[wdobbels]

I'm probably missing something obvious but... Should white skin not reflect sunlight more, and dark skin absorb it more? I agree that northerners have more need to absorb sunrays and hence make more vitamin D, so shouldn't they be black?

[pdonis]

> Should white skin not reflect sunlight more, and dark skin absorb it more?

"White" and "dark" refer to the skin's effects on visible light. Visible light is different wavelengths from UV light; vitamin D production in the skin depends on UV light. "Dark" skin blocks UV light more than "white" skin does.

[BurningFrog]

Yeah, it's confusing.

I think this is how it works: The melanin is not transparent, so it stops the light from penetrating beyond the very surface, and so a smaller volume of skin cells get access to sunlight.

To go a little deeper: Sunlight does two major things in our skin. It creates vitamin D, and destroys folate. Both are essential, so the various skin colors we have reflect the balance evolution has struck in various environments.

[leto_ii]

Nope. Dark skin is an adaptation to exposure to lots of UV rays: https://en.wikipedia.org/wiki/Melanin#Evolutionary_origins

Incidentally you can test this yourself: if you go out in the summer you will tan i.e. your skin will darken. This is adaptation to lower the risk of sunburn and skin cancer.

[hcarvalhoalves]

Vitamin D synthesis happens underneath the skin, and requires UV B radiation. The dark skin is an adaptation to avoid UV damage from the normal UV levels you get at the less extreme latitudes by filtering out most of it.

[jshevek]

Evolution makes adaptations to what it already has, unlike a sentient designer who might create an optimal design then look for ways to implement it.

If you could design a human from scratch, it might be most efficient to give northerners a D synthesizing pigment that's opaque in the necessary wavelength, resulting in the situation you describe.

Instead we have protective pigments near the surface, synthesizing structures underneath, and a need to balance D synthesis against UV damage.

[dreamcompiler]

It's not obvious; this confused me for a long time. The skin we call "white" is fairly transparent to UV, and the skin we call "black" is much less so. Skin is a mixture of transparency and reflectivity at different wavelengths and different depths below the surface. As such it's extraordinarily difficult to model it so it looks right, as any CGI engineer will tell you.

[pygy_]

Melanin is deposited in the epidermis, vitamin D conversion happens deeper, in the blood vessels of the dermis.

[Florin_Andrei]

The dark (or not so dark) layer is quite superficial, not far from surface. But the vit D production happens deeper in. For sun rays to get there, that layer must be light-colored.

Sure, some photons do get reflected, but the benefit is from those photons that get deeper into the skin.

[Raphmedia]

Melanin in dark skin doesn't absorb as much UV radiation. It makes sense for people who live under the sun with relatively little protection to be dark skinned but it wouldn't make sense for people who are rarely exposed to the sun to be resistant to it.

[ahakki]

IIRC from high school biology, the melanin in the upper skin layer absorbs the sunlight thus preventing it from going any deeper into the skin.