Smithsonian Magazine has published a piece on how fire, or rather the discovery of it, helped make us human. By relieving us of the energy we spent chewing and digesting our food, fire allowed us to spend that freed-up energy on other tasks, which then led to the development of tools and skills that catapulted us to the top of the animal food chain.
In essence, cooking—including not only heat but also mechanical processes such as chopping and grinding—outsources some of the body’s work of digestion so that more energy is extracted from food and less expended in processing it. Cooking breaks down collagen, the connective tissue in meat, and softens the cell walls of plants to release their stores of starch and fat. The calories to fuel the bigger brains of successive species of hominids came at the expense of the energy-intensive tissue in the gut, which was shrinking at the same time—you can actually see how the barrel-shaped trunk of the apes morphed into the comparatively narrow-waisted Homo sapiens. Cooking freed up time, as well; the great apes spend four to seven hours a day just chewing, not an activity that prioritizes the intellect.
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