Something they might of done on many other worlds in the past. The idea that they created us and then guided our evolution with 'upgrades' (not ever mentioned in the film, but strongly suggested by early drafts of the script as well as Arthur Max and co.) then it stands to reason that they could of 'engineered' humans on planets all over the galaxy. We might be just one batch of many going back in time.
Still not sure what the goo meant for Earth was intended to do, either kill us or 'upgrade' us. The film to my mind suggests the latter, what with the whole 'destroy to create' thing.