The things that intrigue me most about the human body.

[TheVat]

I wonder if there are multiple reasons - and if any one had particular selective importance - that hair remained thick on the head - sun protection on area most exposed to midday sun, sexual attraction, extra cushioning for blows to vulnerable braincase and nape, etc.

I would guess a hominin having, in youth, a visible growth that somewhat indicates one's quality of nutrition (thick lustrous hair v thin patchy dry) might serve as one signal of health and vitality in a potential mate.  Would be further confirmation on top of other signals like facial symmetry, height, muscular development, etc.

 

[CharonY]

2 hours ago, TheVat said:

I wonder if there are multiple reasons - and if any one had particular selective importance - that hair remained thick on the head - sun protection on area most exposed to midday sun, sexual attraction, extra cushioning for blows to vulnerable braincase and nape, etc.

Absolutely. The problem with these types of questions is that they are very difficult to test. Therefore, narratives alone are pretty much worthless, which is why folks advocating thermoregulation created some models to check whether they make sense (and there has been some back and forth). But as always, the situation is highly multifactorial and a lot of things can play a role, including stochastic effects (e.g. drift) or something that occurred during that time, but for which we won't find any fossil evidence for.

For example, one could speculate that at some point there was some extremely severe skin disease or parasite where somehow loss of functional hair provided a benefit. We cannot prove it either way, but clearly it does require some level of substantiation.

Similarly, sexual selection has been mentioned by some, but as these also do not leave records, it is not a terrible useful model. 

[Genady]

6 hours ago, CharonY said:

Absolutely. The problem with these types of questions is that they are very difficult to test. Therefore, narratives alone are pretty much worthless, which is why folks advocating thermoregulation created some models to check whether they make sense (and there has been some back and forth). But as always, the situation is highly multifactorial and a lot of things can play a role, including stochastic effects (e.g. drift) or something that occurred during that time, but for which we won't find any fossil evidence for.

For example, one could speculate that at some point there was some extremely severe skin disease or parasite where somehow loss of functional hair provided a benefit. We cannot prove it either way, but clearly it does require some level of substantiation.

Similarly, sexual selection has been mentioned by some, but as these also do not leave records, it is not a terrible useful model. 

In short, "just-so stories."

[CharonY]

3 hours ago, Genady said:

In short, "just-so stories."

Exactly.

[Ken Fabian]

18 hours ago, Genady said:

Why is it so? Why could not this trait evolve multiple times independently in separate human populations?

It would be very odd for a trait that appears to offer no special advantage and considerable disadvantage except in exceptional circumstances would be selected for repeatedly in widely different environments and result in all populations evolving the exact same trait, all very recently, all since speciation. I would expect it to be expressed with significant variation in separate human population if that were so but it isn't; it seems to be a trait that has effectively been unvarying - irrespective of how varied adult hairiness is (a secondary sexual characteristic) childhood furlessness is universal. A trait shared universally by all members of a species will almost certainly go back to common ancestors.

18 hours ago, mistermack said:

On the subject of increased nerves feeding hairs, maybe it's a question of leverage. A relatively big hair might give more nerve stimulation than a small fine one, for the same touch event, so the nerves increased to compensate.

I think we really do have greater sensitivity than related apes - and by implication, than our shared ancestors. I can't prove that of course. Doing experiments with chimps and bonobos to compare to humans to determine relative sensitivity seems difficult but doable - they are our best stand-in for our common ancestors but that can't deliver certainty.

The late Mr Montagna seems to have thought the sensory sensitivity of human skin via hairs/hair follicles was exceptional. My own experience is of high sensitivity - but that is subjective. That so many people fail to notice the difference between the sensations from hairs and sensations from direct skin contact and appear to believe they add nothing at all is maybe a question for Psychology.

Arriving at any definitive conclusions is probably expecting too much - I don't claim to have any, barring being confident that we can rule out sexual selection because... the furless child.

I have some hopes that delving into DNA might shed light on it.

[Genady]

4 minutes ago, Ken Fabian said:

It would be very odd for a trait that appears to offer no special advantage and considerable disadvantage except in exceptional circumstances would be selected for repeatedly in widely different environments and result in all populations evolving the exact same trait, all very recently, all since speciation. I would expect it to be expressed with significant variation in separate human population if that were so but it isn't; it seems to be a trait that has effectively been unvarying - irrespective of how varied adult hairiness is (a secondary sexual characteristic) childhood furlessness is universal. A trait shared universally by all members of a species will almost certainly go back to common ancestors.

I think we really do have greater sensitivity than related apes - and by implication, than our shared ancestors. I can't prove that of course. Doing experiments with chimps and bonobos to compare to humans to determine relative sensitivity seems difficult but doable - they are our best stand-in for our common ancestors but that can't deliver certainty.

The late Mr Montagna seems to have thought the sensory sensitivity of human skin via hairs/hair follicles was exceptional. My own experience is of high sensitivity - but that is subjective. That so many people fail to notice the difference between the sensations from hairs and sensations from direct skin contact and appear to believe they add nothing at all is maybe a question for Psychology.

Arriving at any definitive conclusions is probably expecting too much - I don't claim to have any, barring being confident that we can rule out sexual selection because... the furless child.

I have some hopes that delving into DNA might shed light on it.

This "trait" of furless babies might be simply a delay of development of the secondary sexual characteristic. Then, it is not selected one way or another. The one variation it allows for is in duration of the delay until maturity, and this I think in fact varies between populations.

[Ken Fabian]

9 hours ago, Genady said:

This "trait" of furless babies might be simply a delay of development of the secondary sexual characteristic. Then, it is not selected one way or another. The one variation it allows for is in duration of the delay until maturity, and this I think in fact varies between populations.

Other apes have fur from birth until sexual maturity; having fur during the juvenile stage isn't a secondary sexual characteristic but would be a normal juvenile trait. Not having it is the peculiarity. The furlessness doesn't go away with puberty. But it could be a delay or missing out of an earlier growth stage than that.

When puberty starts does appear to vary between populations - the duration of the furless childhood stage might change but otherwise it looks like the same pattern of long hair on head and only fine vellus hairs elsewhere is universal.

And yes, Heterochrony, including Neotony - changes to timing, rate and duration of growth - has been suggested as involved in furlessness and I'm inclined to agree, but it looks more complicated than just that. The gain in follicular nerve supply doesn't appear to be a consequence of a change in timing. Going by Montagna it sounds like it is an exceptional change, but I'm not sure anyone knows; I'm not sure we'll see the kinds of comparative anatomy that Montagna did, that included examination of human and animal skin samples, repeated or extended. Maybe genetics will end up telling us to what extent the factors that affect the size of the hair shafts affected the nerve supply - and what was the result of other evolutionary changes.

 

[Genady]

3 hours ago, Ken Fabian said:

Other apes have fur from birth until sexual maturity; having fur during the juvenile stage isn't a secondary sexual characteristic but would be a normal juvenile trait. Not having it is the peculiarity.

To the list of my doubts, I'd like to add that a conclusion regarding normality or peculiarity of a trait based on comparison of couple dozens of extant apes is weak. There were many times more apes during the last 15 or 20 million years. How many of the extinct apes were furless or had furless babies? Nobody knows. Humans happen to be the only ones surviving today. Even among other surviving apes, large parts of their bodies are in some of them furless or almost furless, such as lower bodies in some gorillas and faces in all of them.

It is possible, that furlessness in humans is coincidental and does not have much to do with traits which "make us human."

[Genady]

@joigus: Here is to answer your question in another thread. I just got a neg rep, above.