Ideologies that seek a return to a mythical purity are flying in the face of hard science. - David E. Reich

About ten thousand years ago there were at least four major populations in West Eurasia — the farmers of the Fertile Crescent, the farmers of Iran, the hunter-gatherers of central and western Europe, and the hunter-gatherers of eastern Europe. All these populations differed from one another as much as Europeans differ from East Asians today. Scholars interested in trying to create ancestry-based racial classifications, had they lived ten thousand years ago, would have categorized these groups as "races," even though none of these groups survives in unmixed form today.

People often think that Europeans are homogeneous group that arrived in a simple way there maybe 40 or 50 thousand years ago maybe based on the archaeology and just kind of sat there until they became the Europeans they are today, but that's probably not true: the Europeans today are a replacement population who came in much more recently and replaced the people who were there originally 40 thousand years ago.

We warn that models in population genetics should be treated with caution. Although they provide an important framework for testing historical hypotheses, they are oversimplifications. For example, the true ancestral populations of India were probably not homogeneous as we assume in our model, but instead were probably formed by clusters of related groups that mixed at different times. (Reich et al. 2009: 492)