Why humans cook

AEvery human society cooks, including those living where fuel is scarce and the effort is considerable. No other animal does. The universality is the first thing that needs explaining, because a practice that costs time, fuel and risk and is nevertheless performed everywhere is unlikely to be merely a matter of taste.

BThe most obvious explanation is safety, and it is partly right and clearly incomplete. Heat destroys many of the organisms that make meat dangerous, which matters, but it does not explain why plants are cooked at least as often as meat, nor why societies with very little fuel still cook their food rather than reserving the fuel for warmth.

CThe explanation that accounts for more is energy. Cooking does to food outside the body part of what digestion does inside it: heat breaks down the structures that hold starch and protein together, so that less of the body's own effort is needed to extract what is in them. Measured carefully, the same quantity of cooked food yields substantially more usable energy than the raw equivalent, and the difference is large enough to matter over a lifetime.

DThis produces a claim that is harder to test and more interesting. A brain is an expensive organ, consuming a fifth of an adult's energy while making up a fiftieth of the body's mass, and the human brain expanded during a period when the human gut was getting smaller. Those two changes are difficult to explain together unless something was making food easier to process, since a smaller gut extracts less from the same material. Cooking is the candidate usually proposed.

EThe evidence for when it began is thinner than the confidence with which dates are quoted. Fire used for cooking and fire used for warmth leave similar traces, and both are hard to distinguish from a fire that occurred naturally. The firmest evidence consists of hearths in places where fires do not start by themselves, with burnt bone and stone arranged in a way that suggests repeated use, and such sites are rare and disputed.

FThere is a cost that is usually left out of the account. Cooking requires fuel, and gathering fuel takes time that could be spent on other things; it requires somebody to do the work, and in most recorded societies that somebody is a woman; and the hours spent at a fire indoors have measurable effects on health that are only now being properly counted. A practice can be an evolutionary advantage and a daily burden at the same time, and the second of these is borne by particular people.

GWhat makes the whole argument unusual is that it runs the wrong way round. Most accounts of human development treat a biological change as the cause and a behaviour as the consequence. Here the behaviour is proposed as the cause: a technique that altered what the body needed to do, after which the body changed. If that is right, cooking is not something humans developed because of what they are. It is part of why they are that.