It's probably because our ancestors were frequently around 300°C metal which badly damages flesh in a fraction of a second because they made fires to cook. I don't see why that would cause natural selection to make that pain so extreme instead of only slight. It's so rare for somebody to die from getting a nasty burn from touching such a hot metal. Is it because those who didn't feel heat pain were frequently holding very hot metal objects and although each one touching made them only a tiny bit less likely to survive, the damage added up and eventually they would have held such hot metal objects so many times that they had a very high probability of dying?
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