The sensory function of hairs: The Omission that made Darwin wrong about "nakedness".

This looks to me to be a case of a Not Even Wrong level of being wrong.

Darwin appeared to be - usually - a very good observer but I have an image in my mind of him swiping and swatting unthinkingly at annoying flies as he wrote that, without noticing that he was reacting to the disturbance of highly sensitive hairs - that the principle way he was feeling them (very annoying) was hairs, not skin.

Note I am not that interested in dumping on Darwin - but he didn't get everything right and did get this wrong. I am more inclined to dump on the very many academics that came after, who sought to understand the evolution of furlessness in humans, who (like Darwin) lived their lives immersed in the sensations hairs provide but never noticed! Which makes another question - how it is possible anyone could not notice, let alone someone who set out to understand how we evolved the patterns of hair growth we do. Or else they didn't want to question Darwin's (sort of) claim that body hairs are useless?

The evolution of human furlessness (usually, misleadingly, "hairlessness") has been a long running interest of mine, arising initially from a "that isn't right" reaction to the claim that human body hair serves no useful purpose. I had (then) recently felt a tick (an Australian Paralysis Tick - nasty things) walking up my leg, because it bumped against the hairs. Avoiding just one bite by one of those makes having hairy legs worthwhile.

The sensory function of those hairs alerted me to it's presence, before it had dug in. Not an absolute preventative of course; I spend a lot of time with my attention elsewhere and ignoring "ordinary" sensations is more usual than paying close attention to them. But it did prompt me to observe more closely - to notice that a fly or small ant walking around on my hand could be easily felt when it walked across hairs yet barely or not felt at all when it was on hairless skin.

It wasn't that no-one had noticed that we feel things via our hairs, even during Darwin's time. But somehow it kept being overlooked, over and over. It managed to rate not even a mention in very nearly all published papers on how our "hairlessness" evolved, with assumptions of no function commonplace; their efforts were purely in explaining absence of hairs (sometimes erroneously assuming actual loss of follicles rather than just smaller hairs).

If you don't know what hairs do for modern humans, how can it be possible to understand how and why they evolved that way?

 

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I think that's needlessly melodramatic. It's more a case of a man well before his time.

It's like you'd call Newton ignorant for not incorporating black holes into his theory of gravity.

Do you subscribe to a distinction between hair and fur? Can you cite a reference?

Is this an actual claim, made by some actual community of anthropo-psysiologists? Because it sounds like a hypothetical - a strawman. I don't know if it is, I just doubt there is a community of scientists that claims that human body hair "serves no useful purpose" (since it's very difficult to prove a negative) - as opposed to a purpose unknown.


Regardless, it seems to me to have the cart before the horse. The proto-human we evolved from already had hair, for whatever reason. So the answer to the question "what purpose does human body hair serve?" is mal-formed. The question is: why did we lose that hair?


And I was of the understanding that one of the primary competitive advantages of relative hairlessness was increased heat loss. One of the reasons we humans could outcompete rivals and prey is because - even though we weren't fast or strong - we could outlast prey in a jog. We would give chase, they would bolt, and eventually tire, but there we were trotting up behind them, never stopping like the psycho-killer from a slasher pic - until after a chase of several days, the prey just gave up. This gave these puny, weak, slow, fragile humans access to an overabundance of rich meat protein they would never have had access to any other way.

 

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Seems to me you both make valid points. First one needs an evolutionary reason for why humans lost most of the hair on their bodies, but not on the head, armpits or groin. Secondly, a reason why retaining some body hair on a largely naked skin has, or may have, advantages.

I too have read that heat loss on the African savannah when chasing game by endurance hunting may have been the reason. The hair on the head I presume was retained to protect from sunburn, while hair in the armpits and groin helps avoid chafing of sweaty skin where the limbs articulate from the body. Retaining some body hair may not need an explanation, as once enough has been lost for heat regulation, the presence of the residue would not matter. But indeed, there may be value in hairs on the skin performing a sensitive sensory function.

 

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Yes, that seems to be the question. Why does a human primate look so different from all the other primates?

This seems implausible. There are a few predators that do this, but all are quite fast runners that burn far fewer calories to maintain a given pace. Wolves comes to mind. They often live in areas where cover is not easily had and the usual stealth-ambush approach doesn't work as well.
Humans have just two legs, and utterly massive ones at that. I cannot think of any prey that we can chase down by endurance. We also lack the ability to take down large prey. The hairlessness and fat evolved well before the invention of hunting tools. We seem to be omnivores/scavengers eating small/abundant things.

