We ought to be careful when ascribing an attitude like xenophobia to all of humanity, because even though it exists and even if we assume it has always been commonj, it's beside the point when it comes to the plausiblity of "race". The question isn't what people say or even think about people who are oustide their group The question is whether they'll *do* individual memebers of outside groups. And when you use genetics to rip the covers off what people have been actrually *doing* (as opposed to saying or even thinking), what you discover is what should be an unsurprising fact: they've been having sex with people they aren't supposed to be doing it with. Lots of it. For a very long time. Everywhere you look.
So you may pick a small number of anatomical or genetic features and find a geographically coherent group where practically everyone has them. But that's *all* you've found: a geographic cluster of certain traits. You can choose a different set of traits that makes the group look very diverse. That means you have *not* found a large group of people who have descended more or less exclusively from some small primordial subgroup of humanity. Such a thing evidently doesn't exist.