I've been assuming in BG-new, it was the other way around: the BG humans are descendants of future Earth humans. Of course, for this to work, if they ever find Earth it can't be 21st C Earth -- there has to have been time between now and then for us to colonize other planets_and_ for them to have forgotten quite where they came from.

It would also be interesting to see how that forgetting has impacted their religion: creationism (and tehrefore religion in general) would be a lot more convincing if there wasn't any pre-human fossil record.