Male pattern baldness is linked to a hormone called dihydrotestosterone (DHT), as well as genetics.
The amount of DHT in your system affects both hair growth and hair loss. DHT is partly responsible for you starting to get more body hair and a beard during puberty.
Oddly enough more DHT in your system is damaging to the hair folics on the top of your head causing them to shrink and eventually stop producing hair all together.
But at the same time DHT stimulates hair growth on your chest, back, armpits, etc.
So the same hormone that makes you bald, also causes you to get hairier as you get older.
Exactly why we have 2 different kinds of hair follicles, and why the sensitive ones are mostly on the top of your head we don’t know.
Androgens, specifically DHT (dihydrotestosterone). Testosterone is converted by 5 alpha reductase to DHT. It’s not how much DHT you have that causes MPB, but your sensitivity to it, which is genetic. DHT binds to the hair follicle and shrinks it, so every time the hair falls out it produces a weaker hair until the hair can no longer break the scalp and there is no visible hair. Now for reasons not entirely known only the follicles on the top and crown of the scalp are susceptible to miniaturization; the back and sides of the scalp are not affected and neither is any hair on the body/face. That is why when people get hair transplants they transplant the hair from the back of the head and place it on the top of the call since that hair does not miniaturize it will stay there for life.
There’s quite a few good mechanistic answers here. In terms of the evolution drivers there’s a few possibilities.
It could be this is a secondary trait that has been selected for. That is, it might be that being hairier on the chest has been selected for, and balding is just a side effect of the way that the mechanism works.
Or baldness itself could confer a fitness advantage. Given that males of other closely related primates develop obvious physical features to show they are older, you could have the same thing in humans. It might be a social signal to indicate ‘I’ve been around awhile. My genes are good and I probably have decent social standing now too.’
Or it could be genetic ‘drift’. That is mutation over time has resulted in baldness, and although there’s no advantage there’s also no disadvantage, so it ends up a part of the population for no reason except that it happened to occur.
There are probably some other possibilities too.
Edit, typos
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