The hairlessness and subcutaneous fat seems to be similar to that of a pig, with similar teeth. OK, the pig, having 4 legs (and far less massive ones) can run considerably faster. Both seem to be evolved for a wet shallow environment. We're swimmers, but not great ones. Our feet are (were) evolving into flippers, not for efficient running, but we moved away from that environment before the job was complete. The aquatic ape hypothesis seems to generate a lot of resistance despite the obviousness of it.

 

Yes, lice and other parasites could be one of the reasons we lost or rather reduced our hair

 

This is relevant https://www.ncbi.nlm.nih.gov/pmc/ar...y primates,the African Savanna mosaic habitat.

Also this. https://pubmed.ncbi.nlm.nih.gov/27000945/

Pubic hair also could be a maturity signal in the tribe, ready for mating.

 

A bit provocative to start with Darwin Was Wrong, maybe, but in this the Not Even Wrong description seems perfectly apt.

It isn't like the sensory function of hairs was something Darwin could not have observed; it was right under his nose, literally, as well as all over his body. It is so ubiquitously pervasive that not observing it is a serious omission. A blindspot? I can understand that; even some extraordinarily accomplished scientists have them. Having a string of biologists coming after and whilst trying to understand how our furlessness evolved persistently failed to notice a principle function of hairs in modern humans - and the principle function of most of the hairs - is more inexplicable. And I do think the sensory function is the principle function of body hair and not just by default, by the loss of other functions but shows signs of providing enhanced sensory capability.

Likewise it was readily observable that modern humans are not hairless ("naked", as Darwin called it) , that hairs are small over most our bodies but not absent.

Sure. Fur does things that human body hairs do not or do not do well, like provide insulation, protection against direct sunlight, display. We have body hairs; they don't add up to fur.

Not unusual to make the distinction. For a reference - one of the contributors to the subject that I have a lot of respect for and will make further references to - Comparative anatomist W. Montagna.

From "The Evolution of Human Skin", 1985 - (the full paper could be paywalled but worth reading if you are interested in the differences in skin (including the hairs) between humans and related primates.) -

Another reference - Nina Jablonki "The Naked Truth" (SciAm - Feb, 2010) -

I favour "furlessness" rather than calling it "hairlessness" because, rightly or wrongly, it suggests absence of hairs and that is not the case.

The incident that incited my interest is, of course, anectdotal, but it included the claim that because human hair serves no useful function they will ultimately be lost entirely, because evolution will favour metabolic efficiency. I don't especially want to spend my time looking for bad examples in science literature but I have encountered that idea as well as some egregious others.

I suggest it is more like a community of scientist had that as an assumption, not necessarily explicitly but by omission, because they were trying to answer "why did we lose that hair?" without having answered "what purpose does the hair we have serve?" first. Speaking of cart before the horse.

Humans have gained exceptional sensory sensitivity by the changes to our patterns of hair growth - a notable gain in follicle nerve supply compared to related apes, notable enough for Montagna to point out that human hair follicles, irrespective of size or location, have nerve supplies more like those of dedicated feeler hairs (vibrissae, whiskers) than the ordinary hairs of related apes. That sounds like it could be adaptation. Understanding how it got that way whilst framing the question as purely about losing hair looks like a sure way to get wrong answers.

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PS I'd like to add a larger quote from Montagna but the PDF I'm using turns out being a collection of scanned images of pages, and needs OCR to lift quotes, which mine does very badly, so would be better as an image file (a snip), not a document. Is there a way to upload an image from my computer?

 

OK, so despite saying you didn't mean to dump on Darwin, you did mean to dump on Darwin.

I think you are arguing with someone long dead. He's not here to defend himself. And it kind of feels like you're putting words into his mouth after-the-fact.

This seems to be needless hair-splitting, designed to belittle him further.

You presume when he said "naked" he meant literally naked, and was not merely a effectively naked by comparison. And you attack him based on your presumption. Again, he's not here to say "Of course it was hyperbole! Can we keep our eye on the ball here?"


We get it - you're smarter than a guy you've got a century of scientific advancement and retrospection on. Regardless, Darwin's intelligence or obtuseness is presumably not a critical component of your thesis, so let's not waste further energy on it.

