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:
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?