The 10,000 Year Explosion

by Gregory Cochran and Henry Harpending

June 19th, 2009

The authors begin with a simple question: why does everybody maintain that human evolution stopped cold once we became human say, 40,000 years ago? Shouldn't evolution be an ongoing process?

The most common answer is that evolution proceeds at a stately pace and that there have not been enough human generations (about 2000 since 40,000 years ago) to permit a lot of change. However, we now know that evolution can proceed much faster, depending on the magnitude of the selection effects. 2000 generations is more than enough to handle a lot of change. Moreover, the authors point out a crucial factor: population size plays a huge role in evolution because a large population provides a larger mutation pool from which to choose. In other words, as humanity has grown in size, its ability to evolve has increased. The authors present the mathematics of this phenomenon to demonstrate that it really does work out.

The second answer is that selection pressures have not been strong enough; society takes care of weaklings in so many ways that small genetic deficiencies are not weeded out. This argument evaporates upon examination. Certainly male humans do a good job of monopolizing sexual access; plenty of studies show that powerful males have more children and more of those children grow to adulthood. Selection pressures, at least among males, are if anything stronger than they were during hunter-gatherer days.

There's a third, unspoken answer: we don't like to contemplate the notion that some people might be genetically superior to others. I find this kind of intellectual cowardice to be despicable. We follow the truth wherever it takes us. Our society spends a great deal of time figuring out who gets the goodies and who sweeps the floors; I doubt that the revelation of a genetic advantage will have much impact. Remember, men are genetically built to be taller than women, but there are a lot of women who are taller than many men. It might work for the group, but it doesn't work for the individual.

After dealing with the general theoretical considerations and demonstrating that humans are likely to be still evolving, the authors turn to specific cases. There are, it turns out, dozens if not hundreds of various mutations scattered through the human gene pool. An obvious one is the sickle-cell gene. If you have one of these genes, it gives you some protection against malaria. If you have two of these genes, you get sickle-cell anemia and die. So the sickle-cell gene is more common in areas with endemic malaria, and less common in other areas. It is nonexistent among the polar populations which never have to cope with malaria.

Another obvious genetic factor is resistance to certain diseases. It was the European resistance to such diseases as smallpox, and the Native American lack of resistance, that led to the die-off of 90% of the Native American population within a century of the initial contact.

Another crucial mutation is the gene that permits the digestive tract to continue production of lactase after infancy. Originally, the body stopped making lactase (necessary for digesting milk) after the child was weaned. There was no point in manufacturing proteins that don't do anything any longer. However, when people started herding cattle, the ability to digest milk (and later cheese and other dairy products) became important. The authors support the hypothesis that the expansion of the Indo-European languages is directly attributable to this. Once a group of people arose who a) raised cattle and b) could digest milk, they had a huge advantage over their neighbors and so conquered everything and everybody in Eurasia. Their language, Proto-Indo-European, became the mother of most languages spoken in Europe, Russia, Iran, and India.

There are a few problems with this hypothesis. It should predict that the Indo-Europeans would have been most successful in areas that supported cattle and least successful in areas that could not support cattle. It's true that Indo-European never penetrated the icy lands of the north, nor did they penetrate the deserts of central Asia, but they were able to cross the Caucasus into the Middle East. Why were they successful in rocky Iran and unsuccessful in verdant Iraq? Possibly Iraq was already supporting a large population by organized irrigation agriculture -- but how did the Indo-Europeans cross the harsh terrain between Iraq and India? That's not good country for cattle. And why did they succeed in conquering Anatolia but failed to penetrate further along the coast of the Middle East? Were they stopped cold by Egyptian power that early in history?

Although there are some weaknesses in the hypothesis, it is the best I have seen to date. I always liked the hypothesis that the agricultural revolution was responsible for the spread of Indo-European. As Indo-European farmers spread across Europe, they brought their languages with them. But that hypothesis has been taking a lot of damage in the recent past, and is now hanging by a thread. The new hypothesis makes much more sense to me.

Another hypothesis the authors take up concerns the Ashkenazi Jews of Europe. Studies have unequivocally shown that the Ashkenazi score on average about 20 points higher in IQ tests than most Americans and Europeans. Studies have also demonstrated clearly that the Ashkenazi Jews are a genetically distinct population, showing many differences with the American and European populations. This suggests -- but does not prove -- that the difference in IQ is attributable to genetic, not cultural factors. The authors present a case that strong selection pressures favored intelligence among the Ashkenazi Jews. They argue that, by being forced into white collar jobs as lenders and brokers, the Ashkenazi Jews had a selection pressure forced upon them. Bright Jews earned a lot of money and raised large families. Stupid Jews didn't earn as much money and so had smaller families.

This doesn't convince me. First, we're talking about an absolute maximum of 1,000 years -- 50 generations for all those changes. That's generous: I doubt that the selection effects took hold until about 1200 CE when the European economy started to use money, and was supposedly obvious by 1800 CE, giving only 30 generations to pull off that selection. To do that, the selection effects would have to be enormous, with stupid Jews being ruthlessly eliminated from the gene pools and smart Jews being blessed with huge families. I simply cannot see that much selection pressure applied that uniformly for that long a period.

I do not question the data showing a) a difference in IQ scores and b) genetic uniqueness among the Ashkenazi gene pool. There has to be a reason for it. I confess that the authors' hypothesis is the best we have right now, but only because it's the only explanation on the table. I'd like to see some alternatives before I plunk my money down on this one.

A final tidbit: geneticists can now track individual genes arising from the mother and the father. All of your mitochondrial DNA comes directly from your mother; if you have a Y-chromosone, all of its contents came directly from your father. So geneticists can trace the genetic heritage of people with much precision using these two genetic samples. One of the not-so-surprising-if-you-think-about-it results was that the great majority of Mexicans have Native American ancestral mothers and Spanish ancestral fathers.

All in all, I enjoyed this book and learned a great deal from it.