That is not a qualitative or objective distinction. It's merely a matter of the degree - and a self-serving one at that.

A wind breaker does not stop being a jacket simply because it doesn't protect form cold like a goosedown parka does. Calling a windbreaker a jacket is not "misleading", as you would seem to have it.

How does that work?
Fully-furred proto-humans couldn't sense black flies as well as their slightly more naked brethren? So the slightly more naked brethren have an advantage?
Fully-furred proto-humans were protected from bug bites by their thick fur. How would losing that protective layer be an advantage in the war on bugs?


Traits and mutations are only advantageous or disadvantaegous after they happen. Mutations happened that caused some of our ancestors to be less hairy than others. For whatever reasons this allowed us to outcompete some our hairy brethren.

Me, I'll put the ability to take down enough protein-rich mega-fauna to feed my entire tribe up against the ability to slap away the occasional black fly.

 

I will add that I am not saying thermoregulation isn't an advantage gained but it can't explain the gain of sensory sensitivity, ie cannot be the whole story.

Less furry, more sweaty (they need to occur together) plus fire, shelter, clothing can add up to advantage from loss of hair, whilst evading some of the disadvantage. So does reduced exposure to parasites. As a naturalist named Belt pointed out to Darwin, less fur makes it easier to find things like ticks and lice. Darwin's dismissal of the idea wasn't one of his best moments imo. Likewise Darwin's conclusion that nakedness was a result of sexual selection doesn't stand up to scrutiny. But some erroneous notions persist - perhaps because the question (however we choose to frame it) has no obvious answer and we are mostly reduced to looking at plausibility of possibilities than dealing in evidence. It invites speculation that is not confined to academia and being a relatively minor area of science, lacks the institutional authority to definitively set any speculation aside.

Some ideas I think can be set aside - eg furlessness as a consequence of sexual selection (Darwin and others), as a consequence of preferential infanticide (Harris), a metabolic trade-off that allows better brain development (Dror, Hopp).

I do think clothing, shelter, fire matter - Darwin dismissed the ectoparasite hypothesis because he could find no other examples where species evolved furlesssness for that apparent reason. But no other species can work around the downsides like tool using, problem solving hominids.

 

It doesn't have to.

Mutations happen. Many mutations are harmful. A lot provide little or no immediate advantage, yet they get propagated anyway. Like blue eyes. Or a second pinky toenail. Or male pattern baldness. Or left-handedness. There are countless examples.

(And you don't know that sensory sensitivity is a directly competitive advantage - as opposed to minor side-effect. It's a hypothesis you have. But I doubt you have hard data to support it.)

Not all mutations need to provide an advantage in order to be passed on. Oftentimes it's just that they are located adjacent to some other strong trait on the strand of DNA. Genes that are closely associated tend to get passed on together. (Perhaps linked to the ability to sweat to to jog, who knows without looking).

A lot of progress has been made on this in the last century. Rather than harkening back to an age when slavery was still commonplace, it might be more relevant and fruitful to look into the works being done in this century on the human genome, and what traits might be linked to hair coverage.

It's time to stop tilting at this windmill. Darwin was the father of evolution, not the god of evolution.

 

I am not arguing with Darwin, I appears I am arguing with you.

On this Darwin was wrong - in and of itself no big deal; if you think the thermoregulation/persistence hunting hypothesis explains furlessness you think he was wrong about human "nakedness" too. I just suspect that the framing of the nakedness/hairlessness/furlessness question itself, as too narrowly about loss of hair and loss of function with the sensory function entirely overlooked, goes back that far. Following mistakes back to their source isn't a bad thing and Darwin had a kind of pervasive unintended influence on what came after.

I think the sensory function of our hairs is a valuable one. The presence of bugs, puffs of breeze. But also feeling close passes by hard objects, say dodging tree stumps or boulders whilst chasing prey and better judging just how close. Or feel a thrown rock pass close by.

You don't have to agree but I think something that gives so much sensory sensitivity that a tiny bug can cause children to scream "get it off me, get it off!" is going to be important. We have extraordinary sensory sensitivity - able to feel even the air vibrations of buzzing flies when they get near us; a hungry hominid might find those black flies delicious. If it were a tse-tse fly and sleeping sickness were around, avoiding illness would be a bonus.

