>Humans have on the order of a hundred thousand genes, depending upon how you count. So if more than one part in a hundred thousand of some individual's inheritance tree was extraterrestrial, then they likely have some genes deriving from them. In other words, a single ancestor 16 generations back (~500 years) would suffice.
>Even with a "minimal mixing" assumption, I would expect most of the population to have at least 0.1% extraterrestrial ancestry after the first thousand years. More likely a much greater amount, but 0.1% is already something like a hundred times the required threshold. Then there's another few millennia for those genes to diffuse.
By that logic, we now on Earth IRL are all a lovely shade of brown, with no particularly distinctive subgroups, given that we've had upwards of 50 000 years of sharing a common planet since the emergence of Homo sapiens sapiens. Why do you assume that human mating is some sort of brownian motion process that happens entirely randomly, without any guidance from the conscious minds humans are equipped with? Why do you assume that all of Terra is some kind of Coruscant-like city planet with no isolated populations? Why do you assume that extraterrestrial immigration was both extremely massive and completely unrestricted?