the article, and the scientists behind it seem to be saying there is a line to draw, between Denisovia and Humans and Neanderthal. There is a line because you

the article, and the scientists behind it seem to be saying there is a line to draw, between Denisovia and Humans and Neanderthal. There is a line because you can differentiate between almost any two groups of people using DNA. Ashkenazim Jews versus other Jews for example. That obviously does not make them different species, it just means you can separate the two groups with enough math, like principal component analysis. That's why the quoted scientist in the article says: "It was the first time a new group of distinct humans was discovered" A new group of humans. what are the metrics in DNA used That information is hopefully in their paper. Usually it's some fairly straightforward statistics. how can we call them different species if we were still interbreeding 40k ago? I think we call them different species because the status quo used to be that we never interbred with them. We couldn't find any Neanderthal mtDNA in humans, despite the strong archeology evidence that we had contact with them. As DNA technology kept improving, we were able to recover more DNA from ancient samples, and looked at the Y chromosome, and surprise you might have enough Neanderthal DNA to count as full grand, grand father! So I think at this point, we can't call them a separate species any more. But inertia being what it is, we keep doing it. Also it makes for more eye catching article headlines. while human/denisovians could never produce fertile offsprin That was just my example about how we define species. According to the article: "...but the evidence also suggests that Denisovans and humans interbred." can we estimate the odds of that being mutally independnt mutation Yeah, that's kind of what they are doing. It is heavily based on statistical estimation and that's why when then guess the age of a sample it is between A LOT of years. They know their error margin is large.