It looks like a species of primate body lice didn't survive the changes to hominid body hair; if it were also a disease vector the natural selective implications are made greater than merely the avoiding some parasitic metabolic load. We don't know if furlessness evolved incrementally or came as a distinct mutation. I'm inclined towards a distinct mutation myself, a sub-type amongst the furred. Under such conditions the furless mutants will do better and the relative distribution of furred to unfurred can change very fast. Where it is recessive that would be when a whole lot more furless get mated with furless. Unlike other animals, that would revert back, because being furless without clothing or shelter or fire is not likely to persist.

 

It may have seemed gratuitous or provocative to start with Darwin but, whilst this omission goes back that far it isn't really about Darwin. It is an omission that, with a couple of exceptions, has persisted all the way to the present. This function exists. It has not been well described or the sensitivity well quantified, which doesn't make it not matter, let alone not exist - not Darwin's fault but is an omission that goes back that far and persists to this day.

I admit I don't get this as an objection. Yes it is a hypothesis - an aspect of an ectoparasite hypothesis if you like - but presuming sensory sensitivity has no (or insignificant) role or that enhanced sensitivity gives no advantage or to fail to consider it at all looks like a good ways to end up wrong; not sure to be wrong but if we don't know what this function does for us we can't know that any explanation is right.

The claim that humans gained sensitivity by gaining more follicle nerves is William Montagna's, not mine; unlike others interested in how we evolved to be how we are, I just haven't passed over that anatomical evidence and ignored it. Actually I started as ignorant as everyone else, but I had already concluded that we probably gained some sensitivity - not by greater nerve supply but simply because smaller hair shafts can be displaced and moved by smaller impulses and, being (proportionally) more sparse there will be less dampening of them from hairs laying against each other. Add in what happens when alarmed or aroused - times when sensory sensitivity matters most - and it is further enhanced, by hairs standing on end, extending further from the skin, by reducing even further any dampening by other hairs and by the goosebumps reflex where small disturbances of some hairs can cause sympathetic responses in surrounding hairs, standing them on end, amplifying sensation.

Speculation? Sure but I think it is built around observation. There are plenty of ways to demonstrate that our hairs are very sensitive but less ways to show they are better than related apes or thanour common ancestors'. The hairs on my nose and close to my eyes are so small that I need a magnifier as well as mirror to see them at all but even small disturbances of them provokes unthinking swiping, scratching, swatting and it takes deliberate effort to NOT react. According to Montagna those tiny hairs have follicles exceptionally rich in nerves above and beyond what a chimp or bonobo would have, but I can't know how they would experience it. But I don't need to know that to know they work well, providing a very useful service.

So far as I am aware the only paper published since (or ever) that directly addresses the potential role of hair sensitivity for avoiding ectoparasites has been Dean and Siva-Jothy's "Human fine body hair enhances ectoparasite detection". I think it is a start but not definitive - it demonstrates better detection of one specific pest, bedbugs, but didn't seek to quantify hair sensitivity thresholds in any direct way. The subjects used all appear to be adults and there is well known wide variation in hairiness in adult humans, whilst children, that show little variation, were not tested. It is a useful contribution. A belated start.

It has remained an omission through all this time - did you really give it any thought until this thread?

Hardy's Aquatic Ape had nothing about it. Wheeler, who advanced the thermoregulation/persistent hunting hypothesis effectively declared body hair functionless - "The Evolution of Bipedality and Loss of Functional Body Hair in Hominids" - (Wheeler appears to have originated the "Hairlessness" naming which further suggests absence of any function). Jablonski, as an example of a much more recent contributor to the subject lists multiple functions for hairs in mammals in The Naked Truth article, in what appears a genuine effort to be comprehensive - and failed to include the sensory one, at all.

Jablonski, The Naked Truth -

Whilst not specifically naming humans it is clear that she does include humans as one of those lineages. Jablonski's SciAm article was published a long time after Montagna's comparative anatomy showed exceptionally rich follicular nerve supply in humans compared to related primates along with his claim that is the principle sense mode of human skin.

It is a function that clearly exists, significantly as dedicated feeler hairs as well as being a common function of hairs across mammalia and arguably may have been the very first function hairs had in the earliest mammals.

So I am not claiming a new and novel function for hairs in humans, rather that it has always been one of it's functions in mammals and evolution can enhance that function as well as (if we actually were hairless over our bodies) take it away.There is anatomical evidence human hairs did not just grow smaller but gained the anatomical elements for enhanced sensitivity.

