Hi! Our host invited me to take a stab at this one, so here we go.
Before I had a chance to read all the replies, I had a few ideas.
- Homo sapiens is some sort of attractor in biochemical phase space. This is essentially the convergent evolution hypothesis that has been previously explored and rejected, with an extra wave of the hands towards a justification.
- A species which has transcended biology travels through space as silicon-mediated minds, then synthesizes biological bodies using native species as templates in order to interact in a way that is comfortable for the locals. I like the incarnational aspect of this one. It turns out to be basically a variation on ideas already proposed by jongarvey and Argon.
- The Bekenstein bound says that the maximal amount of information needed to describe a physical system is proportional to the radius of a sphere bounding that system. For finite systems, that puts a finite bound on the information needed to describe them. Some have postulated this implies that if you go far enough out into space, you’ll find an identical Earth because there are only finitely many possible contents for Earth-sized spheres. So maybe these folks have traveled far enough to be from our doppelgänger planet.
That last one does not appear to have been already proposed, at least in that form, so that’s my guess for now.