Wednesday, June 13, 2018

Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past I

This book, by David Reich, summarizes research in the new field of the genomics of ancient human DNA. I'm about halfway through, and at first I thought it would be boring. However, it is turning out that this is science at its best, and Reich and his colleagues are making serious headway in areas that have been of interest to me for some time. By analyzing the DNA of archaeological human remains representing different populations at different locations at different times, a far more detailed picture of human migrations and interbreeding is emerging than was available a few years ago. Improvements have been made in DNA extraction, its processing and the use of statistical models. The first major result has been the demonstration that humans and Neanderthals interbred directly and that Neanderthal genes now comprise about two percent of the genome of most modern humans. Modern humans are one of five known human populations that have lived on the planet during the last seventy thousand years. The others were the Neanderthals, the Siberian Denisovans, the Australo-Denisovans and the "hobbits" of Flores island. There is evidence of interbreeding directly and indirectly between these groups. With carbon dating of the remains and genome sequencing, it is possible to map the distribution and prevalence of certain genes by location and time and associate them with specific populations, earlier and later.

The main picture that has emerged so far has been that human populations have been on the move constantly and have interbred, producing non-sterile hybrids along the way. The people whom we associate with certain geographic locations commonly have lived thousands of miles away within the last few thousand years and have interbred with other populations repeatedly. It is even possible that, contradicting the prevailing theory that most of the evolution of modern humans occurred in Africa, some of the evolution may have occurred outside Africa, with non-African populations returning to Africa over 300,000 years ago.

The appearance and disappearance of populations in Europe has been common until recently. Early groups of hunter-gatherers and farmers in Europe were replaced by descendants of the Yamnaya people from the steppe of eastern Europe about 5000 years ago. The Yamnaya were early adopters of wheels, carts and horses. The Yamnaya's descendants were the Corded Ware people, and until recently these two groups had been thought unrelated due to differences in their artifacts. However, genomic studies indicate their relatedness, which also applies to most modern Europeans. This genome is associated with Indo-European languages, the exact origin of which is unclear. Reich speculates that they may have originated in Armenia or Iran.

I skipped ahead to the chapter, "The Genomics of Race and Identity." Here, Reich attempts to sort out the controversies that arise from racist-sounding language in a politically correct environment, and he finds errors in both camps. He chides not only Henry Harpending, one of the authors of the study of Ashkenazim intelligence cited earlier, for making an unsubstantiated racist statement about Africans, but none other than James Watson, one of the discoverers of the double helix, for assuming that Ashkenazi Jews are smart. On the opposite side, he criticizes Richard Lewontin for popularizing the idea that all living humans are essentially the same, an idea that is not supported by current evidence yet is defended with such great fervor that unbiased scientists such as Reich have to edit their language carefully for fear of being attacked. Reich, himself an Ashkenazi Jew, does not believe that the Ashkenazims' susceptibility to certain diseases is related to intelligence, but he leaves the door open to the possibility that a genetic origin for high intelligence may be found. There is currently little evidence connecting specific genes with intelligence, and the genetics of cognition has not been studied thoroughly, though this is not to say that connections won't be identified in the future. In Reich's view, there are significant genetic differences between populations; the problem is that, in a reaction to the ideological oppression of political correctness, some writers have overstated their cases without sufficient evidence, hence betraying racist biases. To show that differences aren't necessarily bad, Reich uses the example of men and women as people who are very different genetically yet are able to get along. At first this seemed useful to me, but on reflection it seems like a poor example. I find that the interests of men and women are usually at significant variance, and were it not for certain biological imperatives, such as the need for sex and the desire to procreate, men and women might have little reason to interact. There are millions of men who would prefer women to live obediently in harems, and there are millions of women who think of men as ATM's. In my experience, men and women spend most of their lives complaining about each other. Gender differences aren't really the best model for a harmonious society even if men and women do manage to cooperate.

Reich does not specifically mention The 10,000 Year Explosion, which I discussed earlier, though Harpending was one of its authors. It appears that the general thrust of that book may be correct, but that many of the details may be wrong. The Yamnaya and Corded Ware cultures were not farmers, yet they seem to have displaced farmers. It is possible that they were immune to the bubonic plague or other diseases, while the earlier farmers in Europe were not. It is possible that evolutionary changes may have given the Yamnaya an advantage, but this may not have played out exactly as described in that book.

There are other chapters of interest left, and I'll have more to say when I've read them.

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