(CNN)Early modern humans originated in Africa and started spreading around the world about 80,000 years ago. As they traveled, they came across other ancient humans, including Neanderthals, who had already populated Europe and parts of Asia. Some of them had sex and gave birth to children -- current-day human DNA still carries echoes of these prehistoric sexual encounters.
How Neanderthal DNA affects human health -- including the risk of getting Covid-19
The evidence that Homo sapiens interbred with Neanderthals first emerged in 2010, after Swedish geneticist Svante Pääbo pioneered methods to extract, sequence and analyze ancient DNA from Neanderthal bones. Pääbo mapped the entire Neanderthal genome, and thanks to his work, scientists can compare Neanderthal genomes with the genetic records of living humans today.
Most living people can trace a very small percentage of their DNA to Neanderthals -- and that likely includes Africans, who until recently had been thought to have no genetic link to Neanderthals.
Scientists have been trying to figure out what, if anything, this genetic legacy means. Research has found links between Neanderthal DNA and fertility, how people feel pain and immune system functionality. Neanderthal DNA may affect skin tone and hair color, height, sleeping patterns, mood and even addiction in present-day Europeans.
What's more, Neanderthal DNA may play a small role in swaying the course of Covid-19 infection, recent research has shown.
"It's exciting for us to find out that Neanderthals had things that are important for us 50,000 years after they went extinct," said Pääbo, director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who has been studying Neanderthal DNA for three decades.
Pääbo and his team have turned their attention to medical science's most pressing concern, the coronavirus pandemic, to see if Neanderthal DNA can shed any light on Covid-19.
The scientists examined a strand of DNA that has been associated with some of the more serious cases of Covid-19 and compared it to genetic sequences known to have been passed down to living Europeans and Asians from Neanderthal ancestors. The Neanderthal DNA strand is found on chromosome 3; a team of researchers in Europe has linked certain variations in this sequence with the risk of being more severely ill when infected by Covid-19.
"We've worked for 25 years on Neanderthal genomes, and suddenly they seem very relevant for a big medical emergency. It was really a huge, huge surprise," Pääbo said.
The study also revealed considerable differences in how common this genetic risk variant is in different parts of the world. It's particularly common among people in South Asia, where about half of the population carry the Neanderthal risk variant. In Europe, one in six people carry the risk variant, while in Africa and East Asia it is almost nonexistent.
Commercial DNA tests can tell you what percentage of your DNA is Neanderthal. However, it's not something individuals need to worry about when it comes to Covid-19 risk and the other diseases and traits that are associated with Neanderthal DNA.
"We can't blame Neanderthals for Covid. That's a damaging response, and that's why I want to emphasize so much the social and environmental factors are the real things that people should be worrying about," said Tony Capra, a geneticist at the University of California, San Francisco's Bakar Computational Health Sciences Institute.
"That said, as a geneticist, I think it is important to know the evolutionary history of the genetic variants we find that do have effects on traits. The effects of Neanderthal DNA traits are detectable, but they're modest."
Capra said that his work had found links between Neanderthal DNA and psychiatric disorders like depression and addiction.
"We've found associations with sensitive neuropsychiatric diseases and addiction (and Neanderthal gene variants), which are primed for people to emotionally connect with when they're looking for an explanation for why they or their loved one is struggling," he said.
"I think we have a duty to explain to the public that you can't blame Neanderthals for any of these associations that we've found."
Scientists are able to look for links by comparing Neanderthal DNA with information on living humans mined from large biobanks used for medical research that contain information about an individual's genetic makeup as well as other details in their medical records.
"These big cohort studies -- the UK biobank is the biggest one -- where one studies hundreds of thousands of people. ... This has opened up this (field) to make it easier to study," Pääbo said.
While many links between Neanderthal DNA and characteristic traits are largely of academic interest, some may have medical consequences in terms of identifying people at risk of dise