Psychedelic Dinosaurs, Four-Dimensional Hummingbirds, and How We Got Our Vision: Color, Consciousness, and the Dazzling Universe of Tetrachromacy
By Maria Popova
Without color, life would be a mistake. I mean this both existentially and evolutionarily: Color is not only our primary sensorium of beauty — that aesthetic rapture without which life would be a desert of the soul — but color is how we came to exist in the first place. Our perception of color, like our entire perceptual experience, is part of our creaturely inheritance and bounded by it — experience that differs wildly from that of other species, and even varies vastly within our own species. In that limitation lies a glorious invitation to fathom the fundaments of our humanity and step beyond ourselves into other sensoria more dazzling than our consciousness is even equipped to imagine.
That is the invitation Ed Yong — one of the most insightful science writers of our time, and one of the most soulful — extends in An Immense World: How Animal Senses Reveal the Hidden Realms Around Us (public library), appropriately titled after a verse by William Blake:
How do you know but ev’ry Bird that cuts the airy way,
Is an immense world of delight, clos’d by your senses five?
A quarter millennium of science after Blake — a quarter millennium of magnifying delight through the lens of knowledge — Yong writes:
Earth teems with sights and textures, sounds and vibrations, smells and tastes, electric and magnetic fields. But every animal can only tap into a small fraction of reality’s fullness. Each is enclosed within its own unique sensory bubble, perceiving but a tiny sliver of an immense world.
With an eye to the Umwelt — that lovely German word for the sensory bubble each creature inhabits, both limiting and defining its perceptual reality — he adds:
Our Umwelt is still limited; it just doesn’t feel that way. To us, it feels all-encompassing. It is all that we know, and so we easily mistake it for all there is to know. This is an illusion, and one that every animal shares.
Nothing can sense everything, and nothing needs to. That is why Umwelten exist at all. It is also why the act of contemplating the Umwelt of another creature is so deeply human and so utterly profound. Our senses filter in what we need. We must choose to learn about the rest.
We are insentient to myriad realities readily available to our fellow creatures — the temperature currents by which a fly, Blake’s supreme existentialist, navigates the air; the ultrasonic calls with which hummingbirds hover between science and magic; the magnetic fields by which nightingales migrate. With the perspectival felicity that science singularly confers, Yong writes:
The Umwelt concept can feel constrictive because it implies that every creature is trapped within the house of its senses. But to me, the idea is wonderfully expansive. It tells us that all is not as it seems and that everything we experience is but a filtered version of everything that we could experience. It reminds us that there is light in darkness, noise in silence, richness in nothingness. It hints at flickers of the unfamiliar in the familiar, of the extraordinary in the everyday, of magnificence in mundanity… When we pay attention to other animals, our own world expands and deepens.
No corner of the house of the senses is more fascinating — for its aesthetic gifts, its evolutionary convolutions, and its almost spiritual effects — than color.
“Color itself is a degree of darkness,” Goethe wrote in his poetic theory of color and emotion. Although the theory was falsified by science and revised by the very scientists whom it inspired, this particular statement from it stands as an apt description of the evolutionary history of color vision.
To see at all, ancient animals developed a type of protein receptor called opsin, which patrols the surface of the cell that contains it — a type of cell called a cone — and grabs at light-absorbing molecules, forming a partnership that sparks the chemical reaction of electrical signals that carry vital information to neurons — information which resolves in what we call vision. Some 500 million years ago, once our primordial ancestors moved from the depths to the shallows of the sea, they confronted something profound confusing from the vantage point of a creature with primitive monochromatic eyes only capable of distinguishing degrees of darkness: sunlight dancing on the surface of the rippling waves, rapidly refracting into the water. Suddenly, a single patch of visible space could vary in brightness a hundredfold from moment to moment under the flickering rays. Against this strobe assault, it became impossible to detect predator or prey.
To cope with the dangerous disorientation, our monochromat ancestors needed a way to not only detect binary variations of brightness and darkness, but to compare them. Cones and their opsins grew more and more specialized, with different types emerging to absorb different wavelengths of light — long, which we perceive as red, medium for green, and short for blue. A complex neural network emerged to compute these comparisons — neurons excited by some cones but inhibited by others, allowing creatures to detect particular colors, indistinguishable by degrees of darkness in monochromatic vision — certain shades of red and green can (and do, to the red-green colorblind) look identical in grayscale.
This process, known as opponency, is the basis of all color vision. Different animals have different types and numbers of opsins, unmooring the perception of color from its physics and making it an inherently subjective experience.
