Cultural innovation has been a driving force behind biological change in humans for a long time—certainly since the first use of tools some 2.5 million years ago. Natural selection acting on the hominid brain made those early innovations possible, and the innovations themselves led to further physical and mental changes.

Biological and cultural co-evolution was slow at first, at least by modern standards, but gradually things sped up. The archaeological record shows that our capacity for innovation continued to increase until, about 40,000 years ago, we were primed for what has been called the "human revolution" or the "creative explosion" of the Upper Paleolithic in Europe and northern

Asia. This sudden spurt in technology and art occurred shortly after modern humans expanded out of Africa—and it too must have involved biological changes, changes that we suspect were driven in part by genes stolen from the Neanderthals and other archaic humans, the previous occupants of Eurasia. Behavioral modernity led to even more change: Men made better tools and then, in turn, were reshaped by those tools over many generations.

With the development of agriculture, both cultural and biological evolution accelerated even further because that way of life made new demands on humans. Before agriculture humans had always been foragers: The huge population increase associated with agriculture resulted in more favorable mutations as well as more new ideas. This rapid evolution of our species following the spread of agriculture is indeed a 10,000 year explosion.

The explosion is ongoing: Human evolution didn't stop when anatomically modern humans appeared, or when they expanded out of Africa. It never stopped—and why would it? Evolutionary stasis requires a static environment, whereas behavioral modernity is all about innovation and change. Stability is exactly what we have not had. This should be obvious, but instead the human sciences have labored under the strange idea that evolution stopped 40,000 years ago.

All this means, has to mean, that biological change has been a key factor driving history. It has certainly not been the only factor, and it has been strangely intertwined with more traditional influences. Genetic changes like lactose tolerance have arisen and spread because of cultural innovations (such as the development of agriculture) as well as the random occurrence of the right mutations, and those genetic changes have in turn had their own cultural consequences. The expansion of the Indo-Europeans, the successful European settlement of the Americas and Australia, the failure of the "scramble for Africa," the entry of the Ashkenazi Jews onto the intellectual stage, possibly even the industrial revolution and the rise of science—all appear to be consequences of this endless dance between biological and cultural change.

If researchers in the human sciences continue to ignore the fact of ongoing natural selection, they will have thrown away the key to many important problems, turning puzzles into mysteries. Cortés, with 500 men, conquered and held an empire of millions. Try to explain this without invoking biological differences in disease resistance caused by ongoing evolution—it can't be done.

Thucydides in the fifth century BC said that human nature was unchanging and thus predictable, and many scientists today believe that human nature stopped changing tens of thousands of years ago. Historians seem to make the same assumption. In so doing, they're ignoring tremendous opportunities: not just in decoding the past, but in shaping the future as well. Continuing evolution over human history has been a vast natural experiment, an experiment that promises big payoffs in understanding, and then fighting, disease and mental illness. Limone sul Garda hid an important clue about human disease. With a million villages in the world, there must be many more such clues. Some of the results of history's experiments may even aid us in more ambitious efforts aimed at increasing human life spans and cognitive abilities.

It's time for researchers in the human sciences to shrug off the chains of dogmas like evolutionary stasis and "psychic unity." There's no time to lose—and there's a world to win.

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