I understand that the function of melanin is to protect individuals from sunlight, and that people living in sun exposed areas are darker due to increased production of melanin. But why is the melanin pigment black in color? Wouldn't it have been more useful if melanin was white?

From my understanding, black absorbs more sunlight than white, so if melanin is black, it will tend to absorb more heat (which isn't a good thing in areas with a lot of sun exposure). This will lead to production of more melanin (tanning).

Instead, evolutionarily wouldn't it have made more sense if melanin were white, since not only would the pigment protect the person from sun exposure, but also prevent more sunlight from being absorbed?

  • $\begingroup$ As phrased, the question is not a biological one, but one for chemistry or physics: melanin is black because of the way its chemical structure interacts with photons. Perhaps you meant to ask how mammals evolved to produce melanin, and how humans (among others) have evolved to use it as protection against the harmful effects of intense sunlight? $\endgroup$ – jamesqf Nov 11 '18 at 17:41

Let's do a quick mind experiment. Imagine a ray of UV light hitting a white molecule in the skin. White generally reflects light, so that UV light would bounce off, and potentially could hit something much more critical. Black, on the other hand, absorbs light, so the energy in the UV light would be absorbed by melanin rather than other, more sensitive molecules. According to this article:

eumelanins are capable of dissipating >99.9% of absorbed UV and visible radiation through nonradiative means.

Eumelanins are the most commmon type of melanin. Incidentally, not all melanin is black.

Another quick note, somewhat relevant: Try not to think of evolution as having a goal or purpose. Melanin being black is a result of random changes that happened to result in a beneficial phenotype. It didn't conclude that something was needed to counteract UV light and try to reach that goal.

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