Monkeys see fewer shades of blue than humans

We take it for granted how we see the colorful world. Apart from those who suffer from color vision, we are so-called trichromats, who have three different types of cones in addition to the light-sensitive rods. These color receptors differ in their sensitivity in the red, blue and green spectrum. The perception of colors is very different in different animal species. While most mammals are dichromats – they only have two kinds of cones for green and blue – birds and many reptiles, fish and insects are tetrachromats, which also have a UV receptor. How many colors can actually be perceived and distinguished depends on how the brain processes things.

Color perception has probably evolved several times over the course of evolution. Early mammals, likely nocturnal, lost two of the original four color receptors, but with the transition to diurnal activity 30 to 40 million years ago, a third type of cone emerged through gene duplication in Old World primates. This development was probably at the expense of the sense of smell, as color vision is impaired in those primates that still have a good sense of smell.

The results of a new study, published in the journal Proceedings of the National Academy of Sciences (PNAS), now suggest that humans can perceive a wider range of shades of blue than monkeys. However, this wording is a bit misleading, because homo sapiens belongs to the great apes according to biological taxonomy, along with orangutans, gorillas and our closest relatives, the chimpanzees.

The ancestors of today’s great apes split from the lineage that led to the other modern Old World apes about 25 million years ago. Even further back in primate evolutionary history—about 46 million years—is the split of the apes into Old World apes (catarrhini) and New World monkeys (platyrrhini), which are also called narrow-nosed monkeys or wide-nosed monkeys.

The research team – Yeon Jin Kim, Dennis Dacey, Orin Packer from the University of Washington School of Medicine in Seattle (US), Paul Martin and Ulrike Grünert from the University of Sydney (Australia), and Andreas Pollreisz from the Medical University of Vienna – investigated the retinal nerve cells and their connections in humans and one representative each of the Old World monkeys (macaques) and the New World monkeys (marmosets). Their goal was to compare the neural wiring responsible for color perception between humans and apes.

Using a new three-dimensional electron microscopy technique, the scientists examined the fine nerve connections in the retina. They focused on that small part of the retina that is densely covered in cones and is responsible for sharp vision and color vision – the visual pit or central fovea. The color information obtained from the different types of cones is processed by neural circuits that interpret the data.

The analysis revealed that a specific short-wavelength (blue-sensitive) cone circuit found in humans is absent in marmosets. Although it occurs in macaques, it differs from that of humans. It also showed that there are features in the human neuronal connections involved in color perception that were unexpected based on previous studies in non-human primates.

This finding suggests that humans may have developed special neural connections in the retina that improve color vision. This could be the result of evolutionary adaptations that have occurred phylogenetically relatively recently. The study authors suggest that these differences in mammalian visual circuitry are influenced, at least in part, by their adaptation to specific ecological niches.

For example, marmosets live in trees, while modern humans are adapted to life on the ground. Adaptation to different habitats may have put selective pressure on specific color-sensing circuits; for example, the ability to recognize ripe fruit in the forest’s changing light may have conferred a selective advantage on certain neural circuits. However, the precise effects of environment and behavior on color perception have yet to be studied. (i.e)