We were the second evolution of their form. From what I gather they purposely seeded Earth so that mammals would evolve the same way that they did. Hell, maybe they were the ones who wiped out the Dinosaurs.
Which is also a huge load of crap. It was stupid when TNG did it, it was stupid when Stargate did it, not just because it's bad science but because it also gives people a deeply wrong impression of how evolution works.