They’re human. I don’t think it’s been fully covered how this happened, but there was one interesting piece that didn’t get published.
It combines Lucas’ various other movies like THX-1138 and Indiana Jones. Earth is overrun with an AI-driven society in THX, and a group of humans get on a ship to escape. They fall through a wormhole and end up in the Star Wars universe, becoming the first humans there. Han and Chewie travel back through this wormhole, and crash land on Earth in a forest. Chewie survives, and him walking around starts a bunch of stories about Big Foot. Indiana Jones investigates, finds the remains of the Falcon and Han, and wonders why this guy looks familiar.
I think American Gothic was in there somehow, too.
Even if it did get published, I can’t imagine it being taken seriously as Legends canon. Chewie was already killed off in the Yuuzhan Vong stuff with Han surviving. But that’s the closest to an answer we ever got.
As it stands, Courscant is often believed to be the original human homeworld in-universe, and whatever the truth is has been lost to time. Star Wars is interesting with how old the universe feels–which is more of a Tolkein-like property than traditional science fiction–and this is a pretty good example.