Evolution of colour vision
The colour vision of many herbivores allows them to see fruit or (immature) leaves which are good to eat. In hummingbirds, particular flowers are often recognized by colour. Predators also use colour vision to help them find their prey.
All this applies mainly to animals in the daytime. On the other hand, nocturnal mammals have much less-developed colour vision. For them, space on the retina is better used with more rods since rods collect light better. Colour differences are much less visible in the dark.
Apart from vertebrates, the only land animals to have colour vision are arthropods. Aquatic arthropods such as crustacea also have colour vision. As with vertebrates, the details differ, but the molecules which do the work – the opsins – are very similar.
- "rods and four spectral classes of cone each representing one of the five visual pigment families. The complement of four spectrally distinct cone classes endows these species with the potential for tetrachromatic colour vision".
- "...two cone opsin gene families appear in contemporary eutherian mammals and, with the exception of some primates, none of these animals derive more than a single photopigment type from each of their two gene families".
Many primates do live as daytime animals, and one group – the Old World monkeys – has developed trichromatic vision. The anthropoid apes and humans are descended from this group of monkeys, and also have good colour vision. So it comes about that most monkeys and humans have good colour vision, but most other eutherian mammals do not: They have only two opsins, and are bichromatic.
Birds, turtles, lizards, many fish and some rodents have UV receptors in their retinas. These animals can see the UV patterns found on flowers and other wildlife that are otherwise invisible to the human eye.
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- Jacobs G.H. 2009. Phil Trans Roy Soc B. 364 (1531) 2957-2967. 
- Kemp T.S. 2005. The origin and evolution of mammals. Oxford University Press.
- There was gene duplication of one of their opsin genes.
- The group is called the catarrhines.
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