It is a vestigial reaction back from when humans used to have much more body hair.

Goosebumps cause our body hair to stand up on end. When we were much hairier, this had two effects:

• Trapped more air near the skin, warming us up (like a natural sweater).

• Made us look larger and less attractive to preditors.

While the hair has evolved away, the goosebump reaction remains when we get cold or scared.

Apparently goose bumps are a relic of our ape-like ancestors. Back when our ancestors were covered in fur, the goose bump reflex would make their fur rise and make them look bigger. This was useful in scaring off predators.

It was also useful when cold, because it would create a layer of air for insulation. This reflex is common in many mammals, and is most obvious in porcupines, since they have long quills.

Now that our hair is sparse it doesn't do us much good. But we still have some vestigial body parts (like the appendix) or reflexes (like goose bumps) that used to be more useful.

You know that thing where a scared cat gets all puffed up to look big?

Imagine instead of cool cat hair you just had tiny little arm hairs. That’s a goosebump

Even shorter- you know how a cat puffs out its hair when it's scared or cold? Humans do the same thing except we're not furry like that anymore.

More info:

There are little muscles in hair follicles that make them stand up. It's called piloerection. It's controlled by the sympathetic nervous system.

but what's the actual mechanism?

something in the follicles?, muscular, or fluid moving around? why does the skin feel tighter? where does whatever the bumps are made of go afterwards, why didn't that mechanism go when we lost the hair - does it serve another purpose most of the time?

There are small muscles attached to each hair follicle. When those muscles contract, the skin tightens up and the hair stands on end. When they relax, the bump goes away.

As for why they are still there, evolution doesn't have a goal - it rewards what works. There was never a benefit in the reaction going away, so it never did.

kinda mind blowing there is a tiny muscular infrastructure there still, like almost everywhere, just doing nothing most of the time.

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