Actually the spaceships are unmanned. They have sensors good enough for virtual reality of all the existing species.

Earth at this time is sort of a time-reserve within a time reserve. The previous human civilization got very advanced and was met by the existing multi-species civilization. That human civilization moved about one billion year in the future to allow the earth to be time-reserve for that time, with minimal messing up of the fossil record.
However, humans already existed before they evolved. Humans are one of the species who have an apparent origin. Most "alien" species have an apparent origin too, but a few do not seem to have an origin at all. Strangely enough, the Dublis seem to have two apparent origins on two totally different planets, with totally different evolutions. There are more pets who also seem to have multiple apparent origins.