Yes, there are many theories that can be entertained. My favourite is the mythos that, over a hundred thousand years, perhaps humans did evolve to the point they had technology to leave the planet, tidying up after themselves as they did, and its just us stragglers left behind that have been slowly devolving the tools they also left behind .. 'what if' doesn't have to be disregarded, just because its preposterous.

I mean, I never once mentioned two aliens in a UFO, but hey .. sure, why not.