this is a good one. we’re tooling along as one happy species and some parasites pair up with a few of us, while the other parasites decide to infect the rest of us (excluding the other ones of course) and voila: they evolve us into two different species.
oh well, we all taste a little different to the little buggers don’t we?
It isn’t anywhere that easy. In fact, it reaches into the realm of stupefying complexity.
For example, in the modern world there is an abundance of “auto-immune” diseases, from asthma to arthritis, quite a list. And the current theory is that, to a great extent, these cases of our immune systems attacking our own bodies may lie in the *absence* of the parasites that used to afflict us.
That is, living better without parasites has confused our immune systems, so it assumes it just cannot see the parasites and attacks us as if they were there.
And this may have been the case going way back. While the royalty of ancient Egypt, for example, were known to have suffered from arthritis, most of the common people did not.