Why primates see in colour

The mammals that lost their colours

Colour vision is old, and mammals are the odd ones out. The four cone pigments found in many fish, reptiles and birds go back to early vertebrates, and the pattern was present in every major vertebrate branch except the mammals.¹ Birds still have four cone types, one of them sensitive to ultraviolet, and are thought to see colours that people cannot.¹

Early mammals gave half of that up. A 2018 study of opsin genes, the genes that build the light-sensitive pigments, argues that mammals lived through a long "nocturnal bottleneck", active mostly at night from before 215 million years ago until about 66 million years ago, when the non-avian dinosaurs disappeared. Along the way they lost two of the four cone pigment genes entirely.² In the dark, rods do most of the work and colour matters less. The result is that most mammals today, dogs, cats and mice included, are dichromats, with one cone type for short wavelengths and one for medium or long ones.³ They are not blind to colour, but they distinguish far fewer hues than we do, in a way that resembles common red-green colour blindness in people.

A gene that copied itself

The Old World monkeys and apes, humans among them, got a third cone back by an unusual route. In these primates the gene for the long-wavelength pigment sits on the X chromosome, and at some point it was duplicated. The two copies, lying head to tail next to each other, drifted apart through mutation until one became the pigment of the L cone and the other the pigment of the M cone.⁴ They are still about 98 per cent identical in their DNA.⁵ With a short-wavelength cone already in place, the result was three cone types, or trichromacy.

Dating the event is harder. A figure of roughly 35 million years ago is often given.⁶ A 2020 preprint that compared the opsin genes of many species argued for something different, that apes and Old World monkeys each acquired their duplication separately, somewhere between about 29 and 20 million years ago, though its authors accept that a single shared duplication could produce a similar pattern in the data.⁷ The question is open.

The close similarity of the two genes has a cost that shows up today. Because they sit side by side and match so well, they can misalign when chromosomes swap material, producing missing or hybrid genes. Jeremy Nathans and colleagues showed in 1986 that this kind of unequal recombination accounts for most inherited red-green colour deficiency, which affects about 8 per cent of men of European descent.⁵ Men have only one X chromosome, so they have no spare copy to fall back on.

The monkeys of the Americas

New World monkeys show what the in-between stage may have looked like. Most have a single opsin gene on the X chromosome, but it comes in several versions tuned to slightly different wavelengths. Every male is a dichromat. A female who inherits two different versions, one on each X, ends up with two different cone types alongside the short-wavelength cone, and sees in three colours.⁴ Howler monkeys are the exception. They have their own gene duplication, separate from and probably more recent than the one in Old World primates, so both sexes are trichromats.⁸ Trichromacy, in other words, evolved more than once.

An experiment in 2007 suggested how quickly a new cone could be put to use. Gerald Jacobs at the University of California, Santa Barbara and Jeremy Nathans at Johns Hopkins gave mice a human L pigment gene. Females carrying both the mouse and the human versions could tell apart colours that ordinary mice could not, choosing the right panel about 80 per cent of the time, against the one in three expected by chance.⁹ The mouse brain had made use of the extra signal with no other rewiring.

Fruit, leaves or faces

Why the third cone was worth keeping is the long argument. The oldest answer, associated with the Cambridge vision scientist John Mollon among others, is fruit. Ripe reds and oranges stand out against green foliage far more clearly to a trichromat.⁹ A 2013 study from Cardiff tested people searching for fruit in a bush and found that normal trichromats did best compared with colour-deficient viewers at the longest distance tested, 12 metres, suggesting that the advantage lay in spotting a fruiting tree from afar more than in picking from it.¹⁰

In 2001 Nathaniel Dominy and Peter Lucas, working in Kibale Forest in Uganda, proposed leaves instead. Four trichromatic primate species there chose young leaves that differed from mature ones mainly in red-green contrast, and those leaves were higher in protein and less tough. Leaves are what these animals fall back on when fruit is scarce, and the authors argued that this could be what keeps trichromacy in place.¹¹ In 2006 Mark Changizi and colleagues at Caltech suggested a social reason. The spacing of the L and M cones suits detecting small changes in skin colour caused by blood oxygen, as in blushing or pallor, and trichromatic primates tend to have bare faces and rumps.¹²

A 2017 review of the evidence concluded that no single hypothesis explains trichromacy in every case and that different pressures may have mattered in different species.⁸ Each idea has support, and none has seen off the others.

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1. Mary Caswell Stoddard et al., Wild hummingbirds discriminate nonspectral colors, Proceedings of the National Academy of Sciences, 2020.
2. Rui Borges et al., Adaptive genomic evolution of opsins reveals that early mammals flourished in nocturnal environments, BMC Genomics, 2018.
3. University of California, Santa Barbara, Genetic modification enables mice to see new colors, UCSB News, 2007.
4. Kanwaljit S. Dulai, Miranda von Dornum, John D. Mollon and David M. Hunt, The evolution of trichromatic color vision by opsin gene duplication in New World and Old World primates, Genome Research, 1999.
5. Jeremy Nathans, Thomas P. Piantanida, Roger L. Eddy, Thomas B. Shows and David S. Hogness, Molecular genetics of inherited variation in human color vision, Science, 1986.
6. Wikipedia, Evolution of color vision in primates.
7. Phylogenetic analyses suggest independent origins for trichromatic color vision in apes and Old World monkeys, bioRxiv preprint, 2020.
8. Livia S. Carvalho, Daniel M. A. Pessoa, Jessica K. Mountford, Wayne I. L. Davies and David M. Hunt, The genetic and evolutionary drives behind primate color vision, Frontiers in Ecology and Evolution, 2017.
9. ScienceDaily, Genetic studies endow mice with new color vision, Howard Hughes Medical Institute release, 2007.
10. Aline Bompas, Grace Kendall and Petroc Sumner, Spotting fruit versus picking fruit as the selective advantage of human colour vision, i-Perception, 2013.
11. Nathaniel J. Dominy and Peter W. Lucas, Ecological importance of trichromatic vision to primates, Nature, 2001.
12. California Institute of Technology, Old-World primates evolved color vision to better see each other blush, study reveals, Caltech, 2006.