This statement has always struck me as less of a hard scientific truth than a hazard warning, an expedient intellectual Here Be Dragons. Surely it was only a matter of time before the 'controversial' thoughts it was planted to ward off would be being thought again?
Cochran and Harpending appear to give it short shrift. There's a similarly sizeable variation within dog breeds than between them, but does that mean that the differences between a Great Dane and a Chihuahua are 'skin deep' too?
Their book sets out to inform us how, far from arriving on an evolutionary plateau, our first modern ancestors to break out from Africa were about to embark on one of the most accelerated phases of human biological change. Through a process of genetic introgression they began by stealing some genes from the European natives, the Neanderthals, an exchange which the pair believe may have kick-started the sudden leap-forward in artistic and technological capabilities which ensued.
Yet it was the advent of agriculture which heralded a whole new set of selection pressures, such as new diets and disease risks. Cochran and Harpender duly map the spread of lactose tolerance to the ascendency of the Proto-Indo-European language and show how new patterns of social organisation favoured alternative heritable psychologies.
So far so credible. Indeed I have a great deal of sympathy for the book's broad analysis, it's just that some of the detailed explanation is at best flimsy and at worst blood-pressure raising. This is a complex web of may haves and could haves, with the occasional must have thrown in for good measure, and one is never quite sure how the dependencies work. In other words, how long a string of may haves is holding up that must have.
We're told that humans who have been growing stuff for longer have had more time to hard code an understanding of the underlying economics into their wetware. So, it's hardly surprising, we're then informed, that 'Amerindians' find it harder to get their heads around the benefits of neo-liberalism when it comes to wealth generation.
Aside from the gobsmacking political bias behind this little hypothesis, everything we know about the Maya surely gives the lie to any notion that America's indigenes are somehow congenitally soft-headed when it comes to numbers. The Maya might have domesticated corn rather later than Europe's classical cultures had access to wheat, but it was the Romans, ingenious as they were, who had to make do without a zero, wasn't it?
I take no issue with the data showing that Jews of European origin win more Nobel prizes and score higher in IQ tests than any other distinct group of people on the planet. But Cochran and Harpender's explanation for how this state of affairs might have come about is also neither quite good science nor good history.
Yes, the Ashkenazim were a closed community forced by their host culture to specialise in finance and management. But how exactly would the selection pressure have worked in practice? Unless the less brainy Jews were somehow more prone to die off before marriage, or indeed were comparatively less likely to marry, then the whole process would depend on the cleverer sort having more children than both the intellectually-mediocre and outright dumb, something which we know to be generally counter-factual from contemporary research. (It also rather depends on intelligence being something that is passed down the male line only!)
And as ever with these retrospective arguments from natural selection, I can all too easily concoct an alternative model of my own: in the middle ages Christian education was controlled by the Church and the most promising pupils would naturally have been siphoned off into a profession which would (mostly) take them out of the procreation business. So instead of the Jews getting smarter, the rest of Europe was steadily becoming thicker!
Anyway, as ever the real problem here is the term Evolution itself. We might be able to move beyond seeing Great Danes and Chihuahuas as only superficially different, but which one is more evolved? While Cochran and Harpender are surely right to question the conventional wisdom of human evolutionary stasis, can we really say that the various detectable adaptations to the multiplicity of different environments that human beings have taken up residence within over the course of the last 10,000 years, are qualitatively the same thing as the series of species transformations which occurred before the emergence from Africa?