In light of David Reich’s interview I have been thinking about how genetics will shed light on many questions in the near future, and what my particular expectations are. The interview prompts me to collect some of my thoughts into one place, and outline a tentative thesis that I’ve been pointing to for the past few years. My friend Greg Cochran wrote The 10,000 Year Explosion: How Civilization Accelerated Human Evolution in the late aughts, and some of his predictions have come to pass (e.g., Neanderthal admixture, and likely adaptive introgression according to many analysts of the data).

Others have not, and one of those that needs to be heavily modified was the idea that a mutation for lactase persistence allowed for the Indo-European expansion. That is, the original Indo-Europeans were simply biologically superior at extracting calories from the land, and so succeeded due to that advantage (at least in large part). The ancient DNA tells a different story; the Indo-Europeans may have originated the mutation, but it came to be at higher frequency after their demographic replacement and absorption of the European first farmer populations. That is, genetics post-dated the cultural shift, rather than initiating it.

Though understanding the biological basis of human behavior remains important to me, over the past decade or so I have become more and more convinced that the missing piece of the puzzle of the last 10,000 years is about how cultural evolution produced civilization and altered patterns of human genetics, rather than the other way around. This is somewhat a change in tack for me. One of the reasons I refer to Richard Klein’s Dawn of Human Culture so much is that ten years ago the book’s thesis that a biological change in our cognitive architecture allowed for the “Out of Africa” expansion was moderately persuasive (also see Steven Mithen’s Prehistory of the Mind). My acceptance was probably inadvertently tempered by the fact that Klein seemed to have only a rudimentary idea as to the details of formal evolutionary theory, appealing as he did to punctuated equilibrium.

It may be that anatomically modern humans changed in some fundamental way 50,000 years ago. But the bigger picture seems just too complicated to reduce in this fashion right now. Rather than focus how human culture was shaped by the genes, I am now more curious about how genes were shaped by human culture.

Last year a paper was published, A recent bottleneck of Y chromosome diversity coincides with a global change in culture, which reported that it seems that a major recent change had occurred in the composition of Y chromosomes of humans. These are basically records of paternal transmissions across the generations. Ancient DNA shows that many of the very common lineages only appear to have risen in frequency ~4,000 years ago. This was of course thousands of years after agriculture. One can’t reduce this simply to a shift in mode of production, and the demographic excess of farming societies.

I’m sure most of you can anticipate where I’m going here. The rise of pastoralism, and the emergence of a mobile arms-bearing males changed civilization. It wrecked civilization, but it also created civilization as we know it. If you read The Horse, the Wheel, and Language: How Bronze-Age Riders from the Eurasian Steppes Shaped the Modern World, you would already suspect that (or, books going back to the early 20th century). But even the author of that book was shocked by the demographic impact of the Indo-Europeans as evident in ancient DNA. And, anyone who looks at star-shaped phylogenies such as that for R1a1a would have a hard time explaining what might have caused such an explosion in anything but the vaguest detail.

My answer comes from Peter Turchin in Ultrasociety: How 10,000 Years of War Made Humans the Greatest Cooperators on Earth. In War in Human Civilization Azar Gat reports that more numerous armies are more likely to win any pairwise conflicts. But in Ultrasociety Peter notes that Lanchester’s laws indicate that superiority with long range weapons on flat territory gives a much greater likelihood of victory to those groups who are more numerous than with simple near engagement on foot. The combination of this with horses to aid in mobility, and I believe you had a revolution on the Eurasian steppe where the outcomes of inter-group competition between coalitions of males became “winner-take-all” affairs.

Because of the inevitability of the drafting of the horse as a beast of burden and transport it was inevitable that the early adopters would undergo a cultural revolution, and trigger a high stakes series of inter-group competition. The winners of that elimination tournament are the Y chromosomes we see around us.* But between 2000 BC and 0 AD the winners decided to cash out, as a new stable equilibrium emerged. “Higher religion,” a shift toward monogamy, and reduced inter-group warfare due to the emergence of state monopoly on violence, was an exit strategy from the melee of the transition between the Neolithic and Iron Age (again, Peter Turchin has discussed this at length). The patriarchy forged on the steppe at the tip of the spear and on the chariot now decided to mature and accrue more cultural adaptations to prevent itself from eating its own young.

* Something similar happened between 1650 and 1850 in Europe. Who have guessed that by the 20th century English would have been the international language? First the British vanquished their Dutch commercial competitors, and slowly ground down preeminence of French political, military, and cultural power on the continent. A dynamic Europe was engaged in competition on a massive scale, and the victorious British obtained the empire upon which the sun never set.