I’m still not certain how the Science of it works. My understanding—and granted I am no geneticist—is that hybrids are sterile. Animals born of the breeding of two different species cannot reproduce. But it appears our Cro-Magnon ancestors could and did interbreed with Neanderthals, because the Neanderthals were our ancestors too, a little bit. Most people alive today have small amounts of Neanderthal DNA. It’s a fact. And there have been studies suggesting those with more in the way of Neanderthal DNA are more susceptible to things like depression and other chronic illnesses. Now there’s new evidence that people who are getting sicker than most with Covid-19 have just such a larger sampling of that ol’ Neanderthal genetic material.
From the linked article: “A group of genes passed down from extinct human cousins is linked with a higher risk for severe COVID-19, researchers say. When they compared the genetic profiles of about 3,200 hospitalized COVID-19 patients and nearly 900,000 people from the general population, they found that a cluster of genes on chromosome 3 inherited from Neanderthals who lived more than 50,000 years ago is linked with 60% higher odds of needing hospitalization…While the study cannot explain why these particular genes confer a higher risk, the authors conclude, ‘with respect to the current pandemic, it is clear that gene flow from Neanderthals has tragic consequences.’”