Page "Trichromacy" Paragraph 3

Humans and other mammals have evolved trichromacy based partly on pigments inherited from early vertebrates.

In fish and birds, for example, four pigments are used for vision.

These extra cone receptor visual pigments detect energy of other wavelengths, including sometimes ultraviolet.

Eventually two of these pigments were lost ( in placental mammals ) and another was gained, resulting in trichromacy among some primates.

Humans and closely related primates are usually trichromats, as are some of the females of most species of New World monkeys, and both male and female howler monkeys.