There is a remarkable thing about human evolution. We always tend to think that humans have just had a continuous surge in brain size over the last two million years, but actually over the last thirty thousand years brain size has decreased by 10 to 15 percent. The standard explanation for this is that we became more gracile at the same time — that we became thinner boned — which meant that we were lighter in body weight. And because there tends to be a correlation between body weight and brain weight, then maybe this explains our smaller brains. But I don't see any reason why brain size should be correlated with the amount of meat we carry on our bodies. This gracility is exactly the same pattern we see in the evolution of dogs from wolves, or bonobos from chimpanzees, or domesticated foxes from wild foxes. In all these cases an increasing gracility of the bone is an incidental effect.

I think that we have to start thinking about the idea that humans in the last 30, 40, or 50 thousand years have been domesticating ourselves. If we’re following the bonobo or dog pattern, we're moving toward a form of ourselves with more and more juvenile behavior. And the amazing thing once you start thinking in these terms is that you realize that we're still moving fast. Tooth size, for example, is extremely strongly genetically controlled and develops with little environmental influence, and is continuing to decline fast. I think that current evidence is that we're in the middle of an evolutionary event in which tooth size is falling, jaw size is falling, brain size is falling, and it's quite reasonable to imagine that we're continuing to tame ourselves. The way it's happening is the way it's probably happened since we became permanently settled in villages, 20 or 30 thousand years ago, or before.

People who are anti-social, for example, have their breeding opportunities reduced. They may be executed, they may be imprisoned, or they may be punished so badly that they're kept out of the breeding pool. Just as there is selection for tameness in the domestication process of wild animals, or just as in bonobos there was a natural selection against aggressiveness, here there's a sort of social selection against excessively aggressive people within communities. This puts humans in a picture of now undergoing a process of becoming increasingly a peaceful form of a more aggressive ancestor.

EDGE: Why is understanding these episodes in evolutionary history so important for us today?

WRANGHAM: We tend to think of the problems that have given rise to Al Qaeda, for example, as being concerned primarily with economic and political conflict, and obviously those are hugely important. Nevertheless, in order to understand why it is that particular countries and particular people within those countries find Osama bin Laden's wild schemes attractive, we have to think in terms of rather deeper differences among groups and sexes.

Think of it this way: Why is it that Western civilization is threatening to the people who support the Al Qaeda philosophy? And not just the Al Qaeda fighters themselves, but more importantly the great masses who are buying the Al Qaeda t-shirts in the Middle East? It's true that U.S. hegemony over oil and support for Israel in the Palestine conflict are general economic inequities that are going to contribute to people's resentment, but there are reasons why those men in particular resent Westernization.

Men in the Middle East come from a society in which there is polygany — one man having many wives — and even though polygany can never be very wide-spread within a society because there aren't enough women, it has the enormous effect that women marry upwards. Polyganous marriages are always concentrated in the upper socio-economic strata. This means that in the lower socio-economic strata you have a lot of men with very few women, and they use the typical systems for getting wives that are used in polyganous societies, which include gaining control over women. In a polyganous society, women want to marry into the polyganous society because that's where all the wealth and the opportunities are to get good food and survival opportunities for your kids. Consequently, they allow themselves to be frustrated, to be veiled and put in the burkha, to be given rules that mean they can only stay inside the house and have to blacken their windows. They allow themselves to be totally controlled by men.