Abstract

The 'golden age' of racial admixture is long past us.
It occurred after the Ice Age and before the alphabet, on the edge of human history.

Main Text

Aeon has an interesting essay up, The future is mixed-race[1]​.
The subtitle is "And so is the past.
The Migration and mingling are essential to human success in the past, the present and into the future." It seems that the essay is adapted from the author's book, Future Humans: Inside the Science of Our Continuing Evolution[2]​.
Much of it is solid science, but there is one element that I think is missing: awareness of how ancient DNA has revolutionized our understanding of human population structure.
The thumbnail sketch is rather simple.
Between the end of the Ice Age, and more likely the beginning of agriculture, and ending two thousand years ago with the Bantu Expansion, a series of demographic pulses and admixture events (and replacements and assimilations) reshaped the entire landscape of human genetic variation, to create the patterns we take for granted today.
For example, it looks as if Northern Europe was totally reshaped between 4 and 5,000 years ago[3]​.
Indian population genetic structure as we understand it today seems to be of a similar date[4]​.
Meanwhile, in the Near East there also seems to have been a major change in the last 5,000 years[5]​.
One thing clear from ancient DNA is there was once far less gene flow between nearby populations.
The genetic variation between groups was relatively high.
With the transition to history we see an incredible amount of gene flow and admixture between nearby populations.
Some of this was replacement, and some of this was assimilation.
By the time that "history" as we understand it is in full the landscape was set, with only minor changes in comparison.
Aside from the Bantu Expansion the migrations mentioned in the Aeon piece are not very significant in comparison to the prehistoric events.
What happened with history to change the rate of change? First, there is only so much genetic difference you can homogenize.
Once the between population genetic variation drops to ~0.01 you may be near equilibrium for some distances.
Second, I believe that the emergence of complex cultural systems which allowed for elites to extract rents out of agricultural populations incentivized conquering groups to allow the local groups to persist, as opposed to exterminating or absorbing them.
Another major issue with the Aeon piece is that the census sizes today are so large, that I believe that the admixture you see around us simply can not be as large an impact relatively as the ones that occurred during the mid-Holocene, with humans numbered in the tens of millions at most.
Today there are more than one billion Han Chinese.
There are simply too many Han, too many South Asians, for a panximia to result in the predominant change of the demographic character of the human race.
Note here I am not saying that admixture will not occur, or that the rate is not going up.
We will see more diversity in the 21st century than we've ever seen in the history of the human race because of international travel.
But, the mid-Holocene homogenization is something that we'll probably never recapitulate.
Imagine, for example, that all Northern Europeans and Han Chinese mixed together, so that there were no "pure" Northern Europeans and Han Chinese left, and a mixed population created a new identity.
That is what occurred during the Holocene homogenization....