Summary
Highlights
The video introduces the special case of color vision in primates, contrasting it with other animals. While most terrestrial mammals have two cones (dichromatic vision), humans and some primates possess three cones (trichromatic vision). A brief review of color vision basics covers the role of the cornea, lens, retina, rods, and cones, and the molecular mechanism involving retinal and opsin proteins. Cones are responsible for color vision in bright light, while rods provide grayscale vision in dim light.
The video details the three types of cones: S-cones (short wavelength, blue), M-cones (medium wavelength, green), and L-cones (long wavelength, red). It notes that cones developed before rods and that vertebrate lines diverged with different complements of cones. Most mammals retained two cone types, making them dichromatic, while primates generally developed three-color vision. This trichromatic vision provides a significant advantage, especially for primates whose diet includes fruit, enabling them to distinguish ripe from unripe fruit.
Primates are divided into Old World (Africa and Asia) and New World (Central and South America) groups. Old World primates, both male and female, predominantly have three-color vision. In contrast, New World primates show variability: two-thirds of females have three-color vision, while one-third of females and all males have two-color vision but with polychromatic potential due to multiple long-wavelength pigment choices.
The video explains the genetic underpinnings. Ancestral mammals were likely nocturnal and dichromatic. With the extinction of large predators, mammals became more active in daylight. The S-cone gene is on chromosome 7, while the L and M cone genes are on the X chromosome, often located close together. The concept of X-chromosome inactivation in females is crucial: one of the two X chromosomes is randomly inactivated, which can lead to diverse color vision capabilities in New World female primates if the two X chromosomes carry different alleles for the L gene.
Two main hypotheses explain the evolutionary divergence of primate color vision. Story A suggests that an ancestral primate had blue and red pigments. In Old World primates, the L-gene duplicated, followed by a mutation creating the M-pigment. In New World primates, mutations in the single L-gene created different spectral sensitivities. Story B, a leading theory, proposes that the common ancestor had three L-gene alleles (like current New World primates). In Old World primates, an unequal crossing-over event led to the duplication of L and M genes on one X chromosome, establishing trichromatic vision, while New World primates retained the ancestral allele arrangement. The video concludes by noting that modern humans often have more than one copy of each pigment gene on the X chromosome, contributing to variability in human color vision.