You probably remember from science class about the light spectrum, but did you know that compared to creatures such as dogs and cats, humans can distinguish millions more shades of color? The ability derives from a basic feature of eye anatomy. Each eye contains a retina at the back that interprets the information from light-sensing cells called rods or cones and sends that information to the brain. Most animals only have two types of cones, blue and green, which help them distinguish a certain range of the light spectrum. However, humans add a third type of cone, red, which allow us to see much more.
How do humans produce red cones, though, when so many other mammals don’t? Initially, it was thought that the development of red cones was a relatively random process dependent on the release of hormones from the thyroid that would cause the production of new red cones. However, a study looking at lab-grown retinas determined that in fact, the substance responsible for the third cone is retinoic acid, a substance derived from vitamin A that helps cells grow and develop. More retinoic acid in the early stages of development, and you will have more green sensors, less in the later and you will have more red.
Scientists have begun comparing the ratio of red to green sensors in adults and have discovered a diverse range with seemingly little effect on vision quality. More research is needed, but understanding our third cone could help us better treat conditions such as color blindness and the gradual loss of vision with age.
