I'm curious as to what it means to be colour-blind. Do you simply view the colours you don't perceive as "grey" or some other colour?
It's much more complicated than that. There are different types of color blindness.
I have a strong protan color vision defect so I tend to confuse red with green and blue-green. On the test at
http://www.archimedes-lab.org/colorblindnesstest.html I scored only A and I correctly. This page also shows how a color wheel looks like to some severely colorblind people.
I do have difficulty seeing some wild berries in the grass and on bushes, especially reddish or purplish berries on green plants. My mother or sister will say "The berries on that bush/plant are coming along nicely," but I cannot see them (which tends to frustrate my mother ha ha
"They're right here, can't you see them?" hee hee). If survival depended on wild berries I would be in some deep doo. This is yet another reason why there is no place for people like me in a vegan/vegetarian world. I seem to be a living rebuke to their world view. I not only appear to do much better on a meat-heavy diet than a plant-heavy one, I have difficulty even seeing many of the plants that vegans/vegetarians think were so essential for our ancestors to find, gather and eat.
There is a hypothesis that color blindness survived because it enabled grassland hunters to distinguish camouflaged khaki-colored animal from the slightly-different-colored khaki grass it is hiding in:
"About 6-8% of humans today are red-green color blind. Most of them are men. This X-linked inherited condition known as deuteranomaly is due to opsin pigments that are normally sensitive to green light behaving more like the red-sensitive ones. This results in a difficulty in distinguishing between colors in the red and green wavelength ranges. However, people with this condition are at an advantage in differentiating slight variations in khaki colors. This could have been a benefit in the dry grassland environments of East and South Africa where humans first evolved." (PRIMATE COLOR VISION, ttp://anthro.palomar.edu/primate/color.htm)
It's interesting that color blindness mostly occurs in men--traditionally the hunters for whom color vision would be less important than for gathering females. Color vision deficient people also have a tendency to better night vision. This would presumably be an advantage for hunting or warring at dusk and night. (Ishihara Color Blindness Test,
http://www.archimedes-lab.org/colorblindnesstest.html)
However, I haven't seen any evidence that color blindness was present in the Paleolithic (has anyone else seen that?), so it's also possible that it is a Neolithic mutation from antigenic diets rather than an evolutionary adaptation.
[/quote]And are there any humans only able to see black/white and grey like some wild species?
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Yes, a small number, as was said above.