In today’s Classic column about why green and blue are not naturally occurring human hair colors, Cecil states:
But that seems the most relevant part of the question: why red and not green or blue?
Variations of melanin is the primary pigment in human beings. It ranges from palest blond through various brown to darkest possible brown/black. As is happens, there are also a couple of variations that yield shades of red. So, really, it takes only a minor change to get red from the typical brown.
You see this in other mammals, too, where you have a red option for hair/fur color in addition to the usual browns, tawny-blond, and black. It’s only a small change from the brown sequence, which is why it has appeared over and over among mammals.
Blue, however, is typically a structural color and not a pigment (although there are probably exceptions). It is frequently achieved in nature by refractive effects, which require a certain stiffness such as possessed by feathers, scales, and insect exoskeletons. Hence we see it in fish, reptiles, arthropods, and birds - but rarely in mammals. As noted in the column there is an ape with blue pigmentation but it’s very much an exception to the general rule. Blue pigment effects are rare and apparently hard to achieve. Human skin and hair don’t have the refractive properties for achieving it as a structural color.
Green in birds and reptiles frequently involves overlaying the blue structural color with a yellow pigment… which again, humans don’t really have. So not only would you need major changes for a structural color, you’d need to evolve (or genetically engineer) a new pathway to produce a new pigment.
Mutations in birds that negate production of a color show that such colors are produced by separate genes. For example, my bird Sydney, a cockatiel, lacks the ability to produce yellow pigments, so the only “colors” he has are the browns/greys/blacks. Instead of a normal cockatiel head he has a white one (hence, the source of the term “white-headed cockatiel” to describe this mutation). There is another variation, where a cockatiel produces yellow pigments but no melanin/brown ones… resulting in a very pale bird accented with yellows and some orange. Black mask African lovebirds are normally green and yellow with a dark head, but there is a blue/white/black variation and another that is yellow and green but without the darker shades and no “black mask” (Note how the persistence of structural color. Only if MULTIPLE mutations affecting coloration occur can you get a white/albino bird.
So, basically, because we’re mammals we pretty much have only the melanin continuum of color to work with - many variations, but all of them brown to one degree or another with short step to a yellowish variant and a red one giving us all the hair color variety we currently enjoy, and no more.
Now, it is true that human irises can be grey, blue, or green in addition to the usual brown… but oddly enough not red (the “red” irises of albino humans are actually colorless, allowing you to see the color of the eye’s blood vessels). I’m not sure if human irises have some sort of structural color thing going on, but it wouldn’t surprise me as it would be easy to go from that to green by an overlay of yellowish pigment (a very pale variant of melanin) to very dark brown by simply adding more/darker pigment.
TL:DR - it easier to go from brown to a red color rather than investing a whole new pigment pathway for people.
Yup, human blue eyes are structural blue, and green eyes are structural blue combined with yellow pigments, just like for bird feathers.
It might be significant that, so far as I know, these hair colors aren’t found in other mammals either. Maybe the right pigments just never evolved.
Sorry. I didn’t the last, more detailed comment. I’m still a little new at this.
This (well, your whole post, not the TLDR) is what I expected to see in the column! Nice work.
Powers &8^]
Agreed. That was some impressive ignorance-fighting!
Yes, your response is much better than the column. Which was my point.
Edit: nevermind.