Science desk
< June 29	<< May \| June \| Jul >>	July 1 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

June 30

Doubt on Melanin

A black body is a perfect absorber. The name "black body" is given because it absorbs all colors of light according to Black body article. Then why Melanin changes skin color to Black instead of white in skin tone of African diaspora which clearly orginated from Eqautorial region where Sunlight concentrated more than other regions? Rizosome (talk) 16:50, 30 June 2021 (UTC)[reply]

Did you try reading the article on melatonin that you linked to? You'd see that it has absolutely nothing to do with skin color and is not a skin pigment at all. It's a hormone related to the sleep-wake cycle. Please format your question in a way that a 1 second glance at the article wouldn't already show you are wrong. --OuroborosCobra (talk) 16:54, 30 June 2021 (UTC)[reply]

Thanks for pointing my mistake, Now I fixed it. I am waiting for answer. Rizosome (talk) 17:03, 30 June 2021 (UTC)[reply]

Rizosome to keep it simple: melanin absorbs and dissipates the harmful UV rays that would otherwise cause extremely high rates of mutation in the cells underneath those containing the melanin. So it's better to have darker skin (with more melanin) in sunnier regions.

The reason humans who left Africa lost melanin is because less melanin facilitates the synthesis of vitamin D from UV light, so in regions where there's not enough sun to cause problems, lighter skinned people experience a benefit. ᛗᛁᛟᛚᚾᛁᚱPants Tell me all about it. 17:14, 30 June 2021 (UTC)[reply]

All of which is in the melanin very easily, had they chosen to read it, but seeing as they didn't even read the melatonin article that they had originally linked to, I guess we can't expect that. Rizosome, a reminder that the reference desk is NOT a place for random questions that are incredibly easily answered by a cursory glance over the relevant article. You have one of the largest and best encyclopedias humanity has ever made at your fingertips, please at least try to use it before coming to the reference desk. --OuroborosCobra (talk) 02:34, 1 July 2021 (UTC)[reply]

According to physics, Black Body is good absorber, then why it's better to have darker skin (with more melanin) in sunnier regions? You first line says "melanin absorbs and dissipates the harmful UV rays".This single line doesn't describe the reason why darker skin in those region? Rizosome (talk) 03:24, 1 July 2021 (UTC)[reply]

And also I have found this source saying white paints are good reflectors of sunlight. Rizosome (talk) 03:32, 1 July 2021 (UTC)[reply]

Melanin is not paint. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:29, 1 July 2021 (UTC)[reply]

Also, the skin colors called "black" are not the same as the general color black and neither is the same color as what a true black body would appear. --Khajidha (talk) 11:48, 1 July 2021 (UTC)[reply]

The melanin absorbs the harmful sunlight. If the melanin was not there then the sunlight would be absorbed into the body's tissues (e.g. the skin). But now the sunlight is getting absorbed into the melanin. So the melanin is protecting the body from the sunlight by absorbing the sunlight. So yes, the melanin is a good absorber of sunlight and it is a dark colour so that it can protect the rest of the skin and body from the sunlight. Does this make sense? 163.202.51.17 (talk) 11:52, 1 July 2021 (UTC)[reply]

OK, it is clear that a black pigment is good because it means light is well absorbed over a wide range of frequencies, protecting the tissue underneath. But there does remain an interesting question that is not being answered here: why use a pigment that absorbs rather than a surface that reflects the light? It need not be a mirror-like reflection, although some fish and insect have managed to engineer such surfaces. Guanine is a widespread chemical in the body that is used to reflect light in some animals. Jmchutchinson (talk) 13:09, 1 July 2021 (UTC)[reply]

The question "why use a pigment that absorbs rather than a surface that reflects" implies that evolution is somehow consciously driven. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:19, 1 July 2021 (UTC)[reply]

No it doesn't! If you prefer, why did sun protection based on absorbance evolve rather than protection based on reflection? But that is just a longer way of asking the same thing. There is no teleology implied in my original phrasing, which is absolutely standard amongst academic biologists. The answer to the question might be that one method has a selective advantage, or it might be that we can envisage some biological constraint preventing evolution towards the better solution. Jmchutchinson (talk) 14:43, 1 July 2021 (UTC)[reply]

