As a rule of thumb, evolutionary development is often dependent on parasites. The more parasites, the faster the adaptation to them. This also applies to pathogens.
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?