There is not much original in this book, which summarizes the results of ancient DNA studies over the past decade. Reich has been in the thick of those discoveries, first as part of the team that showed Neanderthals and modern humans had interbred, then as the leader of a lab that has read and analyzed a huge amount of DNA. His writing is not great but it is good. You may find some of his technical explanations hard to follow; I did, and have been reading about this stuff for years. My advice is that if you get bogged down in a particular passage, just skip it and go on. If you do that, you should be able to get a huge amount of information out of this book however minimal your background in genetics or statistics.
I have been writing here all along about the big discoveries, which I would say are these:
- Modern humans interbred with Neanderthals, and outside of Africa human genomes are on average 1 to 2 percent Neanderthal.
- Modern humans in East Asia also interbred with another lineage of archaic humans we call Denisovans, although as Reich shows in this book the identity of Denisoavans is a complex story.
- Our modern races are not ancient branches of the human tree, but the result of mixing that has taken place since the origin of agriculture; 20,000 years ago Eurasia was home to a different assortment of races, and the first farmers of Syria were as different, genetically, from those of Iran as Chinese and Welsh are today.
- There are in the Eurasian family tree groups with no clear obvious descendants today, such as "ancestral West Eurasians"; we call these "ghost populations".
- Modern Europeans are descended from three quite different ancient groups: the Paleolithic hunter-gathers of old Europe, the farmers who migrated from the Middle East beginning around 9,000 years ago; and invaders who came in from the Steppes during the Bronze Age.
- The people of India are a mix of an ancient population ("Ancestral South Asians"), farmers who migrated from Iran during the Neolithic, and West Eurasians; the West Eurasian invaders seem to have come in multiple waves beginning in the Bronze Age. As you would expect by looking at people in India, the mixture of West Eurasian ancestry is much greater in the north, but there is some admixture even in the far south. This mixture skews male, indicating that the invaders were majority male, or else that they did their most successful breeding with local women.
- East Asians are a mixture of an ancient Yangtze Valley ghost population, which spread after the invention of agriculture, with numerous other ghost populations, in particular one in the Yellow River Valley; modern Han Chinese originated within the past 5,000 years from a mixture of those two and possibly other groups.
One of the anti-racist points Reich makes in both his book and his Op-Ed is that modern races are all mixtures of ancient races. There are no pure racial types; miscegenation is our legacy and our origin. It is true that some groups have remained relatively pure for the past 3,000 years or so, but compared to the vast sweep of human evolutionary history that just isn't very long.
In other words, the vast movements and upheavals of modern times – migrations, conquests, the rise and fall of empires, the disappearance of old peoples and the appearance of new ones – are not an aberration. The medieval world seems to have experienced a few thousand years or so of comparative stasis and endogamy, leaving us a legacy of people who think they belong to a pure race (Koreans, Japanese, Han Chinese, Germans, Jews, Celts). But that only goes back a certain distance, and we now have the tools to peer back much farther. In that long view, migration and mixing are the human norm.