Question in the title, but to be clear, the question is not about the physiology which produces little melanin in those areas. Rather it’s about why people would have evolved in such a manner.
Any answer is going to be pure speculation, so I’m going to make up a hypothesis on the spot: early hominids communicated extensively with hand signals, and having pale palms helped with visibility. The pale soles come from shared common genes during development.
Why do you say this?
Melanocytes exist in abundance in the dermal layers of your palms and soles, but are buried beneath the more heavily keratinized callous layers and rarely receive enough photostimulation to produce melanin.
At least I learned that in my residency’s dermatology rotation a while back.
Isn’t there an extra layer of skin there, the Stratum Lucidum?
Becasuse speculations on specifically why specific traits evolve are mostly just so stories.
Then why should you make up such a story? Especially as a first answer in GQ.
Qadgop has given the answer. There is no adaptive evolutionary reason for it. Instead it is just a secondary consequence of the keratin layer in our palms and soles.
Many physical features are like that. They have no adaptive value themselves, but instead are a consequence of some other feature that is adaptive.
He gave the answer the OP didn’t want: “the question is not about the physiology which produces little melanin in those areas.” and claiming that there is definitly no adaptive reason for it is entirely as speculative as suggesting one.
There is a similar “just so story” for why people show the whites of their eyes.
And, the “thicker skin” answer is factually incorrect–people have five times fewer melanocytes on their palms and soles, and nobody knows why:
However, virtually nothing is known about mechanisms by which melanocytes stop migrating in the skin in palmoplantar areas during human embryogenesis and why the palms and the soles are generally hypopigmented.
Except for a small scientific principle called Occam’s Razor. Don’t invoke adaptive reasons for a feature if you don’t need to.
And there was certainly no reason to invoke the absurd explanation you made up.
No more absurd than the eye one. I stand by it as as good an evidence-free speculation as others.
Am I correct that humans are the only primates that have hypopigmented palms? As far as I can see, chimps, gorillas, etc. have dark palms. That would imply that the trait has evolved fairly recently.
I don’t see any evolutionary advantage to lighter palms and soles.
We don’t evolve because it makes sense, we evolve because a change gives us an advantage.
Qadgop posted htat callous layers cover our palms and soles.
I was looking at some videos with chimps recently which coincidentally included a lot of closeups of their hands. I noticed that the skin coloration on their fingers was very uneven or blotchy. Some areas of some fingers would be about the shade of a typical Caucasian human, whereas on the same individual much of the rest of their hands would correspond to a rather dark shade of African human.
I agree that unlike dark-skinned humans, their palms did not seem much lighter than their general coloration. Net of the blotchiness I mentioned.
And I posted a journal article covering the fact that Qadgop was wrong about that being the reason for lower pigmentation. Another quote:
The melanocyte density in palmoplantar human skin (i.e., skin on the palms and the soles) is five times lower than that found in nonpalmoplantar sites. Palmoplantar fibroblasts significantly suppressed the growth and pigmentation of melanocytes compared with nonpalmoplantar fibroblasts.
In other words, it is not a result of skin thickness and an actual developmental adaptation.
Good human and chimp hands together photo on this page:
Good pic and an interesting book review too. That particular chimp has rather more uniform hand coloration than the ones in the vid I watched.
In fact, there is an Uncle Remus just-so story on this very subject.
Warning for people who may not be familiar with Uncle Remus:
Uncle Remus is the fictional title character and narrator of a collection of African American folktales compiled and adapted by Joel Chandler Harris and published in book form in 1881. [snip] He wrote his stories in a dialect which was his interpretation of Deep South Negro language of the time. For these framing and stylistic choices, his collection has encountered controversy.
The story includes the n-word and dialect that many would consider offensive.
This is interesting–a hairless chimp with the blotchy hands but non-blotchy (except for lots of freckles) body:
https://m.riverfronttimes.com/newsblog/2009/02/18/rip-cinder-saint-louis-zoo-chimp-dies-at-fourteen
It seems to me that the physiological reason and the evolutionary reason would be tied together. Yes, the thicker, more keratinized skin @Qadgop_the_Mercotan mentioned would be a physiological reason… but there are obvious evolutionary reasons for those two spots to have more keratinization.
Another factor would be that the palms of hands and especially the soles of feet are exposed to less sunlight, and so wouldn’t need as much melanin. Though that wouldn’t explain why they do have less melanin, only why they can.