I phrased my earlier answer along the lines that you are thinking (i.e. that light skin is somehow fundamental and dark skin is a mutation that was selected to protect against the sun) because that's how the OP was framed. But I think the current consensus is that the first humans were dark skinned and the ones who migrated away from the equator evolved lower pigmentation so as to gain from the *helpful* aspects of sunlight (e.g. vitamin D). 163.202.51.17 (talk) 14:50, 1 July 2021 (UTC)[reply]

Loss of melanin as we migrated away from the equator is quite straightforward to explain. More interesting is the evolution of skin colour as we became largely hairless, because up to then hair shaded the skin. I think chimps have pale skin, so that might well have been the original condition of our ancestors before we lost our hair. Jmchutchinson (talk) 15:08, 1 July 2021 (UTC)[reply]

Back to paint. (I know melanin ain't no paint, but that is not an argument.) Suppose you have a transparent sheet, say a windowpane, and you want to shut out daylight. You can do this by covering it with paint. You have a choice between two paints, a black paint and a white paint. Which do you choose? Not necessarily the one that is the most reflective. What matters is how much of the incident white light is let through. To minimize that, you want to maximize the sum of the amount reflected and the amount absorbed. Consider this. If you use red paint, the pane will look red from both sides. The paint absorbs the higher frequencies of the spectrum (towards blue); the remaining light, mainly red, is partially reflected, partially let through. If you use blue paint, it is the same story except that now another part of the spectrum is absorbed. With most paints, the light that is let through has the same spectral distribution as the reflected light. To darken a room using white paint on the windowpanes you have to slather it on rather more thickly to achieve the same effect as with black paint.
Perhaps there is a pigment that is biosynthesizable at a not excessive cost and reflects more UV than melanin absorbs. If so, and if evolution was a matter of carefully pre-planned design, it might have been chosen by the Engineer instead of melanin. But that is not how evolution works. It works through optimization by hill climbing in small tweaks of trial-and-error experiments until a local maximum is reached. --Lambiam 14:47, 1 July 2021 (UTC)[reply]

Lambian, you are absolutely right that evolution may not find a global optimum, and right to point it out to this audience. Indeed in this case it seems likely that selection acted on melanin concentration because this was a pigment already produced in skin tissue. Having said that, it is the sort of explanation that is a bit too easy. Biologists have come a long way in understanding life by applying the modus operandi of only using such constraint-based explanations as a last resort if adaptive explanations fail.

What your answer also made me think is that, although it is not so important whether the harmful UV wavelengths are reflected or absorbed, it would be advantageous if the other longer wavelengths were reflected rather than heating up the body. I don't know about the reflectance spectrum of skin in the infra-red but at least the absortion spectrum of melanin shows a strong decline as wavelength increases. Jmchutchinson (talk) 16:48, 1 July 2021 (UTC)[reply]

Trying to bring some science to that poorly-formulated question, I wondered whether the different skin tones (keyword: Fitzpatrick scale) did have very different absorption spectroscopy. I did not find anything in the IR range. However, there is some data for the visible and UV ranges, for instance this. I also found that one where figures 3 and 11 are somewhat funny, even if the science is a bit dubious (it is OK-ish to round up the lab students for your pet spectrometry project even if you will not get any decent statistics out of it; however, "Asian" is not a sufficient descriptor if I want to reproduce your spectra, and they themselves admit the spectrometer was not well-calibrated).Tigraan^{Click here for my talk page ("private" contact)} 14:49, 1 July 2021 (UTC)[reply]

As a broad observation, if one assumes that human biology could have permitted the development of a reflective, rather than a light-absorbing, skin pigment, it would likely make humans more rather than less (or at least no more) visible in the landscape. They would therefore have been more easily seen by prey they were hunting, and also by predators stalking them, so natural selection would be unlikely to favour it. {The poster formerly known as 87.81.230.195} 2.122.0.163 (talk) 17:53, 1 July 2021 (UTC)[reply]

As noted in the Sunburn article, melanin absorbs the damaging rays, but even those people with maximum melanin are still subject to sunburn. ←Baseball Bugs ^{What's up, Doc?} carrots→ 22:51, 1 July 2021 (UTC)[reply]

Lambiam, based on your line: Not necessarily the one that is the most reflective. Then why are they trying to use ultra white instead of ultra black here? My ultimate question is: why use a pigment that absorbs rather than a surface that reflects? Rizosome (talk) 01:14, 2 July 2021 (UTC)[reply]

