Why do we have goosebumps?
The quick answer is that goosebumps are a remnant of our hairy past.
The longer answer is that at one point, we were all pretty hairy. Completely hairy. While most of us have lost the bulk of our body hair, the internal wiring is still present, as evidenced by goosebumps. (some call them goosepimples)
Goosebumps are an automatic reaction of the nervous system that flexes the piloerector muscle found at every hair follicle. Piloerector literally means making the hair stand on end. This reaction is caused by one of two things… either you are cold, or you are scared.
Looking at cats, or anything else more furry than us, they can puff up their fur when it’s cold to trap and hold more air around their body, thus insulating and staying warmer. Our goosebumps are trying to do the same thing.
As for being scared, the fur puffing reaction is seen in many animals as an attempt to make themselves appear larger and more threatening than they are. For animals, size generally equals power, and whenever an animal has the capacity to make itself larger, it stands a better chance of scaring away whatever may be attacking or harassing it.
On a funny note, even though we call them “goose” bumps, only mammals have what is known as “horripilation” (the scientific term for goose bumps). Although most species of birds do have a similar system in place, they do not have hair, and thus also lack those small muscles, which lay below the skin level.