Saturday, 26 September 2020

Evolution News - Homo Sapiens May Have Saved Early Neanderthals From Extinction

Upper molar of a male Neandertal (Spy 94a) from Spy, Belgium.
© I. Crevecoeur
Y chromosomes of Neandertals and Denisovans now sequenced | Max-Planck-Gesellschaft

An international team led by Martin Petr and Janet Kelso of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, has succeeded in sequencing the Y chromosomes of two Denisovans and three Neanderthals.

The surprising finding is that the Neanderthal Y chromosome appears to be closer to the Y chromosome of modern Homo sapiens than to Denisovans, although analysis of the autosomal DNA had shown Neanderthals and Denisovans were closer than either were to H. sapiens.

This suggests early Neanderthals probably benefitted from an ingression of H. sapiens Y chromosomes some-time between 370,000 and 100,000 years ago. The thinking is that small, isolated groups of Neanderthals and subsequent in-breeding, may have reduced breeding success due to accumulated deleterious genes in the Y chromosomes, so an ingression of H. sapiens Y chromosomes would have given the descendants considerable advantage over the early Neanderthals, given the essential part Y Chromosomes play in reproduction and fertility.

As the Max Planck Institute's press release explains:
By comparing the archaic human Y chromosomes to each other and to the Y chromosomes of people living today, the team found that Neandertal and modern human Y chromosomes are more similar to one another than they are to Denisovan Y chromosomes. “This was quite a surprise to us. We know from studying their autosomal DNA that Neandertals and Denisovans were closely related and that humans living today are their more distant evolutionary cousins. Before we first looked at the data, we expected that their Y chromosomes would show a similar picture,” says Martin Petr, the lead author of the study. The researchers also calculated that the most recent common ancestor of Neandertal and modern human Y chromosomes lived around 370,000 years ago, much more recently than previously thought.

It is by now well established that all people with non-African ancestry carry a small amount of Neandertal DNA as a result of interbreeding between Neandertals and modern humans approximately 50,000-70,000 years ago, quite shortly after modern humans migrated out of Africa and started spreading around the world. However, whether Neandertals might also carry some modern human DNA has been a matter of some debate. These Y chromosome sequences now provide new evidence that Neandertals and early modern humans met and exchanged genes before the major out of Africa migration - potentially as early as 370,000 years ago and certainly more than 100,000 years ago. This implies that some population closely related to early modern humans must already have been in Eurasia at that time. Surprisingly, this interbreeding resulted in the replacement of the original Neandertal Y chromosomes with those of early modern humans, a pattern similar to what has been seen for Neandertal mitochondrial DNA in an earlier study.

At first, the complete replacement of both Y chromosomes and mtDNA of early Neandertals was puzzling, as such replacement events are quite unlikely to occur by chance alone. However, the researchers used computer simulations to show that the known small size of Neandertal populations may have led to an accumulation of deleterious mutations in their Y chromosomes which would reduce their evolutionary fitness. This is quite 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 of the Y chromosome in reproduction and fertility, the lower evolutionary fitness of Neandertal Y chromosomes might have caused natural selection to favor the Y chromosomes from early modern humans, eventually leading to their replacement” says Martin Petr.

The team's findings were published yesterday in Science


Ancient DNA has provided new insights into many aspects of human history. However, we lack comprehensive studies of the Y chromosomes of Denisovans and Neanderthals because the majority of specimens that have been sequenced to sufficient coverage are female. Sequencing Y chromosomes from two Denisovans and three Neanderthals shows that the Y chromosomes of Denisovans split around 700 thousand years ago from a lineage shared by Neanderthals and modern human Y chromosomes, which diverged from each other around 370 thousand years ago. The phylogenetic relationships of archaic and modern human Y chromosomes differ from the population relationships inferred from the autosomal genomes and mirror mitochondrial DNA phylogenies, indicating replacement of both the mitochondrial and Y chromosomal gene pools in late Neanderthals. This replacement is plausible if the low effective population size of Neanderthals resulted in an increased genetic load in Neanderthals relative to modern humans.

Contrary to creationists' claims that the Theory of Evolution has no predictive powers and is therefore not testable and not scientific, these findings enable a firm, testable prediction to be made - that, when eventually sequenced, the Y chromosomes of early Neanderthals - such as those at Sima de los Huesos in Spain - will show that early Neanderthals were indeed closer to Denisovans than to H. sapiens.

Watch this space!

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