There are two (or three) puzzles in European prehistory:
How the robust, low-skulled, broad-faced hunter-gatherers became more high-skulled, narrow-faced and gracile?
How the latter became brachycephalized until early modern times?
Why they have become partially debrachycephalized in the most recent of times?
The answers to these questions turn to a great extent on the questions:
1. How much is facial structure genetic and how much is it a product of one's diet?
2. How rapidly can selective evolution take place in facial structure?
3. Is the two phased punctuated change in skeletal features quoted above an accurate characterization of the fairly rich body of ancient human remain evidence in Europe?
I'm probably less skeptical of Dienekes' propositions that there are punctuated changes in skeletal structure, and that these changes are due more to genes than to diet or rapid selective evolution within a population, than I was a few years ago.
In other words, the evidence I've seen over the past few years makes the hypothesis that a group of people with a distinctive physical appearance were to a great extent replaced (with a modicum of admixture blunting the trend) by a new group of people with a different distinctive physical appearance who had a technological and/or cultural advantage over the indigenes, on multiple occasions in prehistory, including one or two in Europe.
Plop an average 21st century American a la "The Magic Treehouse" books into a setting at the boundary between hunting-gathering lifestyles and farmings ones at the Neolithic transition in Europe (or for that matter, Bantu expansion in Africa), and he'd say that the newcomers and the existing population belonged to different races that were distinguishable on sight, at least, prior to significant admixture of the populations. Indeed, the differences in appearance were probably at least as stark as those between any two modern human populations in existence today.
Sure, there have been instances of dramatic cultural and/or linguistic change that are elite dominates and culturally diffused (Hungary's adoption of the Hungarian language, and Turkey's adoption of the Turkish language, and various mass religious conversions to Christianity, Buddhism and Islam are all examples).
There are also some examples of relatively equitable mergers to two different ethnic/racial groups, such as the merger of Austronesian mariners from Borneo and Bantu farmers from West Africa into the roughly evently admixed population of Madagascar (although linguistically, the Austronesians were dominant).
But, the transition from hunting and gathering to farming increasingly doesn't look like one of those historical moments in most of the places that we observe it.
The second, somewhat more subtle, transition that Dienekes' identifies is a plausible fit to a Bell Beaker/Indo-European transition era stone age to copper age transition in Europe. I've made the case in previous posts that this transition is the source of the Y-DNA R1b v. R1a divide in Europe, and that both haplogroups were probably rare before the transition, when Y-DNA haplogroups like G2a were more common. I've also pondered, although not with very solid evidence, if Y-DNA haplogroup I might be a strong candidate for a leading Paleolithic Y-DNA haplogroup.
The physical anthropology discipline of studying differences in skulls and other skeletal traits has something of a tainted reputation in the field, not the least of which because of its embrace of race-like concepts that American academics have memetic antibodies to considering seriously as science given the disasterous results that flowed from taking these ideas too seriously in the 18th and 19th centuries. But, that doesn't mean that the concept of migrations by and conflicts between "Peoples", rebranded as "demic diffusion," is an entirely useless construct as a first order approximation of the historical reality.
The abstract Dienekes cites on the latest study in this field is suitably cautious in framing its conclusions and transparent in revealing the assumptions upon which the conclusions are based.
Human Biology vol. 84
Cranial variation and the transition to agriculture in Europe
Ron Pinhasi, Noreen Von Cramon-Taubadel
Debates surrounding the nature of the Neolithic demographic transition in Europe have historically centred on two opposing models; a 'demic' diffusion model whereby incoming farmers from the Near East and Anatolia effectively replaced or completely assimilated indigenous Mesolithic foraging communities and an 'indigenist' model resting on the assumption that ideas relating to agriculture and animal domestication diffused from the Near East, but with little or no gene flow.
The extreme versions of these dichotomous models have been heavily contested primarily on the basis of archaeological and modern genetic data. However, in recent years there has been a growing acceptance of the likelihood that both processes were ongoing throughout the Neolithic transition and that a more complex, regional approach is required to fully understand the change from a foraging to a primarily agricultural mode of subsistence in Europe.
Craniometric data have been particularly useful for testing these more complex scenarios, as they can reliably be employed as a proxy for the genetic relationships amongst Mesolithic and Neolithic populations. In contrast, modern genetic data assume that modern European populations accurately reflect the genetic structure of Europe at the time of the Neolithic transition, while ancient DNA data are still not geographically or temporally detailed enough to test continent-wide processes. Here, with particular emphasis on the role of craniometric analyses, we review the current state of knowledge regarding the cultural and biological nature of the Neolithic transition in Europe.
The quote from the paper that Dienekes' analyzes and comments upon is also a pertinent one that clarifies the study's conclusion:
"Nonetheless, the craniometric analysis allows us to discern certain patterns. For example, the ‘Forest Neolithic’ specimens are clearly much more similar to other Mesolithic hunter-gatherers than to Neolithic farmers in terms of their craniometric shape, suggesting a large degree of cultural diffusion in this region. However, it is also evident that the earliest potential colonisers of southeast and central Europe are very similar to the Anatolian Çatal Höyük population, congruent with an initial demic diffusion from the Near East/Anatolia."
The "Forest Neolithic" included pottery-using groups of eastern Europe (hence Neolithic, since pottery is one of the hallmarks of that period), but should not be confused with the early agriculturalists who apparently practiced farming without pottery early on in the Near East and Greece, and then acquired pottery and expanded with it into the rest of Europe, together with their full "package" of domesticated crops and animals.
Thus, in the case of the forest people, there was cultural diffusion, while in more prime farming lands, immigrants largely replaced the native populations.