Our color vision is based on three different types of visual pigments. This is called trichromacy and as a matter of fact, it is quite unusual in the animal kingdom.
The following information is a compilation of the article Color Vision: How Our Eyes Reflect Primate Evolution.
Animals have either only one visual pigment, like a few nocturnal mammals. Or they are dichromats, which means they have two different pigment types. This is true for almost all mammals—except the primates, which are most often trichromats. And some birds, fish and reptiles even have four different types which makes them sensitive also for ultraviolet light sources.
The questions arises now: How comes that we are trichromats?
1st step: It looks like that the short-wavelength (S) pigments are the most ancient ones, as they are found in almost all vertebrates.
2nd step: Similar forms to our medium- (M) and long-wavelength (L) pigments are also found quite often—and therefore very old. But only a few primates have both of them, so this has to be a recent evolution.
3rd step: Let’s say the M pigment existed (we actually don’t know). And then through some mutation of a few acids in the DNA the L pigment evolved. Now a first interesting step happened: Some female primates inherited one X chromosome encoding the M pigment and one X chromosome the L pigment and they became thrichromats! This way only female primates could be trichromats as only they have two X chromosomes (male = XY).
4th step: Now the New World primates evolved away from Old World primates. They still are carrying the information encoding color vision as described above. But in the lineage of Old World primates a next interesting development happened: A female primate merged the M and L encoding into one single X chromosome through mutation! This way also male primates became trichromats (thanks!).
5th step: This genetic mutation had such a strong advantage (seeing more colors = finding more good food) that X chromosomes encoding only one pigment were wiped out of the genetic pool.
This five steps of the evolution of color vision sound very interesting. But there are also some questions which arise when reading through, which make me think that we didn’t get to the bottom of it yet. Here are my questions:
- How comes that our ancestors suddenly could see the new colors only because of a new pigment type?
- Can really only one female primate be the source of our trichromatic vision?
- And why the hack are there still so many color blind people? Why is color vision deficiency in so many forms still that widespread?
The authors of Color Vision: How Our Eyes Reflect Primate Evolution have some answers ready. But I’m not sure if I can believe what they are writing.
And there is still no answer for my most important question. Maybe you have one? — Why is color blindness still such a common disease?
Why is color blindness still such a common disease?
Maybe because the female carriers of the genes have no evolutionary disadvantage.
1. How comes that our ancestors suddenly could see the new colors only because of a new pigment type?
2. Can really only one female primate be the source of our trichromatic vision?
3. And why the hack are there still so many color blind people? Why is color vision deficiency in so many forms still that widespread?
1. The detection of wavelength ranges and the perception of color are different things. Color perception is taking place in the brain, so if a new range of signals arrives there’s a new opportunity and demand to interpret these new signals – as new colors, for example. But it’s not as simple as “red” cones make “red” colors. It’s a more complicated process that generates the signals we know as opponent colors by taking the signals of two or more cone types and sum these up or subtracting them (roughly: red+green=brightness, red+blue-green=red-green coding and red+green-blue=blue-yellow coding)
2. This question can be asked for almost everything that developed evolutionally. Genetic evidence gives that impression that something like this is the case.
3. If a woman is a carrier of a “color blindness gene” she doesn’t get color blind herself and for any potential sexual partner there’s no way to rule these women out by seeing any difference from women carrying “healthy” genes.
And if a man (in most cases) suffers from any kind of color deficiency or blindness the disadvantage does not seem to be too bad, as humans are social animals. And some color deficient individuals have certain advantages over “healthy” ones. Protanopes or Deuteranopes (I don’t remember, I guess both) can detect camouflage patterns better than normal trichromats. That has something to do with a phenomenon called observer metamerism. This occurs between all individuals, but dichromats have distinct patterns of color confusion which are obviously good for camouflage detection :). Colors that look metameric (alike) for trichromats need not look the same for dichromats. The third functioning cone type “tricks” normal observers into the perception of seeing the same color by countering the difference that dichromats are able to see in some cases.
3.When were all hunter/gatherers perhaps it was actually an advantage to be dichromat since many animals use camouflage to hide from predators. Since women were believed to the gatherers trichromats would have an advantage to seeing what fruit is ripe etc. Maybe that is why more men, hunters, are dichromats and women, gatherers, mostly trichomats. Of course in modern society being a dichromat has lost it’s advantage if there ever was one.
“But for our purposes, the key question was: Could female mice having two different X chromosome pigment genes use the retinal mosaic of M and L cones produced by X-inactivation not only to sense but to make discriminations within this broader range of wavelengths? The short and remarkable answer is that they can.”
–> This is significant. If this is indeed true, it means that colour blindness is just an “equipment” problem rather than a complex neurological problem. But retina and neurons (optic nerve) are still closely linked, it’s a huge challenge to alter it after the nature has built it.
This reminds me of the sci-fi movie Minority Report where the hero foils retinal scanners by switching his eyes. He might also have altered his colour vision as a bonus :D
fxtl:
“This is significant. If this is indeed true, it means that colour blindness is just an “equipment” problem rather than a complex neurological problem. But retina and neurons (optic nerve) are still closely linked, it’s a huge challenge to alter it after the nature has built it.”
Apart from the genetic reasons color blindness or color deficiency can have multiple reasons. You can suffer from achromatopsia after a head injury/brain lesion, so all your rods and cones are intact, but your color rendering capabilities, taking place in later stages in the brain are damaged. So you see only a gray shaded world. It’s even possible that your field of vision is gray shaded in only one half of your field of vision and the other half is fully colored.
Genetic achromatopsia leaves only your rods functional, so at first sight the effect is the same (gray shaded world). But cone vision has many other side effects, like very low resolution, high sensitivity to light etc.
Regarding the movie “Minority Report”:
Let’s say the transplant of human eyes is possible. Then most certainly the complete vision of the recipient will be scrambled beyond any possibility of recognition. The neurological pathways of two different persons are always different, so there’s no way that the recipient would just walk away from the clinic and enjoy his new eyes. The brain would have to learn all the seeing from the very beginning, making sense of the “pixel-noise” coming from the new eyes. This would be a long process, and I’m sceptical the brain being able to restore the full visual capacity, as it was before the transplantation.
Today, artificial retinal implants do work, but it’s a long learning process for the brain to make sense of the information. The learning process would be the more complicated with getting another person’s eyes. :)