BTW if we assume incremental evolution the objections that small changes won't make a significant difference would also be true for evolving improved hot weather endurance. But I think different things mattered at different times - there were times when avoiding parasites was critical (most critically when they are disease vectors) and times when better hot weather endurance was critical. A role for ectoparasite avoidance doesn't make hot weather endurance wrong or irrelevant - not a case of either/or but of both.

But I do have a problem with that kind of incremental change as being able to explain furlessness; the trait appears to be a developmental one that impacts the progression of human growth and appears to be a developmental delay or truncation. To get from furry to furless means extending infantile traits through to adulthood, with most of the disadvantage of that change loaded onto the young, who, for persistence hunting get benefits only indirectly via parental care (better food supply).

For all that adult hairiness is widely variable the furlessness of human young is universal and has been unaffected. The hairiness of adult Europeans is probably something gained since speciation, not lost. Secondary sexual characteristics do get changed by adult mate selection and the wide variability of hairiness is evidence of it but the fundamental furlessness trait as expressed universally in children appears unchanged by any of it.

 

Sorry, not meaning to overlook other contributions to the discussion.

The first paper you reference - Yesudian's - does mention sensory function in mammals in a general way but imo ultimately falls into the same error of omission when it comes to homo sapiens -

The one about pubic hair is suggesting sexual selection for loss of fur in humans; I just don't think sexual selection can work like that. See the last paragraphs of previous post.

Am interested in what you think of my thinking - that sexual selection seems to be incapable of affecting human furlessness overall, just affect the secondary sexual traits, ie post puberty.

 

These aren't "mistakes"! This is how science works! Theories are put forth, are tested and undergo modification.

You might as well claim Newton was "mistaken" because he didn't account for relativistic effects in his Laws of Gravity.

Theories evolve. There is no question that we are way beyond Darwin's theories now.

If I were a more cyinical man I'd think you're using this as an opportunity to "name drop" Darwin as a way of artificially inflating the importance of your own hypothesis.



OK, so you have a hypothesis that there is at least some survival advantage to proto-humans not losing all their hair. What evidence do you have to support it? I mean, beyond some rationalizing why it might be plausible.

 

In this, the hypotheses are NOT being modified in response to easily obtainable, well established existing knowledge. That is why I've gone historical. Overlooking what is right in front of you - repeatedly - looks like mistakes to me. Call it something else but for any researcher in the present to treat this function as non-existent or presume it to be of little consequence it is a serious failure.

The sensory function doesn't take anything but the most basic of ordinary of observation to identify; humans live immersed in the sensations. We may misidentify the sensations as direct skin touch but simple observation will show that is not the case.

Newton couldn't be expected know about relativity - the tools available were inadequate - but he was fully capable of identifying the sensory function of hairs, because that is what everyone's hairs do. No waiting for better scientific instruments was required - it was and is right under everyone's noses. Darwin's, Hardy's, Wheeler's, Kushlans, Rantala's, Prokov's... Jablonski's. Yours. I expect them all to know about it, because it was/is their everyday somatic experience. Even proponents of ectoparasite hypotheses like Pagel and Bodmar failed to pay it any mind, like it didn't exist. I have no problem addressing objections but think you need to lift the quality of yours.

In this particular matter, no we are not seeing them modified in light of existing or better knowledge.

That is one of my main points and a main reason for me to go historical about it.

Think of it as me championing William Montagna and giving greater consideration to the implications of what he learned from his comparative anatomical studies.

I may have been thinking and talking about this from before Dean and Siva-Jothy's study using bedbugs but I would be surprised if I were first or ever only. But if only published papers counts, then it might be considered their hypothesis but I hope Montagna gets some credit. I don't know what Montagna thought about how our furlessness evolved but I haven't read everything he wrote - some are the required texts for biology courses. He did suggest hairier Europeans could have evolved greater hairiness since speciation. He appeared to have issues with notions that are obviously wrong persisting, specifically claims humans are glabrous over their bodies - ie truly hairless - which must have been common enough to have him include a pointed comment about that needing to be rubbed out. He also dumps on a persistent failure to identify hairs with follicles as the principle "touch sense" - by "most scientists interested in cutaneous innervation". You could argue with him... except he is dead too.

