11/30/2010 07:17 am ET Updated May 25, 2011

The 10,000 Year Explosion: How Civilization Accelerated Human Evolution

Our book, The 10,000 Year Explosion, is mostly about the long-term dance between culture and genetics. Usually these two forces are considered as opponents (one wearing a white hat and the other black) but that's a false picture. They're more co-conspirators.

Culture can shape genetics. After the development of agriculture, humans were eating new foods - food that they weren't well adapted to. Some people, however, had versions of genes that worked better in that new situation - dealt better with those new foods - and carriers of those genes had more surviving children. Eventually, over many generations, those gene variants became common in some groups.

The best-understood example is lactose tolerance. Lactose is a sugar, found only in milk, that accounts for about a third of milk's food value. Babies make lactase, the enzyme that digests lactose. In the olden days, before the domestication of cattle, children stopped making that enzyme around age five - so adults could not digest lactose. This is still the case for most humans. But in some groups, mutations that caused continued lactase production were favored and eventually became common. They've become mampires, creatures that live off the milk of another species. One such mutation became common in Europe and North India, another among the tall cattle-herding peoples of east Africa. Yet another became common in desert Arabs - this likely resulted from the domestication of the camel.

Given time, culture changed human biology. Of course that kind of cultural innovation couldn't have happened without other underlying biological changes. Cows were certainly around half a million years ago, but humans were not yet smart enough to domesticate them - or tame dogs, or invent the bow and arrow. Genetic changes made complex learned behavior possible for humans: those learned behaviors led to further genetic changes.

Widespread lactose tolerance led to other cultural changes: cattle became very important in some of those populations. Consider cattle as bride-price among the Masai, sacred cattle in India, or the Táin Bó Cúailnge (otherwise known as the Cooley Cattle Raid). Not to mention Gunsmoke and Bonanza.

In another example, note that old-style humans had much thicker skulls than we do today. Field researchers have actually mistaken fragments of a homo erectus skull for part of a turtle shell. The educated guess is that those skulls were thick for a reason - they were probably hitting each over the head with clubs. In that situation, people with thick skulls left more descendants.

Over the last few tens of thousands of years, especially in the last ten thousand, skulls have become far lighter. The underlying reason must be that people with thinner skulls outbred those with thicker. The past is another country, but we can have reasonable guesses about the changes that led to this. First, there is reason to believe that violence became less common with the growth of government power over the last few thousand years, just as farmers discourage fighting among their livestock (by dehorning, castration, and breeding for tameness). Second, projectile weapons such as bows and arrows meant that thick skulls were an obsolete defense: a bone helmet didn't protect again an arrow in the belly. Third, artificial head protection like helmets probably made thick skulls redundant. These cultural changes influenced the direction of human evolution, but they certainly didn't stop it. In fact, humans skulls have changed more rapidly over the past few thousand years than ever before.

If we were being all academic, we would say that this book is mostly about gene-culture co-evolution. You could also say it's an excuse to take a look from a different perspective at half of the colorful incidents in human history. We had fun writing it and we hope you'll enjoy reading it.