Sex with Neanderthals protected humans from HIV and made them more vulnerable to covid

There is a tendency to think of human evolution as a story of progress, as if it were a succession of smartphone ranges in which the latest is always better than the previous.

However, it is unlikely that the process that has given rise to our species and all others would be to the liking of a perfectionist engineer.

The genetic variants that allow living beings to better adapt to a changing environment arise after taking advantage of the errors that occur in an organism when it replicates its DNA to stay alive.

Most of these bugs go away, but some are beneficial and make it easier for anyone who experiences them to survive and reproduce.

Other mutations resist because at least they do no harm, and remain hidden in the genome, passing from generation to generation,

In our species, some of these changes have come from ancestors that until recently we did not consider part of the direct family.

In 2006, Bruce Lahn of the University of Chicago discovered that the microcephalin gene, linked to a drastic reduction in brain size in some babies, had appeared in our genome 40,000 years ago.

The date and its characteristics made the researcher think that it had to be DNA of Neanderthal origin that arrived in our genome when some sapiens had sex and reproduced with a member of that disappeared species.

As Lahn told this newspaper, the reviewers of the most prestigious scientific journals rejected the publication of that result because a cross between these two species "was impossible."

Few years later,

Today, the PNAS

magazine

publishes a work in which, once again, the importance of the Neanderthal heritage and the relevance of chance in evolution are recalled.

Its author, Hugo Zeberg, from the Karolinska Institute (Sweden), published in the fall of 2020 together with Svante Pääbo, the main person responsible for sequencing the Neanderthal genome, that the greatest risk factor for severe covid, found on chromosome 3 , was introduced into the human lineage between 50,000 and 70,000 years ago by interbreeding with Neanderthals.

In a second study, Pääbo and Zeberg observed that this vestige of interbreeding with Neanderthals had increased in frequency since the last ice age in an exceptional way.

Compared to the 4% that Neanderthal genes usually occupy, at most, in European populations and somewhat more among some Asians,

Zeberg reasoned that the increase had to be due to some protective effect of that Neanderthal heritage and set out to find it.

In his latest analysis, he points out that people who carry the harmful variant against covid have a 27% lower risk of contracting HIV.

But the AIDS virus did not interbreed with the human species until recently.

The author speculates on the possibility that the factor that could favor the expansion of this protective variant against HIV and harmful when SARS-CoV-2 is contracted could be due to the fact that it also protected against smallpox, a pathogen that appeared more than of 10,000 years.

The emergence of that disease or some other threat to the human species caused the neutral variant that was already there to turn that Neanderthal heritage into an evolutionary advantage.

Cristian Cañestro, leader of the Evolution and Development research group at the University of Barcelona, ​​recalls that "evolution is a matter of balance".

In the case of CCR5, one of the genes in the region of the genome associated with a more severe covid, “a mutagenic variant has been seen that reduces the probability of being infected with HIV and that could protect against other infections in the past. ”, he continues.

"It is possible that this mutation gave some disadvantages because the protein does not fulfill its function well, but if it gives you a better chance of surviving against a deadly virus, you will have an advantage over the rest of the population," he adds.

Cañestro recalls Stephen Jay Gould to recall the key role of chance in evolution.

"We can have the best fish in the world, the most gifted, but if it's in a lagoon that dries up for whatever reason, the fish doesn't survive and doesn't pass on its genes," he says.

"In the end, survival depends on the advantages offered by genetic variants, but also on many chance events," he concludes.

Neanderthal heritage offers further examples of how a beneficial mutation in one circumstance may not be so in another.

A study published in

in 2016 showed how a gene from the extinct species made the blood thicker and, therefore, facilitated the appearance of clots.

For humans with no doctors to stitch up wounds from a bad fall or animal confrontation, such rapid coagulation was a distinct advantage.

For us, much older and with lifestyles that favor heart disease, that same genetic variant is seen as a health hazard.

This ambivalence of genetic variants must also be taken into account when evaluating the possibility of modifying embryos with the intention of creating improved humans.

In 2018, in China, two twin girls were born whose CCR5 gene had been edited to inactivate it.

Asked about the possibility that girls could be at higher risk of severe covid, Zeberg replies: "At the moment we have no reason to think that is the case."

But he argues that these kinds of crossover effects, where a mutation is good news for one disease and bad news for another, should make us "generally humble about our understanding of the genome and genetic variants."

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