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Most species of birds, reptiles and fish have four types of cone cells in their retina, thus they have four independent channels for conveying color information. They are:

  • short-wave (S) cones: sensitive to colors with short wavelengths.
  • middle-wave (M) cones: sensitive to colors with medium wavelengths.
  • long-wave (L) cones: sensitive to colors with long wavelengths.
  • ultraviolet (UV) cones.

This condition is called tetrachromacy. Wikipedia mentions:

It was the normal condition of most mammals in the past; a genetic change made the majority of species of this class eventually lose two of their four cones.

We have only three. My question is, why did we lose our UV cones? I searched a little about this and here's what I found:

Scientists Have Found a Woman Whose Eyes Have a Whole New Type of Colour Receptor

According to estimates, that means she can see an incredible 99 million more colours than the rest of us, and the scientists think she's just one of a number of people with super-vision, which they call "tetrachromats", living amongst us.

They took 25 women who had a fourth type of cone cell, and put them in a dark room. Looking into a light device, three coloured circles of light flashed before these women's eyes.

To a trichromat, they all looked the same, but Jordan hypothesised that a true tetrachromat would be able to tell them apart thanks to the extra subtlety afforded to her by her fourth cone.

Which means some of us still have it. What I don't understand, though, is why this isn't a more common trait? Evolutionarily, is being able to perceive vision using three cones "enough" for humans?

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  • $\begingroup$ we lost more than that, primates only have 3 cones cells due to a mutation creating a new cone cell, other mammals including the ancestors of primates only had two cone cells. $\endgroup$
    – John
    Commented May 23, 2020 at 14:13

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Let's be clear: most mammals, including basal primates, had only 2 cone cell types. Primates evolved a new cone cell becasue frugivores have a distinct advantage being able to distinguish reds and yellows. The mammal lineage may have never had a fourth cone cell, if they did it was lost early.

Eutherian mammals lost the third cones cells likely because we spent several hundred million years as small nocturnal animals during the dominance of dinosaurs. In small nocturnal animals, there is a strong competition between having more rods for better low light vision vs having cones for color vision, so a loss of some cone cells may have been neutral or even beneficial. There was some work on bats that seemed to contradict this, but the researchers failed to control for frugivory in the bat lineages they studied; So there is an issue with their results. There is a lot of variation in color vision across vertebrates so it is no surprise a consistent selective pressure could have a lasting effect.

source

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  • $\begingroup$ The source you cited mentions -"The recent gene duplication and mutations that have given rise to the series of longer-wave pigments in primates also appear to have occurred independently in some teleosts and probably within some reptiles," which clearly means I had some conceptual misunderstanding about the evolution of cones. (sorry for that) Thanks for your quick answer and the amazing source! :) $\endgroup$
    – Bipasha
    Commented May 24, 2020 at 13:43
  • $\begingroup$ the evolution of the different receptor proteins is very complex in the details, there are mutations all over the animal kingdom. worse the gene is fairly easy to duplicate so you often have animals with both the original form of the gene AND and mutant form at the same time. $\endgroup$
    – John
    Commented May 24, 2020 at 15:11

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