First, the conventional wisdom. Because Homo sapiens are a young species and haven't had time to genetically differentiate, we modern humans are all basically genetically identical. Because Homo sapiens figured out ways to use culture to overcome natural selection, human genetic evolution ceased ages ago. Because Homo sapiens are genetically very similar and not subject to natural selection, the differences that we see today among modern human groups are the result of cultural processes, not genes.

Not so say Gregory Cochran and Henry Harpending in their challenging and sure-to-be-controversial new book The 10,000 Year Explosion: How Civilization Accelerated Human Evolution (Basic Books, 2009). We are extraordinarily similar genetically, but the minor differences occasionally had very important consequences: a difference of one or two nucleotides--among billions--could mean life or death for entire populations.These minor differences, where advantageous, were selected for and spread in the ordinary natural selective way: if you developed resistance to malaria in the tropics, you survived and your genes were passed on; if not, then not. Finally, these genetic advantages accumulated: populations that had been put under more "pressure" were more robust than those living in relaxed environments.The more robust populations prospered; the less robust did not.

Interesting for the historian will be the point that most of the enhanced "pressure" was due to historical factors. According to Cochran and Harpending, the transition to agriculture roughly 10,000 years ago in particular forced Homo sapiens to do things that they were not "programmed" (so to say) to do. Those who adapted genetically survived; those who did not, did not. Thus Homo sapiens 1.0 (the hunter-gatherer model) evolved into Homo sapiens 2.0 (the farming model). Cochran and Harpending submit that this process--natural selection due to cultural pressure--has been going on quite recently and may be going on today. For example, they propose that historical factors in the European Middle Ages favored the differential reproduction of brainy Ashkenazi Jews, the results of which can be seen in the differential success of their descendants in brainy occupations.

Hackle-raising stuff to be sure. Is any of it true? Listen to the interview, read the book and decide for yourself.

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