Europeans, as it turns out, are the fusion of three peoples—blue-eyed, dark-skinned Mesolithic hunter-gatherers, Anatolian farmers, and Indo-Europeans from southern Russia. The first farmers largely replaced the hunters (with some admixture) all over Europe, so that six thousand years ago, populations from Greece to Ireland were genetically similar to modern Sardinians—dark-haired, dark-eyed, light-skinned. They probably all spoke related languages, of which Basque is the only survivor.
About five thousand years ago, Indo-Europeans arrived out of the East, raising hell and cattle. At least some of them were probably blond or red-headed. In northern Europe they replaced those first farmers, root and branch. Germany had been dotted with small villages before their arrival—immediately afterwards, no buildings. Mitochondrial variants carried by 1 in 4 of those first farmers are carried by 1 in 400 Europeans today, and the then-dominant Y-chromosomes are now found at the few percent level on islands and mountain valleys—refugia. It couldn’t have been pretty.
In southern Europe, the Indo-Europeans conquered and imposed their languages, but without exterminating the locals—even today southern Europeans are mostly descended from those early farmers.
In other words, the linguists were correct. For a while the archaeologists were too: V. Gordon Childe laid out the right general picture (The Aryans: A Study of Indo-European Origins) back in 1926. But then progress happened: vast improvement in archaeological techniques, such as C-14 dating, were accompanied by vast decreases in common sense. Movements of whole peoples—invasions and Völkerwanderungs—became “problematic,” unfashionable: they bothered archaeologists, and therefore must not have happened. Sound familiar?
The picture is clear now due to investigations of ancient DNA. We can see if populations are related, or not; if they fused, or if one replaced another, and to what extent. We even know that one group of ancient Siberians contributed to both Indo-Europeans and Amerindians.
We also know that modern social scientists are getting better and better at coming to false conclusions. You could blame the inherent difficulty of a historical science like archaeology, where experiments are impossible. You might blame well-funded STEM disciplines for drawing away many of the sharper students. You could blame ideological uniformity—but you would be mistaken. Time travelers bringing back digitally authenticated full-color 3D movies of prehistory wouldn't fix this problem.
Their minds ain't right.