Summary
Highlights
This video delves into the evolution of color vision, building on previous discussions about varying levels of color vision in animals (e.g., marine mammals with one cone, terrestrial mammals with two, and primates with three). The focus is on opsin proteins, which determine the peak wavelength a photoreceptor is sensitive to, thereby controlling the colors we perceive. Rods and cones, located in the retina, are photoreceptors containing retinal (derived from vitamin A) embedded in an opsin protein. It's the opsin molecule that dictates the color sensitivity, not the retinal.
Opsins are broadly categorized into Type 1 (microbial opsins, found in simple organisms like bacteria and algae, also acting as ion pumps) and Type 2 (found in eukaryotes and higher animals, primarily for vision and circadian rhythms). Despite structural similarities suggesting a common origin, recent studies indicate they evolved separately. The ability to sense light is fundamental and ancient, with five opsins (four for cones and one for rods, labeled Rh1) thought to be present in the vertebrate ancestor. These are characterized by their spectral ranges: S cones (blue/short wavelength), M cones (green/middle wavelength), and L cones (red/long wavelength). An additional opsin sensed ultraviolet light.
The evolution of color vision began with a very old opsin, sensitive to medium wavelengths (around 555 nm), similar to our L cone (yellow-green). With only one cone, there was no color distinction, just brightness levels. Around 500 million years ago, a second pigment opsin evolved, derived from the L pigment and sensitive to short wavelengths (UV and blue light). This introduced the ability to discriminate between colors, broadening the range of light sensitivity. For this advantage to be useful, the nervous system needed to develop circuitry to compare information from the two cone types, forming the neural basis for the blue-yellow opponent channel.
The third color opsin pigment, crucial for human vision, appeared more recently (30-40 million years ago) as a modification of the long-wavelength opsin. This mid-wavelength opsin provides humans with a three-dimensional color vision. The brain adapted by potentially using an existing system for spatial vision, leading to a new red-green opponent channel in addition to the old blue-yellow channel. The advantage of three colors over two is evident in scenarios like primates foraging for fruit. While a two-cone primate might struggle to distinguish ripe from unripe fruit, a three-cone primate can easily spot red or orange ripe fruit.
Rhodopsin (Rh1) evolved from a short-wavelength opsin, specifically from a second short-wavelength pigment (SWS2) which gave rise to two Rh pigments sensitive to mid-range wavelengths. Rh1 developed into the rhodopsin present in rods, which came after cone pigments. Rods piggybacked onto existing cone circuitry. This evolution resulted in a duplex vision system with cones for bright light and rods for dim light, allowing adaptation to a wide range of light levels. The video concludes by introducing melanopsin, an opsin from the rhabdom group (typically found in invertebrates) unexpectedly found in mammalian eyes. It senses light but doesn't contribute to vision directly; instead, it's located in a small subset of ganglion cells, providing clues about the association of different neural parts of the retina.