Squid

So it turns out the squid really are aliens. Not invaders or anything like that. They were apparently just left behind some ridiculous amount of time ago. Presumably by whatever seeded life on earth to begin with. Or maybe not? We're similar enough to them it seems likely.

Regardless, we found out they were aliens when the originals came back to see what had happened here. It was a big fiasco when the planet was found to be contaminated. Aliens have red tape to, it turns out, and it took them quite a long time to decide who to blame and decide what to do about it. Then even longer to get back here, since the galaxy is a very big place.

They weren't even supposed to expose this place to microbes, and here they left a whole genus behind. Or maybe it was just one species at the time. We're still working on getting good translations.

So now what? The great answer to “Is there life out there?” has been provided. In fact, it turns out we're the other life. The squid-droppers haven't found anything else, and they've been looking for a lot longer than us.

So yeah, there's a pretty good chance we're all descended from some random bacteria shed off an alien when he violated contamination protocols. Either that, or we're descended from the squid.

I'm not sure which one is better.