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Dart initially echoed Darwin’s theory that bipedalism freed the hands of early hominins to make and use hunting tools, which in turn selected for big brains, hence better hunting abilities. Then, in a famous 1953 paper, clearly influenced by his war experiences, Dart proposed that the first humans were not just hunters but also murderous predators.18 Dart’s words are so astonishing, you have to read them: The loathsome cruelty of mankind to man forms one of his inescapable characteristics and differentiative features; and it is explicable only in terms of his carnivorous, and cannibalistic origin. The blood-bespattered, slaughter-gutted archives of human history from the earliest Egyptian and Sumerian records to the most recent atrocities of the Second World War accord with early universal cannibalism, with animal and human sacrificial practices of their substitutes in formalized religions and with the world-wide scalping, head-hunting, body-mutilating and necrophilic practices of mankind in proclaiming this common bloodlust differentiator, this predaceous habit, this mark of Cain that separates man dietetically from his anthropoidal relatives and allies him rather with the deadliest of Carnivora. Dart’s killer-ape hypothesis, as it came to be known, was popularized by the journalist Robert Ardrey in a best-selling book, African Genesis, that found a ready audience in a generation disillusioned by two world wars, the Cold War, the Korean and Vietnam Wars, political assassinations, and widespread political unrest.19 The killer-ape hypothesis left an indelible stamp on popular culture including movies like Planet of the Apes, 2001: A Space Odyssey, and A Clockwork Orange. But the Rousseauians weren’t dead yet. Reanalyses of bones in the limestone pits from which fossils like the Taung Baby came showed they were killed by leopards, not early humans.20 Further studies revealed these early hominins were mostly vegetarians. And as a reaction to decades of bellicosity, many scientists in the 1970s embraced evidence for humans’ nicer side, especially gathering, food sharing, and women’s roles. The most widely discussed and audacious hypothesis, proposed by Owen Lovejoy, was that the first hominins were selected to become bipeds to be more cooperative and less aggressive.21 According to Lovejoy, early hominin females favored males who were better at walking upright and thus better able to carry food with which to provision them. To entice these tottering males to keep coming back with food, females encouraged exclusive long-term monogamous relationships by concealing their menstrual cycles and having permanently large breasts (female chimps advertise when they ovulate with eye-catching swellings, and their breasts shrink when they are not nursing). Put crudely, females selected for cooperative males by exchanging sex for food. If so, then selection against reactive aggression and frequent fighting is as old as the hominin lineage.22
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Daniel E. Lieberman (Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding)