Why did humans evolve hairlessness? Hair (fur) protects mammals from heat and cold, what would be the benefit from losing this asset?
I think the most commonly held theory is that fur works on quadrupeds, but once you stand upright, it is less effective, and less fur works better. For later time periods, clothing works better than fur because it is more adaptable. Consider that whatever fur-based system human ancestors had was based on needs in the tropics where it does not get that cold, so it is not hard to imagine that clothing is much more effective.
Recent studies of body parasites suggest that body lice unique to humans differentiated genetically only fairly recently, in the range of several tens of thousands of years. This body lice requires clothing … human clothing on human bodies is the habitat for these lice. This suggests there may have been a reduction from a certain level of furriness only with modern humans living in a wide range of environments and using controlled fire, clothing, and some kind of shelter (hut/house) to deal with the elements. So it is possible that the immediate ancestors to modern humans (perhaps Homo erectus?) were actually fairly furry.
As for details of the body hair, this is also interesting. Why do humans have pubic hair but not a lot of other hair? Why to males have more body hair than females in many cases? Why to human males have facial hair? The African Apes have much less facial hair than most modern human males. It has been suggested that this has to do with sexual selection. It is important to distinguish between the idea that the starting condition is a lot of fur and that females may have lost more than males, vs. the starting condition was very little hair and males have added more. The amount of fur, it’s appearance, etc. may be related to testosterone (this is true in males and females but more obvious in males) so facial hair may be a signal of “quality” in males.
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I suspect that (relative) hairlessness is a result of ancestral humans being “grassland apes” with the capacity to run long distances to run down prey. This requires the ability to produce and evaporate large amounts of sweat to prevent hyperthermia. A lack of hair would facilitate this. Certainly, grassland animals have short hair, and some (ie horses) also sweat to cool off. The ability of humans to produce melanin pigment would give protection from sunlight, and further allow selection of hairlessness.
(I read most of this somewhere, but I can’t remember where)
I keep reading that it’s better for upright animals to be hairless, but it is never explained how the hair “knows” the body is upright. Wonder why we didn’t evolve large ears, like an elephant, or just short fur and sweat through that rather than skin bare to scratches and bugbites and a long, easily-tangled mass of head hair. There must be a lot more to the story.
I think you’re explaining something that’s largely a reproductive preference as a survival mechanism. That’s what it is very much today.
There’s probably no tangible survival benefit to not having hair. It’s probably a reverse-ornament and we got away with it because we could fashion clothes.
I ran into some work (sorry, it’s been a while and I don’t have a source) that suggested that high testosterone in males is a side effect of selection for high testosterone in females. In that model hairlessness is assumed to be the “target” state. Just because men are hairy doesn’t mean they have anything to do with it….
Also, the introduction of clothing may have been much, much earlier than the proposed tens of thousands. If the dating at Diring Yuriakh is correct, then humanity must have had clothing by around 270 ky. If this is so, keep in mind that originally clothing may have been too scanty, or too seasonal, etc, to make it worth a louse’s while to move house.
T. Bruce McNeely, I was going to say the same thing. I’ve heard it convincingly argued (to my layman’s ears, at any rate), that our relative hairlessness probably resulted from the massive amount of heat humans need to dissipate when running for long distances, which I gather is rather rare among other animal species.
Here’s a decent article on the topic: http://www.news.harvard.edu/gazette/2007/04.19/99-runners.html
Mr. Laden, how has this hypothesis been received in academic circles? Like I said, it seems sensible enough to me, but I’d like to know what possible criticisms might be.
I’m sorry, that should have been Dr. Laden. No disrespect intended.
As far as I can determine, all the different commonly proposed hypotheses for both bipedalism and relative hairlessness have about equal chances (moderate) of survival. We lack good killer facts for many of these ideas.
Call me Greg.
Clothing had to come no later than cooking. You really, really want a loin cloth — let’s call it a primitive apron — when you’re tending that sputtering roast. 😉
So the aquatic theory is dead.
There is a comment in this thread of the aquatic theory being dead, but I have seen no reason to completely forget it.
The theory that in some stage of human development some predecessors of modern humans have been shoreline-animals feeding mainly from the sea seems still quite plausible to me, even if researchers have been recently sidestepping it.
All mammals that have became seaborne have quickly lost their hair because hair is a hindrance to an animal spending a lot of its time in water.
I can well see that a later drastic change in environment may have pushed these semi-sea-dwellers eventually to a much dryer environment, where the furlessness may have given the benefits described above.
in some stage … some predecessors … shoreline-animals feeding mainly from the sea
That is not the aquatic ape theory. The aquatic ape theory postulates that humans went through a state (not some, but the whole species) in which they lived a good part of their time actually in the water. Gave birth on land, probably slept on land, but foraged mainly in water deep enough that the kids soebt a good part of their time flating around with that extra baby fat for buoyancy and hanging on to mom’s hair, etc. etc.
