Our closest extinct human relative, the Neanderthals, donated genetic material that makes up around one and four per cent of our genome. However, humans and Neanderthals interbred tens of thousands of years ago, so why did so little of their genetic material persist?
According to a new study, published in PLOS Genetics, Neanderthal genetic material was removed as a result of natural selection acting on their weakly deleterious gene variants.
The study used a novel method that estimated the strength of natural selection against the Neanderthal genetic material. From this they deduced that natural selection had removed the Neanderthal alleles that may have had mildly negative effects. Researchers suggest that these negative gene variations were able to persist as Neanderthals had a smaller population size to humans. However, upon introduction to the human genome, these alleles became subject to much stronger natural selection in a larger population.
Battle of the genomes
As one of the first attempts to quantify the strength of natural selection against Neanderthal genes, this study provides a new insight into how our extinct relatives contributed to the human genome. It also confirms that geographical location also relates to the percentage of Neanderthal in your genes. For instance, East Asian people had higher initial levels of Neanderthal ancestry than Europeans.
“For a while now we have known that humans and Neanderthals hybridized,” says Ivan Juric, lead author of the study. “Many Europeans and Asians-along with other non-African populations-are the descendants of those hybrids
"Our results are compatible with a scenario where the Neanderthal genome accumulated many weakly deleterious variants, because selection was not effective in the small Neanderthal populations. Those variants entered the human population after hybridization. Once in the larger human population, those deleterious variants were slowly purged by natural selection.”
The Neanderthal in you
Our current levels of Neanderthal inheritance are therefore a result, in part, to long-term difference in population sizes. Weakly deleterious variants were able to persist in Neanderthals, but could not survive the stronger natural selection acting against them in the large human population.
However, whilst this demographic data is significant in contributing to our understanding of Neanderthal genes in the human genome, the study concludes that it cannot explain everything. For instance, certain deleterious genes may have only acted upon human-Neanderthal hybrids, or other selection pressures (e.g. sexual selection) may have against hybrids, further reducing the presence of Neanderthal gene in the genome. Still, it is interesting to contemplate the ‘what if’ situation. For instance, what if human populations had grown more slowly? Or if Neanderthals populations had been larger? We would probably have a lot more Neanderthal in us today...