|Major geographical axes of continents from Diamond's book.|
One of the big ideas in Jared Diamond's popular book Guns, Germs, and Steel: the Fates of Human Societies is the notion that the primary geographical orientation of continental landmasses (either north-south or east-west) has had a major impact on the historical pace of technological diffusion. Diamond argues that technological innovations (e.g. domestication of livestock and plants) can diffuse faster to areas which similar similar ecological conditions with the source area of the innovation. If you domesticate a strain of wheat in Chicago that produces a lot of reliable food....I might want to try to grow it my back yard too. I would will probably have better luck if I live in New York than if I live in Austin, TX, because ecological changes happen quickly as you go south across degrees of latitude. Simple accidents of geography then, Diamond suggests, have had a big influence on the pace of technological progress and ultimately wealth accumulation on different continents.
Ramachandran and Rosenberg set out to test this idea in an article in early view on AJPA using genetic data from modern peoples. They examined microsatellite loci from several indigenous populations in the Americas and in Eurasia. If (and this is pretty huge if) technological changes have largely occurred by the migration of populations from one place to another, then there ought to be differences in the primary axes of modern genetic variation in Eurasia versus the Americas. In other words, changing ecological conditions across degrees of latitude should have prevented migration (and associated technological diffusion), resulting in greater genetic differentiation along the north-south axis in the Americas compared to the east-west axis in Eurasia. This pattern is exactly what the authors report, which they interpret as support for Diamond's continental axis orientation hypothesis.
Anthropological Sidebar: It is interesting (at least to me) to note that these ideas in Guns, Germs, and Steel have been somewhat controversial in anthropology. Most would agree that Diamond's argument could be described as one of geographical and ecological determinism. His supporters argue that this makes GG&S the ultimate anti-racism book, taking the wind out of the sails of any racists wanting to argue that technological progress is associated with peoples innate intellectual abilities. Others would argue that ecological determinism is a cop-out which doesn't consider the role of racism and colonialism in creating the social conditions which GG&S purports to explain.