In astrophysics, star clusters are birthing centers for stars, fed by the collapse of giant molecular clouds. In genomics, star clusters are a bit different, but have a similar impact on births. The diversity of the Y-chromosome DNA (inherited solely from the male line) is vastly less than that of the mitochondrial DNA (inherited only from the female line). In historical times, star clusters are associated with some famous warlords, for example Genghis Khan, who seems to have passed his Y on to several million living men. His star cluster is the pattern of Y-DNA seen in present day populations.
Such star clusters are the signature of profoundly hierarchical societies, where a small number of men dominate the right to reproduce. Older and larger star clusters are known, which predate history so we can't know exactly whom the fathers were. We do know, though, that most of them lived about 5000 years ago.
From ancient DNA combined with archaeology, we are beginning to build a picture of what this inequality might have meant. The period around five thousand years ago north of the Black and Caspian seas corresponds to the rise of the Yamnaya, who, as discussed in part II, took advantage of horses and wheels to exploit the resources of the open steppe for the first time.20 The genetic data show that the Yamnaya and their descendants were extraordinarily successful, largely displacing the farmers of northern Europe in the west and the hunter-gatherers of central Asia in the east.
The archaeologist Marija Gimbutas has argued that Yamnaya society was unprecedentedly sex-biased and stratified.The Yamnaya left behind great mounds, about 80 percent of which had male skeletons at the center, often with evidence of violent injuries and buried amidst fearsome metal daggers and axes. Gimbutas argued that the arrival of the Yamnaya in Europe heralded a shift in the power relationships between the sexes. It coincided with the decline of “Old Europe,” which according to Gimbutas was a society with little evidence of violence, and in which females played a central social role as is apparent in the ubiquitous Venus figurines. In her reconstruction, “Old Europe” was replaced by a male-centered society, evident not only in the archaeology but also in the male-centered Greek, Norse, and Hindu mythologies of the Indo-European cultures plausibly spread by the Yamnaya.
Reich, David. Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past (Kindle Locations 4159-4172). Knopf Doubleday Publishing Group. Kindle Edition.So how did these relatively few pastoralists manage to sweep away more numerous farmers throughout Europe and much of Asia, including India and portions of China? My guess is that the strongly sex-biased society might have played a role. If all the females are dominated by a few elite males, the only way for the less fortunate males to leave the incel category may have been war and rapine. I expect similar factors were important in the Muslim expansion and perhaps other empires.