The evolutionary advantage of a dark skin is not that it keeps you cool when the Sun is shining. The advantage is that it keeps harmful UV radiation from damaging the underlying tissue. For protection, you want a maximal combined effect of absorption and reflection. It is better to have 20% of the harmful radiation reflected and 50% absorbed, than to have 50% reflected and 10% absorbed. --Lambiam 11:49, 2 July 2021 (UTC)[reply]

Something that no one has mentioned, perhaps because your original question didn't mention heat, is that as well as absorbing a lot of sunlight black bodies are also able to radiate heat effectively, so they cool down faster in the shade. In hot countries you'll see many people dressed in white and also many dressed in black. If you're going to be out in the sun all day white clothing will keep you cool, but if you're going to be in the shade black clothing will keep you cool. Similarly, in hot countries houses are often painted white to keep cool, and it's irrelevant whether the ultraviolet is able to penetrate through the paint into the building material. nagualdesign 01:40, 2 July 2021 (UTC)[reply]

@Nagualdesign: This is not true in general, because the radiative heat transfer shifts in wavelength depending on temperature. "Black" for sun absorption is not the same as "black" for emission. Most of the radiation from the Sun is in the visible spectrum (of course, one can assume it is not a coincidence that earthbound animals evolved vision in the wavelength range that the Sun emits); however, radiation from human bodies close to 300K or so comes at around 7-14μm (which is the usual detection range of thermographic cameras). As a quick and dirty rule of thumb, the emissivity of non-metallic objects (including clothing and human skin) is around 0.9 in that range. (If the 7-14μm "color" was as object-dependent as the visible-spectrum color, thermographic cameras would simply not work.) Tigraan^{Click here for my talk page ("private" contact)} 15:06, 2 July 2021 (UTC)[reply]

While your rule of thumb is correct as a rough approximation, and all non-metallic objects emit infrared radiation, black objects do emit more. If you take two objects of the same temperature, one white and the other black (eg. chalk and graphite), the black one will look brighter in infrared. To complicate things a little, if you clothe the same object (a warm human body) in white you also reflect some of the heat back, whereas black clothing allows the heat to pass through more easily. So if you're in the shade it's a preferable to wear black since your body is the main heat source. And to complicate things a lot, most of the heat in our clothes is actually carried away by conduction with the air, so if the air temperature is lower than our body temperature and there's an ample breeze it's sensible to dress from head to toe in black, loose-fitting clothing even in bright sunlight, like a Bedouin goatherd. nagualdesign 22:51, 2 July 2021 (UTC)[reply]

If you take two objects of the same temperature, one white and the other black (eg. chalk and graphite), the black one will look brighter in infrared. That is only true if the visible-black object is also infrared-black. It might be that more often than not visible-black objects are infrared-black, but it is not a given. For instance, chalk is quite black near 7μm (1400cm-1 in spectroscopy units); according to this article (citing another one that I cannot access) it is due to asymmetric stretching of the CO₃^2- compound. Tigraan^{Click here for my talk page ("private" contact)} 09:20, 7 July 2021 (UTC)[reply]

This two lines answered my doubt: The evolutionary advantage of a dark skin is not that it keeps you cool when the Sun is shining. The advantage is that it keeps harmful UV radiation from damaging the underlying tissue. Rizosome (talk) 01:26, 3 July 2021 (UTC) [reply]

Resolved

Guinea pigs, figurative usage

In the play and film Wit, the central character is dying of cancer, and the doctors obtain her permission to try all kinds of experimental treatments on her. Our film article says that the doctors see her less as someone to save and more as a guinea pig for their treatments, but the words "guinea pig" are linked to our guinea pig article, about the animal. Do we have any article that might be more relevant? Human subject research appears to be about the general concept of scientific research on humans, not someone who submits to experimental treatment, whether excitedly or grudgingly, and human experimentation is just a redirect to human subject research. Nyttend backup (talk) 21:04, 30 June 2021 (UTC)[reply]

I've linked the film article to guinea pig#In scientific research which is the relevant section of that article, and which explains the term used as a metaphor in the second paragraph.-gadfium 21:17, 30 June 2021 (UTC)[reply]