Our own animal experience of color, as fundamental to our consciousness as it may be, came by rather haphazardly, by a glorious accident of evolution. (Then again, we could say the same of consciousness itself, and perhaps of all of life — none of it was inevitable, none part of some grand score for the symphony of chance.) Yong writes:
The first primates were almost certainly dichromats. They had two cones, short and long. They saw in blues and yellows, like dogs. But sometime between 29 and 43 million years ago, an accident occurred that permanently changed the Umwelt of one specific lineage of primates: They gained an extra copy of the gene that builds their long opsin. Such duplications often happen when cells divide and DNA is copied. They’re mistakes, but fortuitous ones, for they provide a redundant copy of a gene that evolution can tinker with without disrupting the work of the original. That’s exactly what happened with the long-opsin gene. One of the two copies stayed roughly the same, absorbing light at 560 nanometers. The other gradually shifted to a shorter wavelength of 530 nanometers, becoming what we now call the medium (green) opsin. These two genes are 98 percent identical, but the 2 percent gulf between them is also the difference between seeing only in blues and yellows and adding reds and greens to the mix. With the new medium opsins joining the earlier long and short ones, these primates had evolved trichromacy. And they passed their expanded vision to their descendants — the monkeys and apes of Africa, Asia, and Europe, a group that includes us.
This accidentally duplicated long-opsin gene suddenly expanded our rainbow by an order of magnitude:
A monochromat can make out roughly a hundred grades of gray between black and white. A dichromat adds around a hundred steps from yellow to blue, which multiplies with the grays to create tens of thousands of perceivable colors. A trichromat adds another hundred or so steps from red to green, which multiplies again with a dichromat’s set to boost the color count into the millions. Each extra opsin increases the visual palette exponentially.
It is easy, then, to imagine that if someone were to wave a magic wand over a dichromat, who sees only 1% of the colors a tetrachromat sees, and add an extra cone, the transformation would be nothing less than a revolution of reality. It would be, were our frames of reference not a stronger determinant of reality than our perceptions. (Thoreau captured this haunting aspect of the animal soul when he observed that “we hear and apprehend only what we already half know.”) When researchers took this fortuitous long-opsin gene that chance handed humans and gave it to a pair of squirrel monkeys, dichromatic by nature, the monkeys gained instant access to a world a hundred times more colorful. But instead of moving wonder-stricken through this new wonderland, gasping at every suddenly green leaf and every suddenly red berry, they went about their ordinary lives in the ordinary way, illustrating the relativity of wonder. Yong reflects:
Seeing more colors isn’t advantageous in and of itself. Colors are not inherently magical. They become magical when and if animals derive meaning from them. Some are special to us because, having inherited the ability to see them from our trichromatic ancestors, we imbued them with social significance. Conversely, there are colors that don’t matter to us at all. There are colors we cannot even see.
One hallmark of our species may be that, unlike our squirrel monkey cousins, we are animated by a restless wonderment about what lies beyond the horizon of the known and the visible. Whether we call it curiosity or imagination, it is the longing that fuels all creativity, in science or in art. And this blind spot of our vision is where the chromatic equation grows infinitely interesting.
It all began in the 1880s, when the polymathic banker turned scientist and philanthropist John Lubbock shone a beam of light through a prism, splitting it into its constituent colors and letting the rainbow fall onto some ants. Predictably, they fled from the light; unexpectedly, they ran not only for the colors he could see but from a patch just past the violet end that appeared completely dark to him. This was the discovery of the ultraviolet range of the electromagnetic spectrum — light with wavelengths between 10 to 400 nanometers, too short for the human eye to detect.
This was an era when science still clung to the dangerous Cartesian binary of human exceptionalism, under which other animals experienced the world either exactly as we do or in greatly diminished ways — non-human animals were thought to either see the same rainbow we do or to be entirely colorblind. The notion that they could see color, and see it differently than we do, and see what we cannot see, was a radical demolition of dogma — too radical to fully accept. For a long while, ants were thought to be exceptional in the animal kingdom — fortunate flukes unrepresentative of the sub-human whole. Eventually, bees joined them.
But then, in a mere century of science — a blink of evolutionary time — numerous birds, fish, reptiles, and insects were reluctantly admitted into the UV-sighted ranks. Still, we excluded mammals from the realm of possibility — this is the history of our species — until, in a humbling testament to Richard Feynman’s insistence that the imagination of nature will always exceed that of the human animal, a team of scientists discovered a short cone tuned to UV light in three species of rodents. Within half a human generation, many mammals — including dogs, cats, reindeer, cows, and ferrets — were discovered to detect UV light with their short blue cones.
