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As I said yesterday, it's been known for decades that it's critical in bone formation, recently was found that it is also critical in neural development and yesterday I read that it is also important in disease prevention and recovery.
And now I read that it has yet another function: it allows macrophages to ignore cholesterol, what prevents these vital defense cells from becoming "foam cells", a central element of atherosclerosis, the degenerative disease that clots blood vessels.
So, well, there are loads of reasons for depigmentation north of the tropics, where winters exist and cause a serious decrease of the natural intake of this crucial vitamin.
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10 comments:
Why are Europeans white?
Newly evolved fur coat a quick hit in Nebraska
Sí, sí...
So you think that depigmentation was a cammouflage adaptation. Maybe that counts for otherwise useless yellow hair but for skin there are more compelling reasons (and therefore more effective).
"If the Agouti mutation occurred in just one mouse, the chances of it spreading rapidly through the whole population would be slim, which is why newly arising mutations like this have been thought to be unimportant.
But Woodruff points out that mutations can occur while an animal is in the womb. If this occurs early enough, the mutation may be present in many of its sperm or egg cells. Then the animal could have several offspring with the mutation, making it easier for the mutation to spread."
One girl being born with blond hair might not have that mutation spread even under strong sexual selection. A family of blonde daughters is a different story
A lot of people is still not blond, even in Northern Europe. Blondisms in Northern Europe are found in about 50% and in Southern Europe in less than 20% (per category: hair or eyes). It doesn't seem to me it's been under too strong selective pressure.
Instead all Eurasians living out of the tropics are not "black" (chocolate color): they range from café-au-lait to cream: the further north the lighter generally. That means marked selective pressure and a major reason seems to be vitamin D.
A lot of people are still not blond, even in Northern Europe.
Maybe
"This is because a potential mate is attracted not just by a bright color but also by a rare one that stands out from the crowd. By enhancing reproductive success, however, such a color will also become more common and less eye-catching. Sexual attraction will then shift to less common variants, the eventual result being an equilibrium that maximizes color diversity (Brooks 2002; Frost 2006; Hughes et al. 1999)" Here
Well, it's a theory. I'm black haired and I like brunettes rather than blonds. My interest in exotics does not normally include blonds (not exotic enough?) - dunno.
Thanks for that link Tod. I too have difficulty with the vitamin D theory. For example if humans in Northern Europe became white-skinned so recently how did they survive all that time with brown skin? And if modern humans have developed white skin to help produce vit D what about Neanderthals? How did they manage with the dark skin they are invariably portayed as having?
I agree with you comment re. sexual selection, 'a potential mate is attracted not just by a bright color but also by a rare one that stands out from the crowd'.
"A lot of people is still not blond, even in Northern Europe. Blondisms in Northern Europe are found in about 50% and in Southern Europe in less than 20%".
Yes, but. That is these days. There has been a lot of migration. Perhaps originally blondism was virtually confined the the far north. There is a story about a Roman emperor ( I forget which one) who remarked on the blondness of some Angles, either English or German, again I forget which.
...what about Neanderthals? How did they manage with the dark skin they are invariably portayed as having? -
Neanderthals were with all likelihood "white". They are even proven to have such an extreme depygmentation trait as red hair, that, at least in our species, is very tightly associated with rather extreme depigmentation. The exact genetics of Neanderthal depigmentation, except for that redhair gene, are not known but it's only logical to think they were quite white.
if humans in Northern Europe became white-skinned so recently how did they survive all that time with brown skin? -
This is a hype on one gene, a gene that might be related with the most recent northwards expansion in the late Ice Age and post-glacial Epipaleolithic. We can't even be sure that their age estimate is correct.
But, anyhow, pigmentation genetics includes many genes (not all known yet), some of whose derived alleles may be shared through all Eurasia, while others are quite specific of West and East Eurasia, suggesting two migration routes northwards with different depigmentation processes. The gene you mention may be only responsible of the most extreme depigmented variants of Northern Europe.
Yes, but. That is these days. There has been a lot of migration.
Meh! Among native populations, not Pakistanis...
Perhaps originally blondism was virtually confined the the far north.
Blondisms are a diverse array of traits, defined by many genes, not all known yet. IMO blond hair was present alredy in the OOA people and that's why you find blonds in Europe and Melanesia/Australia. White skin (western type) is a process that began surely with the colonization of "intermediate" latitudes north of 30-40 degrees, where it's already quite dark in winter. So guess that some 50-40,000 years ago. Even though the adaptation must have needed some time to consolidate, there was already a strong selection for it back then, so the process may have gone faster than you imagine.
I imagine that dark brown (coffee), brown (milk chocolate) and light brown (toffee) skin variants evolved pretty soon in southern Asia. Two of them are already present in Africa (not counting Khoisan, that are much lighter). From the toffee variant to the standard beige of most West Eurasians there is just a step, and this step must have been fulfilled shortly after colonizing the latitudes around 40-50 degrees. Maybe it took some millennia but it happened.
You can see a very parallel process in East Asia up to America (the inverse would happen up to a point as NAs migrated southwards). Just that East Asians have not developed "the ultimate degree" of extreme depigmentation. Instead they have evolved other variants, still not well understood.
You have to understand that, because of the Gulf Stream, Europe is (and was even for Ice Age conditions) the largest, by far, region so far north with a mild climate. Only Europe could hold as much population under pretty dark wintery conditions (very dark in the far north) as to allow for such extreme evolution to happen. This very special climatic situation, extended with the end of the Ice Age, has generated some rare variants.
here is a story about a Roman emperor ( I forget which one) who remarked on the blondness of some Angles, either English or German...
But this is just a relative matter. There is a clear contrast between Italy and Denmark. But that doesn't mean that blondisms are unheard of in Italy. Not at all, just less frequent and normally less extreme. But my Italian granpa (a blue eyed, blond, bluish white "Tatar", so to say) could perfectly pass as one of those Nords, at least in pigmentation - much of the same for my Basque grandpa and great-grandpa. The cranial structure may be somewhat different though.
"Neanderthals were with all likelihood "white". They are even proven to have such an extreme depigmentation trait as red hair"
Maybe they looked like - This .
The big problem is if Neanderthal females looked like that genetic evidence of the very widespread hybridisation would have been discovered - it hasn't.
It seems that modern humans wouldn't mate with them under any circumstances. The century old ideas of Marcellin Boule, (artists impression), are likely more accurate than almost everything that followed.
Whatever the aspect of Neanderthals it was in any case very different from H. spainens, not just in looks but also in behaviour probably (the main barrier for tigers and lions mating is "cultural" in fact: they don't understand each other's courting behaviour). I think modern recreations make them look too much like us. Also we don't know if they were hairy or not: it would have been a handy adaptation for Homo spp. in all the Neanderthal range - but is for real? And finally people tend to marry (or equivalent) within their ethno-cultural circle (tribe or whatever) and I'm quite sure Neanderthals did never belong to our social circles, even if they were living just a few dozen kilometers away.
But they needed to make vitamin D just like we do, so no wonder they had similar adaptations in this sense, even if everything else was different.
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