No, you’re confused by the average age of death. But it was what is called a bimodal distribution that they averaged. What that means is that there were two times when lots of people died: 1) in infancy and early childhood, and 2) in old age, 60s or 70s. If you average those out, it comes to some time in the late 30s. So if someone in that time made it out of childhood, they usually made it to old age. Yes, some people died of accidents along the way, but those were a small fraction of the total number of deaths.
ETA: the old age of 60s and 70s only applies to humans and not to pre-human populations. The pre-human groups probably reach old age earlier, although I don’t think we know enough to say exactly how early. I expect it was a gradual increase in lifespan. But the bimodal stuff applies to all these populations.
I don’t think you can entirely discount sexual/social display - it’s not certain how long people have been grooming/shaping/altering their head hair but it’s been a very long time and some sort of hair styling seems to show up in every human society.
I’ve heard a theory that it’s to reduce chafing when moving limbs, and also a theory that it helps hold onto sex pheromones.
Why do you assume there was a loss? Out closest primate cousins don’t have beards regardless of sex, and there are quite a few human groups where the men have little or no beards/mustaches. How do you know that it wasn’t that men acquired facial hair in some groups?
Male pattern baldness is triggered by certain levels of male hormones. Transwomen who have had their testicles removed or inactivated do not go bald(er) (if they started male pattern baldness prior to gender transition treatment they may retain the level of baldness they had before). A related phenomena was noted by folks in ancient history who noted that eunuchs did not go bald like their uncastrated brothers. Transmen on hormones to treat their condition with the appropriate genetics will undergo male pattern baldness.
Post-menopausal ciswomen, whose hormone profiles change, may also start to experience thinning hair in a male pattern, although usually not as distinctly as their brothers did.
In other words, hormones determine whether or not any hormone-linked baldness patterns will emerge, but it’s your genes that determine that pattern (or lack of it).
The distinction is that almost all other mammals that run, run sprints. Humans are uniquely adapted to run marathons, to basically chase down four-legged game to the point where it is too exhausted to run. A horse can run a very fast mile when it has to, then it sweats buckets and has to “pee like a racehorse” to recover. In an hours long chase where it is constantly harried by humans and cannot stop to eat or drink, it would eventually fail of exhaustion first. Same with all the other savannah big game. But for most other threats, the short sprint is all that is required of such animals - a lion or cheetah will tire just as fast. (And then be too tired for a fight, whereas we would have spears and rocks).
Part of human adaption for that hunting style is the lack of significant body hair to more easily shed the heat generated. I believe in a different thread it was pointed out we have different muscle makeup than, say, a chimp - so we have endurance muscles at the expense of strength muscles compared to some other animals.
one suggestion I read is that in temperate winters men are more likely to be out and about hunting, so baldness helps them with vitamin D production, while women who stayed in the shelter with the children would not really benefit as much - but again, a “just so story”. Certainly don’t disregard the roles of sex selection/distinction in the emergence of certain traits.
And pubic or under-arm hair, just like eyebrows, can play a role in managing sweat - ie. channel it, keeping it around to cool the specific region instead of dripping and annoying. Avoiding chafing and airing pheromones - well, plenty of attributes have multiple functions.
Sorry, but horses can run fast for much longer than a mile, and carrying a human on their back as well. Humans pale in comparison. The other animals are limited by their tactics, not because of hair on their bodies. If humans sprinted they would tire out just as fast as other animals, lack of hair won’t matter at all. Hair is nowhere near as much of a factor as the cardiovascular system. Finally, we do not know how much hair our ancestors had when they started running down game.
