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If you have two X chromosomes, as most women do, it’s incredibly unlikely that you’ll end up being red-green color-blind, whereas roughly 10 percent of men are. If red-green color vision was obviously selected for in diurnal primates, why was it located on the X chromosome? It’s possible this type of color vision was more advantageous for the primate Eve than for her consorts and sons. Perhaps being more efficient at spotting more nutritive foodstuffs (extra-sweet berries, extra-tender young leaves) made a real difference in pregnancy and breast-feeding. If Purgi utilized the same sex-specific parenting strategies as many living primates do, foraging for herself and her infant offspring, then the survival of the young depended far more on the female than the male. In other words, there was more pressure to see red and green on the newly diurnal Purgi than there was on her male counterparts. The second possibility is that Purgi foraged for food with a group, as some of today’s New World monkeys do. In that scenario, it’d be advantageous to have both trichromatics and dichromatics working together, grazing not only in daylight but in the dim light at dawn and dusk, when the dichromats would be better at finding the good stuff. Or both of these things were true: our Eve, as the female, had the most pressure on her to be able to see red and green, but in a highly social species that did some amount of food sharing, it would have been advantageous to have some dichromats, too.
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Cat Bohannon (Eve: How the Female Body Drove 200 Million Years of Human Evolution)