1.Well when you think about it, How long is a "Long time ago"? And Humans have existed up to 40,000 years ago, and a lont time could even bee 20,000 years ago and Humans would still be on earth.
2. Earth could still be in a galixy far far away, but what if this ship was intergalactic? It never showed up in the series, but if the found a human they would normally just take a person back to earth, but they didn't know about earth at the time because they hadn't gone inter galactic yet.
3. I must have not mentioned that he was in chryo for a long period of time on board, they did this to ensure he wasn't taught or didn't learn any more than needed.It also take hundreds to thousands of years to go between the galaxies, and the ships are quite large and fast. They would be in chryo and pick up slaves, goods, rarities from other planets, enough to last them a life time, and on thier way back another ship woul dbe headed there already. So the Travel time could take a while aswell. I'm just trying to make the story interesting, and I could go on and on about how this is possible ^_^
4.Isn't this an alternate universe?