome in reproduction and fertility, the lower evolutionary fitness of the Neanderthal Y chromosomes may have led to natural selection that favored and ultimately increased the human Y chromosomes their replacement leads, “says Martin Petr.
Janet Kelso, lead author of the study, is optimistic that this surrogate hypothesis could be tested in the near future: “If we can retrieve Y chromosome sequences from Neanderthals who lived before this hypothetical early introgression event, such as the 430,000 year old We Assume that Neanderthals from Sima de los Huesos in Spain still have the original Neanderthal Y chromosome and therefore will be more similar to Denisovans than modern humans. ”
Reference: “The evolutionary history of the Neanderthal and Denisovan Y chromosomes” by Martin Petr, Mateja Hajdinjak, Qiaomei Fu, Elena Essel, Hélène Rougier, Isabelle Crevecoeur, Patrick Semal, Liubov V. Golovanova, Vladimir B. Doronichev, Carles Lalueza- Fox, Marco de la Rasilla, Antonio Rosas, Michael V. Shunkov, Maxim B. Kozlikin, Anatoli P. Derevianko, Benjamin Vernot, Matthias Meyer and Janet Kelso, September 25, 2020, science.
DOI: 10.1126 / science.abb6460
