Originally Posted by pr0xyt0xin
Loki fell off the bifrost and somehow ended up in Midgard with no explanation.
I don't think Loki was ever on Earth(except when he came to taunt Thor while he was in custody) before the beginning of TA. Him with Selvig at the end of Thor1 I think was just him astral projecting himself. He needed the cube to be a doorway to physically get to Earth so he influenced Selvig to get to work on it. At least that's how I see it.