If I have any unique "hypothesis" or contribution I tend to think it is by identifying innate problems with sexual selection as a hypothesis, ie a small part in applying a bit of scientific skepticism - "natural selection" - to the wide range of existing hypotheses and eliminating what doesn't work and narrowing the field down to the plausible.

Improved sensory acuity seems advantageous by default. How is it not? I've pointed out a number of ways that can be advantageous. So have Dean and Siva Jothy. Some are so commonplace - like awareness of bugs near eyes - that anyone can observe it... if they don't misidentify the sensations as from skin touch.

William Montagna -

 

You mean because your paradigm-shifting idea isn't getting all the textbooks rewritten?

But you don't have "existing or better knowledge"; you've got an idea that seems plausible - they're a dime a dozen without evidence.

Why not just dial back the name-dropping and puffery and just come here and discuss your idea, and back it up with science?

 

Thank you for your thoughts Dave.

 

Wait, so that's it?

If you're not bucking Darwin or fighting off a conspiracy then you're not interested in discussing the actual science?

Well that's disappointing...

 

The distinct advantage that humans have is the ability to sweat through our skin, rather than through panting. As such we can remain cool far longer than almost all other animals, and travel at distances and speeds that would result in other animals overheating. This meant that humans, as the predator, could effectively run other animals to exhaustion, via persistence hunting. It was possibly with the advent of longer range weapons that this form of hunting became seen as unnecessarily time-consuming.

Nearly all of them, as it turns out, although would possibly/probably depend on climate - i.e. our advantage is bigger when it's hotter. There's also a history of ultradistance runners outlasting horses, covering more miles in a day in such races, or longer, and with horses dying through exhaustion in such races. https://ultrarunninghistory.com/man-vs-horse/

It's relatively easy to take down large prey when they're exhausted.

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There's also the idea that we needed to be able to run long distances to beat other scavangers to a recent kill.

There was a study in 2009 or so that suggested we evolved for efficient running. Something like a 20% increase in toe length doubles the mechanical work of the foot, and the straightness of our big toe also suggests running rather than anything else.
https://www.nature.com/articles/nature03052

Hairloss was likely all part of improving our ability to sweat and keep cool in warm/hot climates, which aided our ability to hunt and scavange, enabling us to eat meat, and all the benefits that has bestowed upon us. Would we sweat the same way, as efficiently, if we were covered in hair, or would that serve to insulate us and actually serve against us in hot climates? For preservation during an ice-age it would be different. But it seems to be all about the sweat!

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Now, would Darwin have known of such a benefit from us losing our hair, and the benefits that efficient sweating may have had? Unlikely, but does that ignorance justify him simply dismissing the change as not evolutionary (or at least "natural selection")?

So this would seem to explain why we lost the vast majority of our hair, compared to the other primates, at least. As to why we retained some, or why it may have subsequently developed to where it is today, well, sure, it helps detect things crawling on us, and all the other sensory reasons discussed. And possibly other reasons. All evolutionary, I'm guessing.

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If so many researchers for so long have thought the sensory function wasn't worth consideration or mention and hot weather endurance as an adaptation that enhanced hunting capabilities is currently what most biologists currently think was most likely how we got to be furless I suppose it makes a kind of sense to think that the odds are they know better than some guy on the internet.

But not all researchers dismiss the sensory function is inconsequential and I consider it a serious lapse of scientific competence to fail to give it due consideration when seeking to understand how we evolved to be how we are. If it had been given due consideration I think it would have been stated. I think it is Omission, that taints any conclusions that fail to give it that due consideration.

Hot weather endurance is one hypothesis but not the only one - the ectoparasite hypothesis was proposed in Darwin's time (without reference to the sensory function and dismissed by Darwin) but I also don't think it has to be a case of either or. I think there were times and circumstances where reduced parasite loads probably mattered and others when hot weather endurance mattered. Perhaps they got the essential sweatiness that combines with furlessness to give hot weather endurance from the start, a spandrel as part of the furless package but no-one knows, so it is also possible it came separately, and for a time there was no advantage in hot weather and probably disadvantage for children in cold weather. No-one knows.

I think it is possible reduced susceptibility to parasite borne diseases could have been the advantage that came first - it doesn't require enhanced sensitivity or anything but the furlessness, but if that did come spandrel-like, as part of the package it seems like it would be useful in further reducing susceptibility. Maybe later a mostly furless survivor of fur-borne contagion got the sweat mutation and with that combination gained hot weather endurance. They are not mutually exclusive.

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