This summarizes the history of soup, taken from "Souper Tomatoes: The Story of America's Favorite Food."

While our early ancestors may have employed hot water to heat foods in natural containers, the cooking technique of boiling was not commonly-used until the invention of waterproof and heatproof containers about five thousand years ago. Boiling was advantageous as a cooking technique. Water turns to steam at a constant temperature that does not exceed F. 212 at sea level. Compared to heating with hot air over a fire, boiling water is more dense and comes more fully in contact with the entire surface of submersed foods. Hot water easily and quickly imparts its energy to the food. In addition to consistency, boiling provides a lower cooking temperature than does frying, roasting, or baking. Before the invention of thermometers, this was particularly significant. Boiling also permitted the fuller use of animal and plant products, and expanded the range of foods that our ancestors consumed. Many animal parts, such as bones, could not be eaten even if roasted. Boiling extracted whatever nutritional value these unused parts possessed. Likewise, some plant parts were inedible in their natural state, but became consumable after boiling. For instance, acorns are edible only after the tannin has been removed by boiling.

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