With the surfeit of genomic data, whether contemporary or ancient, there is a lot of mileage to be gained by description and inference. That is, looking at the data, generating a result, and drawing some conclusion from that result. But another way to skin the cat is construct an explicit model and then test the data. There are details, and then there are generalities.
I’ll offer up a proposition here then: the transition from hunter-gatherers to agriculture and pastoralism has increased the rate of gene flow between neighbor populations. Several years ago Science published ancient DNA results which showed that there was little gene flow between Mesolithic hunter-gatherers and Neolithic farmers in Germany. Those trained in population genetics will know that only a small rate of gene flow can quickly homogenize difference between neighboring groups. Large genetic distances between neighboring populations requires strong taboos in relation to intermarriage.
It also happens that this summer I saw a poster presented by Anders Bergstrom at SMBE where he reported very high genetic distance values in the highlands of Papua New Guinea. Ethnographically we’re well aware that New Guinea is characterized by high degrees of linguistic diversity as well as xenophobia an war between neighboring groups. But a deeper dive into the genetic patterns suggest common descent in New Guinean from a random mating population on the Pleistocene-Holocene boundary, and more recent barriers to contact. New Guinea’s agriculture, which is gardening horticulture, is somewhat different from cereal cultivation. So there may be some differences there which we need to explore.
But something happened in the Holocene. In Game of Thrones “guest right” is sacred. That may seem like a silly observation, but the same principle is the clear in the Bible. The visitation of the angels to Sodom saw an attempt by the natives of the city attempt to violate the hospitality offered by the family of Lot, to the point where Lot offered his own daughters to the men who aimed to rape the angels.
The most recent genetic work suggest that the past 5,000 years or so have seen massing mixing across the world, and reduction of inter-group genetic distances. This is clearly the consequence of rapid increased rates of gene flow. You can take a cultural evolutionary viewpoint for the reason behind this, as Ara Norenzayan does in Big Gods: How Religion Transformed Cooperation and Conflict. Or you can take a more traditional materialistic route which puts the causal agent in the hands of mechanistic processes having to due with increased density and economic complexity. But whatever the reason, we know a transition occurred.