This would then explain (fill in long list of things it explains including bipedalism, relative hairlessness, occasional web footed offspring, females having longer hair than males, etc. etc.)
That some human populations were shoreline-animals feeding mainly from the sea is not the aquatic ape theory, not startling, and not evidenced in the record but plausible.
“Overall, it will be clear that I do not think it would be correct to designate our early hominid ancestors as â??aquaticâ??. But at the same time there does seem to be evidence that not only did they take to the water from time to time but that the water (and by this I mean inland lakes and rivers) was a habitat that provided enough extra food to count as an agency for selection. As a result, we humans today have the ability to learn to swim without too much difficulty, to dive, and to enjoy occasional recourse to the water.” – Vernon Reynolds
Right. Vernon Reynolds is the master of the back pedal. No kidding.
Don’t make me get Jim Moore over here.
I’ve spent plenty of time on this, and come across a number of claims that I haven’t been able to substantiate from peer-reviewed sources. Many sources that I have found I no longer have links to, and probably won’t have time to find them. I’ll pass along what I’ve found anyway:
Humans, chimps/bonobos, and gorillas have a very different configuration of sweat glands than other primates. They have eccrine glands all over their bodies, but Apocrine glands only associated with the “hairy” parts on humans: groin, armpits, etc. Other primates tend to have apocrine glands associated with all hair follicles, and eccrine glands are much rarer or absent over most of the body.
Humans have as many hairs over their bodies as chimps and gorillas, only most of them produce very fine hairs rather than the large, coarse, hairs found on our relatives.
These two facts suggest (to me) that the common ancestor had already evolved a human-style heat loss system. What humans have that chimps and gorillas don’t is the system of subcutaneous fat and enhanced circulation through and above it that allows heat loss without interference from the insulating fat.
AFAIK no studies have been done to determine what chimps, bonobos, and gorillas have in this regard, and whether it could be a degenerate form of something like the human condition. This would be a very interesting study to see.
BTW, I’ve come across references saying the the eccrine glands of chimps are non-functional, but these appear to have come from studies in European zoos, and may not reflect conditions in the wild.
IMO this ties in with the theories of Aaron G. Filler MD, PhD, as documented in The Upright Ape: A New Origin of the Species, in that the environment occupied by apes with an australopithecene shape (upright) would have been very energy intensive for rapid movement. If the common ancestor lineage(s) of all the great apes had consistently had an upright posture, the development by one lineage of a human-style heat dispersal system could have produced a burst of radiation, ultimately producing two brachiating offshoots (who then lost the heat dispersal system once limited to equatorial regions where it had little advantage), as well as the erectus lineage(s).
Note that this would explain the existence of apparently upright apes as much as 7 million years old, despite the fact that the split between humans and chimps/bonobos can be dated to ca. 5MYA at the earliest.
Got another one in the approval queue. What is it, do too many links cause it to dump to the queue? Makes it hard to provide references for assertions, no matter how skimpy.
Whatever the case is I think itâ??s great that human beings have evolved to have less body hair. It really differentiates us from other furry mammals. Can I extend an invitation to post some content on the Canadian BioTechnologist 2.0 Blog? The blog is a communal effort devoted to the productivity of the Canadian Biotechnology sector and the fine people who take part in this profession. We are inviting bench scientists and technologists to contribute content: posters, tools, research, presentations, articles, white papers, multimedia, music downloads and entertainment, conference announcements, videos etc. Additionally, we are interested in publicizing the work of your organization. Generally, we are
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Exactly. Imagine if we hadn’t — we’d all be going around complaining about how unfair it was that evolution hadn’t eliminated all that hair and dealing with the lack of attractive (hairless) mates.
Then there’s the problem of what would have cropped up in place of the Furries — Skinnies? Who knows? That’s a problem for Bill Holbrook to work, I suppose.
Okay, he got me here. 🙂 Jim Moore that is. Let me throw out a little bit of what I know about hair as well as some stuff on the aquatic ape idea.
The dates: This NYT news story by Nicholas Wade on the date of hair changes (reprinted at headlice dot org — really) covers it all pretty nicely. The date for the loss (actually more accurately big change rather than loss) of body hair is about 1.8 mya (and I regret I don’t have handy the range that should be in this date, as that’s always important to remember and tends to get lost in news story and general retelling). And then the recolonization of body hair by head lice is where the date for wearing clothing comes in, and that’s given as 72,000 years ago although the range is something like 70,000-120,000 years ago, if I’m remembering correctly.