Now we know that most animals can perceive ultraviolet, and we are the unfortunate flukes.
Even some human animals — those who have had their lenses damaged in some way — can perceive the UV end of the spectrum as a pale blue, none more famous than Claude Monet and his water lilies, the dazzling product of his refusal to have his cataracts — a progressive clouding of the lens that filters color — surgically removed; instead, he went on painting the world as he saw it, increasingly warping the electromagnetic spectrum into otherworldly colors.
With an eye to bees — tetrachromats with opsins most tuned to blue, green, and ultraviolet — Yong winks at our human tendency toward self-reference and celebrates the supreme gift of science, that of achieving perspective:
If bees were scientists, they might marvel at the color we know as red, which they cannot see and which they might call “ultrayellow.” They might assert at first that other creatures can’t see ultrayellow, and then later wonder why so many do. They might ask if it is special. They might photograph roses through ultrayellow cameras and rhapsodize about how different they look. They might wonder whether the large bipedal animals that see this color exchange secret messages through their flushed cheeks. They might eventually realize that it is just another color, special mainly in its absence from their vision. And they might wonder what it would be like to add it to their Umwelt, bolstering their three dimensions of color with a fourth.
But bees are still trichromats, like us, just shifted along the electromagnetic spectrum. The truly mind-bending part — quite literally, for it flexes our cognitive capacity for imagination beyond the hard-wired perceptual limits of our consciousness — is when we raise color vision by another order of magnitude, to tetrachromacy: the addition of a whole other cone with a whole other opsin. Just as in the leap from dichromacy to trichromacy, a trichromat sees only 1% of the colors available to a tetrachromat. Dinosaurs were almost certainly tetrachromats, walking a psychedelic primordial world. Hummingbirds — those feathered miniature heirs of the bygone giant reptiles — are tetrachromats. They see hundreds of millions of colors and can readily distinguish between flowers that appear to us identical in hue.
But for a trichromat to imagine tetrachromacy is as challenging as for a two-dimensional creature to imagine a three-dimensional world — we inhabit a chromatic Flatland, in which the vision of a hummingbird remains to us as enticing and elusive an abstraction as a Klein bottle.
[Hummingbirds] don’t just have human vision plus ultraviolet, or bee vision plus red. Tetrachromacy doesn’t just widen the visible spectrum at its margins. It unlocks an entirely new dimension of colors.
Picture trichromatic human vision as a triangle, with the three corners representing our red, green, and blue cones. Every color we can see is a mix of those three, and can be plotted as a point within that triangular space. By comparison, a bird’s color vision is a pyramid, with four corners representing each of its four cones. Our entire color space is just one face of that pyramid, whose spacious interior represents colors inaccessible to most of us.
In a wonderfully Dr. Seussian passage, Yong sums up the revolutionary discoveries of violinist turned sensory ecologist and evolutionary biologist Mary Caswell “Cassie” Stoddard, who spearheaded the hummingbird research:
If our red and blue cones are stimulated together, we see purple — a color that doesn’t exist in the rainbow and that can’t be represented by a single wavelength of light. These kinds of cocktail colors are called non-spectral. Hummingbirds, with their four cones, can see a lot more of them, including UV-red, UV-green, UV-yellow (which is red + green + UV), and probably UV-purple (which is red + blue + UV). At my wife’s suggestion, and to Stoddard’s delight, I’m going to call these rurple, grurple, yurple, and ultrapurple. Stoddard found that these non-spectral colors and their various shades account for roughly a third of those found on plants and feathers. To a bird, meadows and forests pulse with grurples and yurples. To a broad-tailed hummingbird, the bright magenta feathers of the male’s bib are actually ultrapurple.
As a violinist, [Stoddard] knows that two simultaneously played notes can either sound separate or merge into completely new tones. By analogy, do hummingbirds perceive rurple as a blend of red and UV, or as a sublime new color in its own right? When they make choices about which flowers to visit, “do they group rurple with reds, or do they see it as an entirely different hue?” she asks. They can tell that it’s different from pure red, “but I can’t articulate what it looks like to them.”
Many more ineffable wonders of perception come abloom on the pages of Yong’s An Immense World. Complement this fragment of it with the great nature writer Ellen Meloy on the conscience of color from chemistry to culture and physicist Arthur Zajonc on the entwined history of vision and consciousness, then revisit cognitive scientist Alexandra Horowitz on how to see reality beyond the habitual limits of our perception.
Published August 10, 2022