“People from a planet without flowers would think we must be mad with joy the whole time to have such things about us.” ― Iris Murdoch
“I must have flowers, always, and always.” ― Claude Monet
“There’s a sunrise and a sunset every single day, and they’re absolutely free. Don’t miss so many of them.” ― Jo Walton
The beauty of a field of blossoms is a wonder to behold and so is the majesty of the setting sun, but just why is that?
We could go in depth about the philosophical reasons we find things “beautiful,” but the fact is that the vast majority of mammals don’t have the equipment necessary to appreciate the rich color palate that you and I see around us.
You may recall from biology that humans have two types of photoreceptor cells called “rods” and “cones.” Rods in humans allow black and white vision in low light while cones allow for color vision in bright light. There are three separate types of cones which combined give (most of) us the experience of three primary colors with varying hues between these colors. These three types of cone cells give rise to the term trichromat. If you are a dog lover like me, then you may be aware dogs are commonly considered colorblind. The reason is that dogs and indeed most of class Mammalia have only two types of cones and are thus dichromats. In fact less than 3% of living mammal species are trichromats.
Now a common misconception in the understanding of evolution is that mammals and humans in particular represent the most complex and “evolved” forms of life on Earth. The idea is that evolution is always marching forward and making things more complex. One clear blow against this kind of thinking is that reptiles, birds, and fish very commonly have FOUR cone type photoreceptors. That’s right. In addition to the full color spectrum you and I enjoy, most of these animals can see well into the ultraviolet AND higher into the infrared range of the electromagnetic spectrum. How is it that more primitive animals routinely have better color vision?
The short answer is that when it comes to adaptations which improve survival ”better” is very subjective. It is commonly believed that the first mammals were small, burrowing, and possibly nocturnal creatures living in a world that was at the time dominated by giant reptiles. These early mammals would have spent a larger portion of their lives in very low light conditions and would not have benefited from the luxury of having tetrachromatic eyes, and much like what is observed in species which have been isolated in caves for many generations, the early mammals began to lose aspects of their vision. This actually happened in two stages.
Texas blind salamanders are only found in one subterranean creek system and only ever encounter daylight if they are washed out of their habitat.
When mammals lost (and re-developed) their cones
Ancestral reptiles as stated earlier had four types of cones of which nearly all living mammals have lost two. First one type of cone cell was lost sometime after mammals first split from reptiles. This is shown by examining the most ancient lineage of the living mammals namely the monotremes of which the playpus and echidna are the only living examples.
In addition to retaining the reptilian quality of laying eggs, these mammals also retain a specific cone type that is not found in any other mammals. Where the next cone type was lost is a little less clear but is generally believed to be shortly after placental mammals (who give birth to fully developed young) diverged from marsupials (who give birth to partially developed young that finish developing outside the mother’s body). Regardless of exactly when it took place, placental mammals almost exclusively exhibit dichromatic vision.
This is a great illustration of how color vision changed over time and is part of an excellent project that shows how we test if animals use their color vision.
Among placental mammals only members of one group have managed to find their way back to trichromatic vision, and that group is the primates. Among primates two groups in particular exhibit trichromacy. They are the Platyrrhini (New World or “American continent” monkeys) and Catarrhini (Old World or “African continent” monkeys). Interestingly gene analysis has proven that these two groups developed their third cone type in different manners the result of which is that among most species of New World monkeys only a portion (less than 50%) of the female population is trichromatic while the rest of the females and all of the males are dichromatic! Howler monkeys are the one exception. They are a New World monkey with fully trichromatic vision just like the Old World monkeys. As it turns out this developed in exactly the same way as the Old World monkeys but at a later time in their evolution.
You, however, are a member of the Old World monkeys and as a result probably have trichromatic vision.
Evolutionary map of primate reacquisition of three-color vision
What does all this have to do with trees?
Though mammals found a niche to exploit underground and at night that didn’t have need of color vision, birds and reptiles with the added benefit of excellent color vision went right on co-evolving with trees in the light of day. What resulted was an elaborate seed distribution system. The plants would overcome their immobility by wrapping their seeds in flesh that would be eaten by more locomotive animals, the maturity of the seeds would coincide with the flavor AND color of the resulting fruit.
The spectacle of an apple on the tree turning from green to a bright red would have been lost on our dichromatic ancestors, however. The four photoreceptors found in most birds, fish, and reptiles have their greatest sensitivity at points on the electromagnetic spectrum that we would associate with ultraviolet, blue, yellow, and orange-red. Dichromatic placental mammals lost the ultraviolet and orange-red photoreceptors. This is believed to result in color vision much like what is experienced in a human with red-green colorblindness and is simulated in the photograph below.
Mammals take to the trees
Around 65 million years ago this all became quite a big deal when suddenly more than 75% of the species on Earth including nearly all large land animals went extinct. Over several million years mammals crawled out of their burrows and began to fill the vacant niches in the world around them.
Some of these mammals climbed into the trees where the ability to distinguish between red and green would have been a huge advantage for an animal looking for ripe fruit and newly-sprouted, reddish leaves. Some of the primates regained this ability by converting the “yellow cone” of their ancestors into two new cones that correspond with blue-green and yellow-green. Thus primate trichromacy was (re)born!
So the next time you look at the fiery hues of the setting sun, the bold beauty of a flower, or indeed the blush of a lover’s cheek, know that the reason you can distinguish those colors at all is a direct result of our species long survival story.
Other Fun Facts
- Marine mammals like whales lost another cone and are monochromats.
- A small portion of the female human population has been shown to be tetrachromats (4 cone types) and can differentiate between 100 million colors while a trichromat can only differentiate between 1 million colors.
- Honeybees and bumblebees lost the ancestral orange-red cone and kept the UV one.
- Some butterflies are pentachromatic (5 cone types) while the mantis shrimp has been shown to have 12 types of cone!