Recently, someone grossly misunderstood why I raised the issue of what is meant by ‘species’ when Neanderthals, Denisovans, and others are said to be of a different ‘species’. After a lot of foolishness, he finally asked.
There are two parts to the answer. The first is simply the concern of a scientist to understand what is actually being claimed; I'm careful.
But there's also a matter of great social importance here.
In the absence of an explicit definition, most people are going to have a sense of ‘species’ inferred from observed use.
In common use, an animal of one ‘species’ is of a fundamentally different sort from an animal of a different ‘species’; in this conception, an animal not of our species is not human.
I've indeed encountered popular-science journalism in which Neanderthals, Denisovans, and other members of the genus Homo are distinguished from those labelled ‘human’.
In that context, when we say that some populations have more or less genetic material from these other ‘species’, readers are going implicitly to imagine that, even if all of these populations are ‘human’, some have more non-‘human’ genetic material and hence are themselves less ‘human’.
Claiming that people have more or less DNA from other species has much the same social effect as claiming that some people are part dog.
That is the sickening chain of inference that shouldn't-but-does get followed.
However, if the term ‘species’ is defined clearly and explicitly, then much of this sort of thinking is preëmpted. Fewer people slide into thinking that Neanderthals and Denisovans were not human; fewer people take it that some present populations are less human than are others.
And, indeed, I was raising the issue in the context of an article reporting that members of one of our present populations have genes of a previously unrecognized ‘species’ of hominid.