According to scientists, about a third of birds and plants actually look different than we think. It's just that people do not distinguish between their real colors. In humans, there are only three types of cones in the eye - light-sensitive cells of the retina, in birds - four. Birds see ultraviolet light and its combinations with other colors. Once upon a time, such a superpower was possessed by our distant ancestors, and in some people it has survived to this day.
Discern the invisible
Compared to birds, we are virtually blind - unable to recognize the true colors of about a third of the plants that some birds feed on. This is the conclusion reached by American and Canadian biologists who studied the behavior of the hummingbird Selasphorus platycercus.
Scientists have placed two feeders in a field at a distance of a meter from each other. One contained sweet water, the other plain. Nearby were lamps mixing the radiation of four LEDs (red, green, blue, or ultraviolet). After the birds, having drunk from the feeders, flew away, they were changed places so that the hummingbirds, when they returned, were guided exclusively by the light of the lamp. So they were taught to associate one of the colors with a reward.
As it turned out, hummingbirds accurately detect not only the three main parts of the visible spectrum - blue, red and green - but also ultraviolet light, which humans cannot see. All this is due to four types of receptors - the so-called cones - in the retina. Humans only have three who are sensitive to red, blue and green.
Proceedings of the National Academy of Sciences Jun 2020.
The color perception of tetrachromats - animals with four types of cones - can be thought of as a pyramid. Its tops correspond to solid colors. They are recognized by individual cones. Colored edges of the pyramid are the axes of spectral colors, dashed ones are non-spectral ones. The color space of a person can be represented as a triangle at the base of the pyramid.
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Also, birds were distinguished by mixed colors - for example, a combination of green and ultraviolet. But how exactly they see shades - by the imposition of pure colors or with a special paint - the authors of the work did not figure out.
Losses and gains of evolution
The distant ancestors of man also had four types of cones, and the world for them was more colorful, American scientists suggest. They found traces of ancient supervision in the genome. These are the regions of DNA responsible for opsins - receptors in the cones. Now humans have three types of such receptors. They are sensitive to long (red), medium (green) and short wavelengths (violet, bluish) optical range. All shades perceived by a person are the result of their synthesis.
This was not the case with the vertebrate ancestors of modern mammals. Apparently, they had four receptors, but with the transition to a nocturnal lifestyle - it is believed that this happened during the time of the dinosaurs - two have lost. In the cones, which are responsible for daytime vision, only receptors for red color and ultraviolet light remained.
Moreover, in the course of evolution in some primates, including human ancestors, the eye lens stopped transmitting ultraviolet light (with a wavelength shorter than 400 nanometers). And his receptor was out of work. But after several mutations that happened between 90 and 30 million years ago, it became sensitive to blue.
In parallel, due to the duplication of the red receptor gene and mutations that shifted its sensitivity to short wavelengths, primates learned to recognize green. According to one hypothesis, evolutionarily it was very beneficial, as it made it easy to see ripe fruits in green foliage. True, it made it difficult to find camouflaged insects.
Female superpower
Until now, in humans, receptors that are sensitive to green and red differ slightly, and the genes encoding them are adjacent on the X chromosome. This explains the prevalence of color blindness - color blindness - among men, because they have only one X chromosome. And a breakdown in these genes makes it impossible to distinguish between red and green.
On the other hand, this situation gives women unexpected advantages. The mutation can lead to the formation of a fourth type of receptor - sensitive to light with a wavelength between red and yellow. Since there are two copies of each receptor, replacing one will result in three receptors encoded in the genome. The result will be four types of cones, and as a result, the eye will be able to distinguish not a million shades, as in all people, but almost a hundred million.
According to experts at the University of California at San Diego (USA), two to three percent of women on Earth have this. British researchers believe that there are many more - about 12 percent.
True, so far only one person has been found who has four types of cones in the retina, and they all function. She is a British citizen, referred to in scientific papers under the pseudonym cDa29. Experts learned about her "superpower" by using a non-standard test to assess color perception.