[AlexanderHowl]

Homo sapiens have been around for 500,000 years, so there is plenty of opportunity for bored aliens to steal Homo sapiens pets and abandon them when their civilization collapses (that is the premise of Traveller, after all). You could have convergent evolution keep two different populations of Homo sapiens from diverging into different species, though Homo sapiens seems to be a remarkably stable species (Australian Aborigines were separated from the rest of humanity for 60,000 years and experienced very little divergence from the rest of humanity). At the apparent rate of divergence, it might take 100,000 years of isolation to make a separate Homo sapiens subspecies and 500,000 years of isolation to make a separate Homo species.