I intended yesterday to write a review of Gregory Cochran and Henry Harpending’s book, The 10,000 Year Explosion: How Civilization Accelerated Human Evolution. It is a book that I heartily recommend – particularly to those who are not already inclined to sympathy with “genetic-deterministic” arguments, precisely because it is a very, very different sort of book from the sorts of books you might have read.
Darwin remains scandalous because the theory of evolution appears to threaten human distinction from the natural world (not to mention, of course, contradicting the literal accounts of creation). But the really scandalous thing about Darwin’s insight was not that he saw kinship between humans and other animals (plenty of others noticed that kinship), nor that he was a materialist (there were materialists all the way back to the ancient world), but that he articulated a simple mechanism by which evolution could take place, and that this mechanism was an exceedingly ugly one: natural selection.
People in Darwin’s own day tended to hear his arguments and think he was explaining how we evolved “up” from “lower” animals, and felt relatively reassured. But there is no “up” in evolution, except in two trivial senses: there is a directional arrow in evolution (later forms evolve from earlier ones), and there is the possibility of increased complexity over time, an apparent reversal of entropy (apparent because it is in fact underwritten by input of energy from the sun). On one level, we’re the “pinnacle” of evolution because we’re at the top of the food chain, are extremely numerous, are able to adapt to numerous environments, and are relatively late arrivals on the scene. But from another perspective, the “pinnacle” of evolution is the horseshoe crab, because it has survived largely unchanged for hundreds of millions of years. More to the point, the very things that we might point to as evidence of our “success” – large numbers, broad expansion across the planet – are precisely the characteristics of “weedy” species that we generally despise in the rest of creation. That fact in and of itself should give us pause in deriving any kind of transcendent value judgement from the workings of natural selection.
In any event, Darwin’s idea was simple and explosive, with repercussions that we are still assimilating. The deepest of these is the very conservative conclusion that victory in biological war is impossible. We cannot defeat disease, for example, merely hold it at bay, because our most successful efforts are precisely those that put selection pressure on microorganisms that will result in new diseases able to thwart our countermeasures. It remains to be seen whether we will be able to defeat our own weedy nature or whether we will ultimately return to the Mathusian world from which we only recently emerged. We may know a negative answer any time, but a positive answer we will never know for sure – certainly not until many generations have been born and given birth in our world of medically-controllable fertility.
So: what is special about Cochran and Harpending’s book is that it tackles human history – the period of the last 10,000 years – from a real Darwinian perspective. This is not a pre-Darwinian story of struggle – how “what did not kill us made us strong” – nor is it a series of “just so” evolutionary-psychology parables. Rather, it’s a story about how chance mutation can have massive historical consequences when that mutation turns out to be meaningfully adaptive.
There are really only a handful of very simple concepts at the heart of the various stories:
- Agriculture increased the overall size of the human population. More people = more mutations. Mutations are the building-blocks of evolution. Hence, with more mutations, all else being equal human evolution should have accelerated, not slowed, in the last 10,000 years.
- Not all individual human beings are equally well-adapted to all environments. But human beings in general are able to survive in a wide array of environments. Therefore, having spread to a variety of different environments, different groups of human beings have been subject to very different kinds of selection pressure by different environments. The fact of human adaptability doesn’t cut against arguments that human evolution has accelerated – our adaptability is a driver of accelerating evolution. And social environments are environments; they impose selection pressure just like any other environment.
- Adaptive mutations can spread with shocking speed precisely because they are adaptive. Relatively small differences in fertility – like relatively small differences in investment returns – compound quickly to massive advantages in population size.
None of these ideas should be especially controversial scientifically, but, like Darwin’s original dangerous idea, none are especially intuitive either. What Cochran and Harpending do with these ideas is explore various ways in which human history may have been shaped by evolution during historic time. This is why I say the book is very different from other books about sociobiology or a Darwinian view of human nature: those books tend to be focused on pre-history, about how human nature as it is today owes this or that to our primate ancestry, not on how we have changed, biologically, within the span of human history.
There are a great many fascinating digressions, but three big speculations stand out:
- That a meaningful genetic inheritance from our Neanderthal cousins materially contributed to the cognitive leaps forward that made possible art, language, agriculture, etc.
- That the development of lactose tolerance was the main driver of Indo-European expansion across much of Eurasia (and, subsequently, Bantu expansion in Africa).
- That high average Ashkenazi intelligence (as measured by IQ tests) is the result of selection pressure caused by the unique social niche of Jews in medieval northern Europe.
All of these are, as I said, speculations – in none of these cases do the authors present anything resembling the kind of evidence that would pass peer-review. Of course, that’s not the point of a book like this – their book is a work of popular science writing, and, as it should be, it is eminently readable and not at all too technical for the intelligent and reasonably informed lay reader. Anybody who could follow Guns, Germs and Steel or Before the Dawn can read this book with pleasure and interest. But precisely because it’s a work of popular science writing, it is somewhat problematic that so much of the subject matter is highly speculative. This is, generally, the kind of book you’d write to summarize developments in a particular discipline, and then conclude with some speculation about where the field might go next. But this field has only barely come into being, and the research needed to validate any one of the big-ticket hypotheses above would cross a number of disciplines and require years of work. Moreover, as Stephen Pinker pointed out in an article about the scholarly paper that formed the basis for the Ashkenazi IQ chapter, these sorts of hypotheses depend on getting a whole series of debatable points right – and they all have to be right for the hypothesis to work out. That makes validation much harder, but it also means that the odds are, pretty much by definition, long.
Which is all the more reason for more researchers to do work in this emerging field, and for scholars in different disciplines – population genetics, history, anthropology, linguistics, archaeology, etc. – to talk to one another. Darwin’s Origin of Species, after all, was written before anyone had any idea what could possibly cause the variation necessary for evolution by selection to occur. With nothing but Mendel to work with, Darwin’s own grand hypothesis was wildly speculative at the time. But had he not speculated as he did, other scientists would not have explored the then-uncharted territory that now we are beginning to map. I think the authors of The 10,000 Year Explosion would be very pleased if their work led to so much research on human evolution in historic time that every one of their speculative hypotheses was roundly refuted, because whatever we did discover would validate their questions, if not their answers.