Why do humans have scalp hair/eyebrows/beards?

Followup on this ancient thread.

Our evolution has dispensed with our hair covering mostly, but we still have hair on our heads. Why? What’s it good for?

Good long healthy shiny hair with lots of natural luster is still an excellent indictator of your mate’s overall health and therefore a better match for your long term genetic material than patchy haired woman and frizzy haired dude.

Well, eyebrows help cushion the brow ridge, helping to protect the eyes. They also provide a handy sweat band, keeping it from running into your eyes and stinging you. Granted, the sweat band effect fails under excessive perspiring, but it works some of the time.

Hair on your head helps protect you from sunburn, it’s the most exposed part of your body and most susceptible, right?


Um, common sense is my cite.

It takes years to grow good, waist length hair in a woman. Major medical problems, malnutrition, scalp infections like ringworm, would all largely prevent that kind of length and luster.

That’s commonly discussed in anthropology books.

Scalps are also most vulnerable to sunburn and the resultant damage it causes. Genetic evidence* indicates that we started losing body hair (or, more correctly, that it stopped growing long) about 1.7M years ago, which coincides with the emergence of Homo ergaster. There is also some genetic evidence* that it’s only in the last 200,000 years or so that our head hair has continued to grow longer than is typically seen in extant apes-- a characteristic as uniquely human among primates as the lack of body hair in general.

*evidence, not what would be considered scientifically definitive

Eyebrows also act as signalling/communication organs. And a full beard helps prevent a rival from ripping your throat out. Like a lion’s mane, you know.

Of course, the mark of a mature alpha male is the shiny scalp and enough back hair for the youngsters in the troop to hold onto .

“Thick, lustrous hair is very important to me”–George Costanza

Don’t recall too many Native American Indians (pick one) with a beard.

The trouble with a lot of the answers about why we have hair, eyebrows and beards (and pubic hair and body hair for that matter) is that the writers are falling into the trap of seeing evolution as an “intelligent” force with “intentional” direction. If we have something there must be a good reason for it. If we could only figure it out.

Maybe not. The answer could be that there is really no reason why humans have hair mostly on their heads. And I would LOVE to hear anyone come up with a good reason we grow hair in our armpits at puberty, for goodness sake!

The answer could be that homo sapiens just happens to have a body that grows hair in some places. A genetic predisposition that has no real reason or use.

The whole point is that evolution is blind and undirected. It did not set out to “go” anywhere, it never intended to “do” anything and it is not going anywhere specific.

We intelligent human beings are not the final masterpiece of an intentional process that “led” to our species.

For example: Since we now know that bipedalism came millions of years before increased brain size, it would now appear that our famous intelligence is just an ACCIDENTAL result of the fact that some ape-like creature had to develop an ability to run fast on the ground and to look over tall grass to spot predators as grasslands replaced jungles. This meant that females had wider pelvic girdles, which just hapened to make it easier for them to give birth to larger-brained babies. But even then it took millions of years for homonids to get smarter.

So Einstein and Beethoven and Stephen Hawking are nothing but the result of an ape not wanting to get his ass chomped!

Nor is our body a marvel of design and efficiency. We are full of weird and useless bits and pieces and design flaws. My urether runs throough my prostate, which in middle age has swollen to the size of a friggin’ watermellon. The prostate squeezes the urether, making me feel like I need to piss all the time and can’t. Talk about lousy design.

Nobody knows why the appendix occurs in apes and humans and no other mammal. What was it for? It serves no purpose in apes or humans. Just a funny gene that went crazy and started growing extra plumbing off our intestines? Mayb so.

And look at that weird little toe!

So maybe the answer as to why we have hair on a few funny spots is “because”.

I’ve also heard that having head hair was to give humans (or rather, our ancestors) something to groom. Grooming is a primary form of socialization in primates. It can reinforce hierarchies. I’ve also heard that since we’ve developed language, one of the social functions of conversation is to act as a grooming surrogate. Even though I can talk, I know that I find having my hair worked on very soothing. Go to any elementary or middle school, and you’ll find you have girls asking each other to play with their hair during recess or assemblies.

While people often do this, I don’t see it being done in this thread. The fact that we are largely hairless except on certain parts of the body is unlikely to be just an evolutionary accident-- ie, a mutation without adaptive value. Brains require a very efficient cooling system in order to work properly. Given what we do know about our evolutionary history is extremely unlikely that there would be a significant change to our cooling system that was not adaptive in the positive sense.

(Armpit and bubic hair probably play a different funtion, most likely associated with trapping odor that is related to sexual preference.)

