Control of fire by early humans
The cooking hypothesis

The cooking hypothesis proposes the idea that the ability to cook allowed for the brain size of hominids to increase over time. This idea was first presented by Friedrich Engels in the article " The Part Played by Labour in the Transition from Ape to Man " and later recapitulated in the book Catching Fire: How Cooking Made Us Human by Richard Wrangham and then in a book by Suzana Herculano-Houzel. Critics of the hypothesis argue that cooking with controlled fire was insufficient to start the increasing brain size trend.

The cooking hypothesis gains support by comparing the nutrients in raw food to the much more easily digested nutrients in cooked food, as in an examination of protein ingestion from raw vs. cooked egg. Scientists have found that among several primates, the restriction of feeding to raw foods during daylight hours limits the metabolic energy available. Genus Homo was able to break through the limit by cooking food to shorten their feeding times and be able to absorb more nutrients to accommodate the increasing need for energy. In addition, scientists argue that the Homo species was also able to obtain nutrients like docosahexaenoic acid from algae that were especially beneficial and critical for brain evolution, and the detoxification of food by the cooking process enabled early humans to access these resources.

Besides the brain, other human organs also demand a high metabolism. During human evolution, the body-mass proportion of different organs changed to allow brain expansion.
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