Modern humans and Neanderthals could be forgiven for having other issues on their minds when they interbred in the stone age. But according to researchers, those ancient couplings laid a grim foundation for deaths around the world today. Scientists have claimed that a strand of DNA that triples the risk of developing severe Covid-19 was passed on from Neanderthals to modern humans. The genetic endowment, a legacy from more than 50,000 years ago, has left about 16% of Europeans and half of south Asians today carrying these genes. The origins of the risk genes came to light when scientists in Sweden and Germany compared the DNA of very sick Covid-19 patients with that from Neanderthals and their mysterious sister group, the Denisovans. The stretch of DNA that makes patients more likely to fall seriously ill closely matched that collected from a Neanderthal in Croatia. “I almost fell off my chair because the segment of DNA was exactly the same as in the Neanderthal genome,” Hugo Zeberg, an assistant professor at the Karolinska Institute in Stockholm, told the Guardian. Zeberg and his co-author, Svante Pääbo, director of the Max Planck Institute of Evolutionary Anthropology in Leipzig, suspect the Neanderthal genes have persisted in modern humans because they were once beneficial, perhaps helping to fight off other infections. Only now – when faced with a new infection – has their downside been exposed. It is unclear how the genes may worsen Covid-19, but one gene plays a role in the immune response and another has been linked to the mechanism the virus uses to invade human cells. “We are trying to pinpoint which gene is the key player, or if there are several key players, but the honest answer is that we don’t know which are critical in Covid-19,” Zeberg said. According to the study, published in Nature, the cluster of genes on chromosome three are most commonly found in Bangladesh, where 63% of the population carry at least one copy of the DNA sequence. “The genes in this region may well have protected the Neanderthals against some other infectious diseases that are not around today. And now, when we are faced with the novel coronavirus these Neanderthal genes have these tragic consequences,” Pääbo said. The researcher, who led the international team that first deciphered the Neanderthal genome in 2010, said his “rough estimate” was that about 100,000 “additional” people have died so far in the current pandemic due to the genetic contribution from Neanderthals. Beyond the Covid-19 risk genes, the Neanderthals have bequeathed other genes to modern humans. Some increase sensitivity to pain, while others reduce the risk of miscarriages. “Some are beneficial and some are detrimental,” Zeberg said. “This has been a double-edged sword.” But Mark Maslin, a professor at UCL and author of the book The Cradle of Humanity, cautioned that the work risked oversimplifying the causes and impact of the pandemic. “Covid-19 is a complex disease, the severity of which has been linked to age, gender, ethnicity, obesity, health, virus load among other things,” he said. “This paper links genes inherited from Neanderthals with a higher risk of Covid-19 hospitalisation and severe complications. But as Covid-19 spreads around the world it is clear that lots of different populations are being severely affected, many of which do not have any Neanderthal genes. “We must avoid simplifying the causes and impact of Covid-19, as ultimately a person’s response to the disease is about contact and then the body’s immunity response, which is influenced by many environmental, health and genetic factors.”