ancestors were tetrachromats, but their nocturnal mammalian ancestors lost two of their four cones during the mesozoic. Most modern primates, however, have evolved to be trichromats. All old world monkeys and apes are trichromats, but new world monkeys are polymorphic trichromats, meaning that males and homozygous females are dichromats while heterozygous females are trichromats (with the exceptions of howler monkeys and night monkeys, who have more and less advanced color vision respectively). There are four prevailing theories as to what the evolutionary pressure was for primates to develop trichromatic vision. The Fruit Theory suggests that it was easier for trichromatic primates to find ripe fruit against a green background. While there is data supporting the Fruit Theory, there is some dispute about whether or not trichromacy was more advantageous for determining how ripe fruit was up close or spotting fruit from afar. The Young Leaf hypothesis suggests that primates with more advanced color vision could better spot younger and more nutritious leaves during fruit shortages, while there are also theories that suggest more advanced color vision was better for recognizing changes in skin tone, allowing primates to better determine the blood oxygen saturation of others. other theories indicate that primates' color vision evolved alongside their sense of smell, though research has shown that there is no direct correlation between concentration of olfactory receptors and acquisition of color vision. Human evolution Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates – in particular genus Homo – and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes. This process involved the gradual development of traits such as human bipedalism and language. The study of human evolution involves many scientific disciplines, including physical anthropology, primatology, archaeology, paleontology, neurobiology, ethology, linguistics, evolutionary psychology, embryology and genetics. Genetic studies show that primates diverged from other mammals about , in the Late Cretaceous period, and the earliest fossils appear in the Paleocene, around . Within the superfamily Hominoidea (apes), the family Hominidae diverged from the family Hylobatidae (gibbons) some 15–20 million years ago; African great apes (subfamily Homininae) diverged from orangutans (Ponginae) about ; the tribe Hominini (humans, Australopithecines and other extinct biped genera, and chimpanzee) parted from the tribe Gorillini (gorillas) between and ; and, in turn, the subtribes Hominina (humans and biped