This is a bit of a pet peeve, the way the range in these studies gets turned into a definite number; there’s always a range in the result from a molecular study. Another pet peeve about human hair specifically is the way we call it hairlessness rather than what it is, which is a big change in body hair length and thickness. I think these things are ultimately important because thinking about how something happened requires we be accurate about what actually did happen — GIGO.
The “radiator hypothesis” does fit the facts about what human body hair is like. But it doesn’t get into the sexual selection part of the feature, and human body hair is either a classic case of sexual selection or something that somehow mimics sexual selection exactly. My site has stuff on this if you’d like to get into it more, but the bottom line is that when you see a feature that varies between peoples, varies radically between the sexes and changes radically exactly at puberty you had better be thinking sexual selection. This is likely also the reason behind public hair, underarm hair, and probably part of the beard question. Scent glands, since we find sebaceous glands in these places, which are also another thing that bears the classic hallmarks of sexual selection, plus being more prevelant in males while females have better scent receptors. Coincidence? Sure. 🙂
Beards probably also have a “power” “bulk” exaggerate size” component, which is a common sexual selection thing too.
As for the aquatic ape idea, my site has an awful lot on why it doesn’t make sense and I won’t try to repeat it here, but let me point out that although the idea’s proponents have for some time now been trying to make the claim that they’re just talking about some waterside rambling and a bit of wading, swimming, and diving, this actually makes their idea even less sensible. The basic problem is that they claim we got the way we did (and they are inaccurate about the features they’re talking about, so they commit the GIGO error) via water use, and the features they claim for us are the features of a very few fully aquatic mammals which have been fully aquatic for tens of millions of years. It never made sense that a few million years of semi-aquaticism gave us those features, but it makes even less sense when you lessen the amount of water use.
The Vernon Reynolds quote mentioned certainly makes sense if you’re talking about humans being able to swim and dive, but that’s not the aquatic ape theory; that’s just regular old human evolution. We can swim and dive pretty well for a primate, about as well as your average macaque, the hairy little buggers. True, the average untrained human can’t hold their breath as well as the average dog, but we manage.
Thank you, Jim. Like that Woody Allen movie…
Greg: this might well be a case where the often scientifically maligned “anecdotal experience” could be valuable.
Is it possible ( and in my mind at least as a hairy bastard) that hair serves the male for purposes of territorial signature? A scenting mechanism?
It seems likely in light of the human “system of subcutaneous fat and enhanced circulation through and above it that allows heat loss without interference from the insulating fat.”
Where there’s fat there’s fire, or in this case sweat–odorous sweat–malodorous sweat if left in a natural state.
Not that I am trying to be confessional or anything…
“some human populations were shoreline-animals feeding mainly from the sea”
Another plug for sushi?…
Yeah, the aquatic ape theory relied on a comparison of humans with other hairless mammals; whales and dolphins, while ignoring otters, seals, etc.
We now know that whales evolved from a common ancestor with hippos and likely developed hairlessness independently of aquaticness.
While searching for something else, I came across an interesting paper: Why do we have apocrine and sebaceous glands? by Alan M W Porter, MD PhD. From the statement of hypothesis:
nah. i think humans evolved without hair to get rid of them head lice, im sure our ancestors tried all their best to evolve hairlessness to get rid of the bugs. lol
The loss of fur in our line of hominids seems to be a long standing puzzle. I got interested in it initially because our remaining body hair has been treated, even by leading academics, as virtually functionless. Having it’s clear and obvious sensory function so thoroughly ignored was seriously irritating – a function that has helped me avoid several encounters with Australian paralysis ticks. A function that is demonstrably useful and so thoroughly a part of our somatic sense that most academics (it seems) fail to distinguish it from sensory input from direct skin contact. A function that is so widespread amongst mammals that it must surely go so far back that it probably predates fur as insulation. Attempting to engage serious academics who actually should know about human hair, it’s functions and it’s evolution hit a solid wall of silence, leaving me with the opiniators of web forums to bounce thoughts off. Need I say that I’m not overly impressed with the level of scholarship surrounding the evolution of (inappropriately named) human hairlessness?
I would like to note that our peculiar furlessness is a trait that appears universal through childhood across our species even if adult patterns are highly variable. Some form of neoteny? Adult variations look to my layperson’s eyes to be something that came later, overlaying that fundamental trait. Even if the diamorphism evident in adult hair patterns can reasonably be put down to sexual selection I’m struggling to understand how underlying furlessness did. I’m inclined to think that it came about as a distinct mutation but I don’t know enough. But then, I don’t think anyone knows enough. Maybe studying our DNA will reveal some answers.