Could be, but unlikely. It takes a lot of energy to grow all that hair. Using up energy is generally maladaptive - unless, like the peacock’s tail, there’s a good “reason” for it, reproductively.

As John Mace says, I’ve often heard it hypothesized that it’s to trap and intensify body odors which indicate sexual maturity. Having a 13 year old boy around and smelling teen funk gain a foothold just as he’s becoming interesting to girls makes me inclined to believe it.

No. We know very well what the appendix does, and it ain’t nothin’. As **gabriela **recently elucidated in this post, the appendix is an immune organ.

Yes, of course “why” is a troublesome question in evolution. Consider, however, the question to be worded: what reproductive advantage, if any, is there to human hair? That we can offer hypotheses for, and there’s some good ones in this thread.

You may not, but I do. All these “explanations” smack of nothing more than guesses – “just so” stories, based on adaptationist principles. Just look at the raneg of explanations offered in this thread: for grooming, for communication, for protection, to prevent sunburn, as an indicator of general health, etc. Could be any of these, could be all, but it could also be none of the above. The truth is, there is little evidence to support any of these, otherwise it wouldn’t be such a common question, nor would there be such a panoply of “reasons”.

There’s no such thing as a “maladaptive” trait. And it takes even more energy to grow that big brain in the first place; any subsequent hair growth is peanuts in comparison.

IF there’s an evolutionary reason for hair growth on the top of human heads, I suspect it has far more to do with sexual selection than any adaptive explanation concerning alleged heat loss / sunburn issues. And yes, that’s a guess. Because there really isn’t any evidence one way or the other regarding a possible “why”.

I don’t see people “falling into the trap” as the quote that I was responding to stated. I think it highly unlikely that head hair is just an accident of evolultion that didn’t have any adapative value, so speculating about what that value might be isn’t the same as assuming that every trait has some adaptive explanation.

Everyone knows the pinkie toe is a device that helps us locate heavy objects and sharp corners in the dark.

There is a correlation of sorts between diet and quality of hair. Someone with distressed hair may be ill-fed, or otherwise have some sort of morbid problem.

But maybe hair is just … hair, with no real deeper meaning in a biological sense.

Do you mean that there is no such thing as a widespread genetic mutation that interferes with reproduction, or that a DNA mutation in a single individual never works to their reproductive detriment? I meant the latter, and agree that hair can’t be one of those, as we all have it and we all still like to have sex with each other.

There are certainly thousands of possible genetic mutations which influence their carriers never to breed, either because they cause death before sexual maturation, or because they make the individual sexually inappropriate. (A bird born not knowing his species’ song due to a gene mutation will most likely never attract a mate, for example.)

I’m sure there’s a better word for what I’m talking about than “maladaptive”, but it was late last night and I was tired. What was the word I was looking for?

Hair, OTOH, doesn’t preclude us from finding mates, so the energy expenditure is “worth it”, or at least not detrimental. If, as proposed, it helps to gain mates by presenting an aura of health (perhaps falsely, perhaps not), then that would be one reason why our bodies spend so many calories growing the stuff. Or not. I’m willing to accept that it was just some random weird thing that happened and wasn’t detrimental, so it stuck around. Could be. I don’t know.

Generally, I think these threads belong in IMHO anyway, as very few of them have GQ answers with overwhelming evidence to support one conclusion or another.

Darwin’s Finch, I happen to think that adaptationism is a pretty good starting place when you want to figure out why a particular trait exists. I’m not talking about “just so” stories. I’m talking about look at long hair, for example, and asking “what adaptive advantage could there be for this?” In most cases, the answer is either that the trait does provide some advantage, or that it is not a disadvantage to the organism, and I think that it’s generally the former and not the latter. The trick with practicing intelligent adaptationism is acknowledging when there isn’t a compelling case for the function of a trait. In the case of hair, I’m not aware of a really good reason to believe any of the various stories - and I agree completely with you in describing them as “just so” stories - told in this thread over the others. Certainly, the idea that eyebrows are for communication is ridiculous. Am I supposed to believe that being able to raise a perplexed eyebrow confers a greater likelihood of passing on more of one’s genes to the next generation? :dubious: In this case, as with hair, sexual selection makes about as much sense as any other suggestion, but there’s no particular reason to believe any more strongly that this is the function of eyebrows.

As for whether any traits have “purpose,” well, that’s tricky. A better word might be “function,” but that’s quibbling. Evolution is a mindless, algorithmic process in a mindless, algorithmic universe. And yet it has given rise to traits that clearly have some kind of function or purpose to the benefit of their possessors. What that means on the grander scale is, well, nothing. But purposeless traits (what a silly idea!) wouldn’t mean anything, either.

Traits have consequences. Not a purpose or function.