The 10,000 Year Explosion

4 minute read

I want to point people interested in recent human evolution to a new book, The 10,000 Year Explosion: How Civilization Accelerated Human Evolution.

The authors, Gregory Cochran and Henry Harpending, are good friends of mine, and I have worked with them on some of the material covered in their book. So, you can hardly expect me to give an unbiased review!

However, I have now heard from a number of people not connected to the authors, who have read the book and enjoyed it. So it’s not just me.

T. J. Kelleher reviewed the book in SEED, bringing out several interesting points:

Cochran and Harpending also find value in such work [as the Genographic Project], but they argue for a fuller appreciation of the geographic distributions of genes, and in doing so, they herald a new era not only in biological anthropology, but also for history. They do not stop with what information about human history can be found in the genes, precisely because many gene variants are not neutral. Where the usual geographical analysis treats the distribution of genes as an effect of history, in Cochran and Harpending's view, the genes themselves are a cause: Two variants in the same gene do not necessarily have the same effect, and the relative selective advantages and disadvantages of them will??not surprisingly, to anyone versed in evolutionary biology??influence the movements of genes through populations over both space and time.

That’s a very ambitious agenda. On the way there, the book covers several topics of great interest to me. Naturally recent evolution by natural selection, particularly in post-agricultural populations, comes to the fore. The possible introgression of genes from Neandertals, as another source of possible adaptive variation in recent human evolution, also gets a chapter.

With this background in place, Cochran and Harpending explore some hypotheses that may link the distinctive histories of human groups to recent genetic changes and exchanges. One is the expansion and dispersal of Indo-European languages, a series of events that anthropologists have tried to connect to a jumble of different factors, ranging from conquering hordes of steppe nomads to conquering hordes of Anatolian farmers. Cochran and Harpending suggest that pastoralism and the resulting population growth connected to milk consumption was the prime mover.

Another hypothesis connects the psychometric literature on Ashkenazi Jewish people to some of the distinctive genetic disorders common in that population, such as Tay-Sachs disease, Gaucher disease, torsion dystonia and others. In a nutshell, Cochran and Harpending suggest that natural selection for general intelligence has occurred during Ashkenazi history, resulting in a distribution of IQ between 0.5 and 1 standard deviation above the European mean.

I can just imagine many readers twisting in their chairs when reading this chapter. And they should: relying upon both documentary evidence and whole-genome surveys of variation Cochran and Harpending puncture several myths about Jewish history, psychometrics, and admixture of populations. In the past, human geneticists have been all-too-willing to believe completely bogus scenarios of population history. The idea that Ashkenazim underwent a severe and prolonged population bottleneck, completely isolated from the surrounding European population, is one of the most pernicious of these scenarios. Cochran and Harpending’s hypothesis, that alleles causing sphingolipid storage disorders were positively selected in Ashkenazi populations of the last 1500 years, is plausible and certainly testable. Plausibly, these alleles may have been selected for their roles in some other function, although none suggest themselves. The bottleneck theory, on the other hand, is not plausible, refuted by both the historical record and the genomic variation of living people of Ashkenazi descent.

I found the book to have a good combination of humor, interesting anecdotes, and description of new science. I’ve read most of the recent popular books about human evolution or genetics. To me, this one stands above the others. Maybe that’s because I’m already thinking hard about the central proposition – indeed the subtitle – that “civilization accelerated human evolution.” Like I said, I’m hardly unbiased.

But I think it’s mostly more fun than most other genetics books. Some science writers cover their tentative approach to genetics by using dark, brooding prose. This book doesn’t suffer from ponderosity, and its organization helps – divided into dozens of little stories with odd historical facts, it’s the kind of book you can stash in your bag for bus rides.

UPDATE (2009-02-19): I wanted to mention that Cochran gave a wide-ranging interview about the book to 2blowhards.

I had fun reading the interview because Cochran’s suffer-no-fools attitude toward purely speculative ideas about recent evolution. He’s looking for testable ideas, not mere generalizations. My favorite quote, with reference to an idea about behavior and mythological characters:

I think this line of analysis is about as sound and solid as Citibank.

Harpending also makes an appearance worth quoting, referring to the question of how much of the book is scientifically established and how much is speculative:

The basics are secure -- population genetics, demography, history, etc. But there are certainly a number of hypotheses we have that are not solidly established. But that is the way science works. If something were rock solid it would be widely known and would be too boring to talk about in the book. We don't spend a whole lot of time for example on malaria defense polymorphisms.
We really hope to see our hypotheses tested, maybe modified, maybe falsified, or not. We don't believe them in any strong sense. Whenever you read a scientist who is deeply committed to his or her ideas, hang on to your wallet!