Goose bumps are a leftover reflex from ancestors who had much thicker body hair.

The longer version

Tiny muscles attached to each hair root contract and pull the hair upright, creating a small mound around it. In animals with thick fur, that erect layer traps a blanket of still air next to the skin and slows heat loss, and it also makes the animal look larger.

Humans retain the machinery but have far less body hair, so the visible effect is mostly the bumps themselves. The response is driven by the sympathetic branch of the nervous system, the same network that prepares the body for action, which is why it also appears during strong emotion and during a sudden chill. The hairs cannot do much insulating work any more, but the reflex remains wired in.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.