This is the closest thing to a teleology I believe in (some of the time). Why do anything? Because the aliens are coming. (I literally think of it using the same verb as you.) And then, death match time. Which may well get settled by utter luck, as likely happened in old world vs new world. But still. The more divergent two populations are, the less likely their meeting is to result in cohabitation. I don't think it's luck that there's only one remaining sentient species on the planet.
See also this excerpt from Peter Watts: http://akkartik.name/post/2012-11-21-07-09-03-soc