A study published a couple days ago in the Proceedings of the Royal Society B shows us that ancient humans, Neanderthals and Denisovans had more genetic similarities than polar bears and brown bears do. In other words, the genetic distance between our ancient relatives and us was smaller than between pairs of species that are known to easily hybridize. Lead author Greger Larson, Director of the Palaeogenomics & Bio-Archaeology Research Network (PalaeoBARN) at Oxford comments,
“Our desire to categorize the world into discrete boxes has led us to think of species as completely separate units. Biology does not care about these rigid definitions, and lots of species, even those that are far apart evolutionarily, swap genes all the time. Our predictive metric allows for a quick and easy determination of how likely it is for any two species to produce fertile hybrid offspring. This comparative measure suggests that humans and Neanderthals and Denisovans were able to produce live fertile young with ease.”
We have only recently identified that Neanderthals, Denisovans and ancient humans admixed. As mentioned above, the authors of this study produced a viability metric based upon genetic distances. They then predicted fertility of the 1st generation hybrids. By correlating the genetic distance with the relative fertility of the hybrid offspring, it was possible to show that the greater the evolutionary distance between any two species, the less likely it is that the offspring between them would be fertile. This challenges the Biological Species Concept, the one that restricts species definitions by reproductive ability. In addition, the team used the distance values to determine a threshold of fertility.
When calculating the genetic distance values between humans, Neanderthals and Denisovans, the team saw that their genetic differences were in fact even smaller than the genetic differences between several pairs of species which we already know to easily hybridize—including polar bears and brown bears, as well as coyotes and wolves. This suggests we could have predicted the existence of Neanderthals and Denisovans in our genomes as soon as the first genetic sequences were generated.
A cool spin off is that this study can help predict the likelihood that any two mammal species can give birth to live hybrids. This is a useful tool that can be used in decisions about whether to place animals together in captivity.