Humans; I like the fact that they're throwing all these hints that it's Earth without outright saying that it is--they're at least treating us with some level of intelligence and not have to spell everything out for us like 5-year old children. If they did, I would feel almost insulted...well, not that much, but it'd be a silly decision. It's the ol' Show-Don't-Tell rule. Also, by not having humans, it calls into question whether humans are still on the planet or not, and personally, I think they are given the transformation of the Forest of Hope and Distant Spring levels to what they are now; but such ambiguity adds to the series instead of just being frustratingly unresolved.
My blog - No BS - Open to Criticism: http://sparx401.blogspot.com/ Pikmin 3 idea almost finished. Current Status: 120 pages