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In its brief evolutionary history, Homo sapiens has come to occupy a larger range than any other terrestrial vertebrate species. Earlier hominins, such as Homo heidelbergensis and Neanderthals, were limited to Africa and the temperate regions of southern Eurasia. Behavior-ally modern humans were living in Africa by 70,000 years ago (Mourre et al., 2010). Between 50,000 and 60,000 years ago, people left Africa, crossing into southwest Asia (Klein, 2009). From there they spread rapidly through southern Eurasia, reaching Australia by 45,000 years ago, a feat that only one other terrestrial mammal (a murid rodent) was able to accomplish (Rowe et al., 2008). Soon after this, people penetrated far north, reaching the latitude of Moscow by 40,000 years ago and the Arctic Ocean by 30,000 years ago. People had spread almost as far south as the southern tip of South America 13,000 years ago, and by 5,000 years ago humans occupied virtually every terrestrial habitat except Antarctica and some islands in Oceania (Klein, 2009). Even the most cosmopolitan bird and mammal species have substantially smaller ranges (White et al., 1994; Bruce, 1999; Wozencraft, 2005).

This global expansion required the rapid development of a vast range of new knowledge, tools, and social arrangements. The people who moved out of Africa were tropical foragers. Northern Eurasia was an immense treeless steppe, relatively poor in plant resources and teeming with unfamiliar prey species. The people that roamed the steppe confronted a hostile climate—temperatures fell to -20 °C for months at a time, and there were often high winds. Surviving in such environments requires a whole new suite of adaptations—tailored clothing (Gilligan, 2010), well-engineered shelters, local knowledge about game, and techniques for creating light and heat. This is just the northern Eurasian steppe; each of the other environments occupied by modern human foragers presented a different constellation of adaptive problems. Ethnographic and historical accounts of 19th and 20th century foraging peoples make it clear that these problems were solved through a diverse array of habitat-specific adaptations (Kaplan et al., 2000). Although these adaptations were complex and functionally integrated, they were mainly cultural, not genetic, adaptations. Much evidence indicates, in fact, that local genetic changes have played only a relatively small part in our ability to inhabit such a diverse range of environments (Richerson and Boyd, 2005; Richerson et al., 2010).

Why are humans so much better at adapting to novel environments than other mammals? There have been many different answers to this question, but the most influential are rooted in the idea that people are simply smarter than other creatures. We have bigger brains and more computing power, and this allows us to adapt to a wider range of environments than other animals. One of the clearest statements of this hypothesis comes from a series of papers by Tooby, Cosmides, Pinker, and collabora-



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