David Reich Speculates

Fascinating podcast interview with paleogeneticist David Reich. The intereviewer asks Reich a lot of questions that elicit the answer, "I don't know" or "this isn't my area of expertise." But Reich must have been in an expansive mood, because after making his qualifiers he goes on to speculate, sometimes wildly, about what might be.

The most interesting theme concerns the role that comparatively small groups have played in human evolution. On the one hand, it seems that modern Eurasians and Native Americans are mostly descended from 1,000 to 10,000 people who left Africa around 55,000 to 60,000 years ago. These people, the model says, outcompeted not only the Neanderthals and the Denisovans but other groups of fully modern humans who had been in Eurasia for 50,000 years. On the other hand, 21st century humans have Neanderthal and Denisovan DNA, and DNA from other groups of modern humans, and, as Reich says about twenty times, the story of our past is always the story of movement and mixing. So what does it mean that we are "descended" from some group or other? Reich makes it clear that he does not understand this at all. The interviewer asks if we know where the group from which we mainly descend was at any point in time, and Reich basically answers no. Nobody knows how to model a past in which we always lived in small groups but somehow shared genes around the world.

He even says at one point that from one perspective you might say that nobody from outside Africa is really a fully modern human. Instead, he says, you could model our past as Neanderthals who are genetically overwhelmed by one wave of modern humans after another, gradually evolving to the point that they became 98% modern human without ever ceasing to be Neanderthals. In support of this view he mentions that while our genome is only 2% Neanderthal, if you go back 50,000 years ago our ancestors are 10-20% Neanderthal. It turned out, though, that Neanderthals had built up a lot of bad mutations over 500,000 years of living in small groups and those variants were quickly selected out.

Reich clearly feels that we do not understand human evolution.

Some other points:

Over the past 10,000 years, our genetic changes have been mostly related to disease resistance and metabolism. Lactose tolerance would be a great example of a metabolic change, but Reich uses another one: we have gotten less efficient at storing fat. This is presumably because farmers have diets that vary less over the course of the year. This was in response to a question about the argument that farming was a big mistake that has made life worse, and Reich basically says there is no evidence for increased dietary stress. In the course of this Reich mentions that there is very little evidence for any recent genetic changes related to cognition; people of 50,000 years ago seem to have had modern brains.

The relationship between culture and genetics is very complex. Reich describes cases in which cultures have been taken over genetically by people from outside, who can be detected only genetically, not because of any cultural or linguistic changes. One is the Polynesians. They came from Taiwan and were genetically East Asian. Then the Melanesian islands they settled were infiltrated by people from New Guinea, who took up Polynesian sailing and so on, leaving a massive genetic imprint on what remained a Polynesian culture.

The evidence seems to show that at the end of the Neolithic, around the time of the steppes invasion, plague was causing more than a quarter of all deaths in Europe. Reich says nobody will say this out loud because it seems so crazy, but it is what the data says.

In India, as in Europe, the population was changed by the steppes invaders who brought Indo-European languages and left a strong genetic signature. But whereas in Europe that percentage is about the same in every modern person, in India there are huge variations, based on caste, because the resistance to marriages between people of different castes has been strong enough to block most interbreeding for 2,000 years.

It is very weird that farming appeared all over the world within about 4,000 years of the end of the last Ice Age, and nobody can explain this. One theory is that the current interglacial era is a uniquely beneficial environment, which to me implies that we are riding a climate lucky streak that can't last too much longer.

There is no chance that there were Sumer-level civilizations before the end of the last Ice Age.

But the weirdest thing concerned attempts to study past epigenetics. Genomes are in practice always being modified by other molecules that shut down some genes and pump up others. Reich says there is some evidence of epigenetics from very old genomes, and the biggest difference anyone has noted is that in us, the genes for language are much more active. It seems that DNA evolution has not been fast enough to give us the language skills we need for our lives, so our bodies have resorted to other tricks