Friday, October 16, 2009

Gregory Cochran and Henry Harpending: The 10,000 Year Explosion

Gregory Cochran and Henry Harpending's The 10,000 Year Explosion talks about how humans have evolved over the period since we've been relatively civilized. They explicitly want to challenge the (sometimes vociferously propounded, but seldom cogently defended) notions that humanity hasn't evolved significantly since homo sapiens emerged, and that there's no significant genetic difference among populations in different places. They open Chapter 1 with quotes from Stephen Jay Gould and Ernst Mayr to demonstrate that they aren't fighting a straw man.

Mostly, this notion seems to be held in order to defend a liberal notion of equality, as if we could only defend equal treatment if we all have equal abilities, endowments, and if none of our observable differences are innate. If this notion isn't supportable, we'll have to be clearer that equal treatment is right for other reasons, and perhaps we'll have to be articulate about what those other reasons are. But that's an argument that Cochran and Harpending leave for someone else.

Cochran and Harpending's argument here is that there are quite a few differences between different modern populations, and that many are clearly genetic in origin. They first go to some pains to show that genetic changes can easily arise in this kind of time period. Particular examples are dogs, which have evolved all their modern variety since separating from wolves only about 15000 years ago, and domesticated plants which have changed enormously since the end of the last ice age 11,500 years ago. There are particular changes in humans that are also clearly of recent origin including skin color, eye color, lactose tolerance, and resistance to various diseases, all of which can be shown to be related to geography and to react to evolutionary pressures on much shorter time scales.

With that as background they make a couple of (they expect) radical arguments, and explain a few things that seemed puzzling before. Their first radical argument is that modern humans probably interbred with Neanderthals in Europe, and that therefore, the populations that left Africa probably got a significant contribution from them. I thought they did a reasonable job of demonstrating opportunity, plausibility, and some indications from recent genetic studies that some variations were introduced in the right time frame to have come from Neanderthals. This argument was presented as if the authors expected people to be "outraged at the charge", but it seemed sensible and plausible to me.

The radical argument that I expected to have trouble with is the claim that there's something genetically different about the Ashkenazi Jews. One of the members of the reading group I attend has been making this point for years, and I've been passively resisting it for just as long. Well, Cochran and Harpending put an end to that, easily and without much fight. They showed that the time-frame isn't extreme, that there were sufficient environmental pressures to push for particular changes, and that there's a reasonable case that the Ashkenazi were genetically isolated for long enough for the hypothesized changes in intelligence and susceptibility to diseases to have arisen. They add in some evidence that the diseases specific to the Ashkenazi are tied to genetic changes in neuron development to hammer home the point that the changes in brain function and disease susceptibility are probably tied together. The fact that we already knew (even if we didn't admit it in discussions of evolution) that Tay-Sachs is specific to Ashkenazi and is of genetic origin helps cement the case that evolution has continued into the modern era.

If there had been any remaining doubt that there are genetic predispositions to varying intelligence by race, this pretty clearly puts them to bed. I still agree with the sentiments Les Earnest expressed in his 1989 article Can Computers Cope with Human Races? It's not clear that there's anything useful to be done with this fact, and it's pretty clear that many people mis-apply the fact, but it's a fact none-the-less.

Explosion makes a fairly strong case that evolutionary stasis doesn't happen without a static environment. Humanity hasn't been static over the last 100,000 years--there have been many changes in our way of life over that period, and the changes keep accruing faster than evolution has been able to catch up. Our bodies are still adapting to changes in diet since the agricultural revolution and continuing changes in the sources of our food. Our susceptibility to disease has varied dramatically across populations, and the differences haven't settled down yet. There are still vast differences in hygiene between first world and developing nations and many places in the world where populations are sparser and provide fertile ground for new diseases to arise or transfer to human hosts. There is less of a case for sufficient continuing genetic isolation to drive differing evolutionary pressures for intelligence, but there is certainly pressure for differing intellectual capabilities than were selected for 200 or 500 years ago, much less 1000 years ago.

We haven't finished adapting to the civilization that surrounds us, and the form of our civilization continues to change. We shouldn't expect continuing evolution to be visible on a human time scale, but we shouldn't be surprised that many of the differences among people can be explained as the effect of different evolutionary pressures on our ancestors. In some cases, like disease susceptibility, we can take advantage of it if we stop treating it as tainted information. In others, as race, we should in most cases pay more attention to the abilities of individuals, rather than to the predispositions predicted by apparent racial categories.


William H. Stoddard said...

It surprises me to see sparser populations mentioned as the breeding ground for new diseases. I haven't studied epidemiology in any depth, but my understanding was that cities tend to be the big breeding grounds for new diseases, for several reasons: they have large populations crowded together, they engage in trade and have regular foreign visitors, they're often build in unhealthy river bottom environments, and they keep livestock whose diseases organisms can mutate and afflict human populations (such as swine flu and bird flu). What factors do Cochran and Harpending think make low-density populations disease sources?

Chris Hibbert said...

Hmmm. This was my own loose summary of the continuing evolutionary environment they describe. I can't find a place where they talk about lower population density as a wellspring of new or newly introduced diseases.

But I think this is right, even though the accepted version is that denser populations are reservoirs of diseases, and locations where existing diseases recombine and mutate. But I think it's away from major cities where new diseases arise or cross over to human hosts.

People aren't living in close proximity to their livestock in major cities, though it is poor farmers, not pastoralists who act this way. I think if you look at the examples of AIDS and swine flu, you'll see that they weren't introduced to humanity in the cities, though the cities are where they breed and spread the best.

Cochran and Harpending do point out that pastoralists have lower resistance to disease because they aren't exposed to the virulent conditions you cite. This may contribute to their ability to host new cross-overs before they've evolved features explicitly adapted to attacking humans. They also point out that when the old world explorers visited the new world, new diseases traveled both directions (Syphilis is the primare example.) The pastoralists had so much lower resistance in general so the consequences were more devastating.