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Perhaps the achievement of ephemeral sophisticated industries in Africa before 70 kya, and later more permanently in western Eurasia, depended upon larger populations, leading to the ability to accumulate more innovations. Rather than a bottleneck around 70 kya as mitochondrial coalescence data suggest, perhaps human populations were chronically rare before 70,000 kya. Imagine that humans were competing in a rather crowded guild of top carnivore species: lions, leopards, cheetahs, and other large cats; hyenas; wild dogs; wolves; and bears. More variable environments, to which humans could adapt culturally, might have given our species a competitive advantage. An increase in millennial- and submillennial-scale climate variation might thus have led to the spread of moderns out of Africa, and to population densities high enough to lead to Upper Paleolithic and similar industries (Richerson et al., 2009).

Selection from the Late Pliocene to Middle Pleistocene

Events deeper in the evolution of hominins are naturally even more opaque. The interesting high-frequency part of the paleoclimate record is seriously deficient for this period. During the long period from about 2.6 to 1 million years ago, when the low-resolution record was dominated by the 41,000-year cycle, early members of our own genus Homo enter the fossil record, particularly H. erectus sensu lato. These populations had relatively modern postcrania and brain sizes relative to body sizes intermediate between Australopithecines (and living apes) and anatomically modern humans and Neandertals (Ruff et al., 1997). Many, if not most, of the major genetic changes between humans and the rest of the apes probably occurred in this period, during the transitions from Australopithecines to H. habilis and from H. habilis to H. erectus. According to some interpretations, H. erectus had a rather modern physique and an enlarged brain relative to body size (McHenry and Coffing, 2000). Subsistence activities might have included a considerable ability to acquire meat and fat from hunting. For example, an important component of human hunting is the ability to run down large- and medium-sized game. Humans from H. erectus onward could probably run efficiently and sweat to keep our body temperature down during extended exercise. H. erectus hunters could thus probably have run medium-sized herbivore prey until they were exhausted or overheated or both, and then dispatched them with unsophisticated weapons (Bramble and Lieberman, 2004; Liebenberg, 2006; Jablonski and Chaplin, Chapter 9, this volume). We can expect to find genes related to a large variety of specifically human traits to have evolved in this period, but in most cases we will have to entertain the hypotheses that they evolved earlier in Australopithecines or in post-H. erectus hominins.

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