This story appears in the February 2016 issue of National Geographic magazine.
“If you ask people what animal eyes are used for, they’ll say: same thing as human eyes. But that’s not true. It’s not true at all.”
In his lab at Lund University in Sweden, Dan-Eric Nilsson is contemplating the eyes of a box jellyfish. Nilsson’s eyes, of which he has two, are ice blue and forward facing. In contrast, the box jelly boasts 24 eyes, which are dark brown and grouped into four clusters called rhopalia. Nilsson shows me a model of one in his office: It looks like a golf ball that has sprouted tumors. A flexible stalk anchors it to the jellyfish.
“When I first saw them, I didn’t believe my own eyes,” says Nilsson. “They just look weird.”
Four of the six eyes in each rhopalium are simple light-detecting slits and pits. But the other two are surprisingly sophisticated; like Nilsson’s eyes, they have light-focusing lenses and can see images, albeit at lower resolution.
Nilsson uses his eyes to, among other things, gather information about the diversity of animal vision. But what about the box jelly? It is among the simplest of animals, just a gelatinous, pulsating blob with four trailing bundles of stinging tentacles. It doesn’t even have a proper brain—merely a ring of neurons running around its bell. What information could it possibly need?
In 2007, Nilsson and his team demonstrated that the box jelly Tripedalia cystophorauses its lower lensed eyes to spot approaching obstacles, like the mangrove roots that it swims among. It took them another four years to discover what the upper lensed eyes do. The first big clue was a free-floating weight at the bottom of the rhopalium that ensures that the upper eye is always looking upward, even if the jellyfish swims upside down. If this eye detects dark patches, the jellyfish senses that it’s swimming beneath the mangrove canopy, where it can find the small crustaceans that it eats. If it sees only bright light, it has strayed into open water, and risks starving. With the help of its eyes, this brainless blob can find food, avoid obstacles, and survive.
The box jellyfish’s eyes are part of an almost endless variation of eyes in the animal kingdom. Some see only in black and white; others perceive the full rainbow and beyond, to forms of light invisible to our eyes. Some can’t even gauge the direction of incoming light; others can spot running prey miles away. The smallest animal eyes, adorning the heads of fairy wasps, are barely bigger than an amoeba; the biggest are the size of dinner plates, and belong to gigantic squid species. The squid’s eye, like ours, works as a camera does, with a single lens focusing light onto a single retina, full of photoreceptors—cells that absorb photons and convert their energy into an electrical signal. By contrast, a fly’s compound eye divides incoming light among thousands of separate units, each with its own lens and photoreceptors. Human, fly, and squid eyes are mounted in pairs on their owners’ heads. But scallops have rows of eyes along their mantles, sea stars have eyes on the tips of their arms, and the purple sea urchin’s entire body acts as one big eye. There are eyes with bifocal lenses, eyes with mirrors, and eyes that look up, down, and sideways all at the same time.
At one level, such diversity is puzzling. All eyes detect light, and light behaves in a predictable manner. But it has a multitude of uses. Light reveals the time of day, the depth of water, the presence of shade. It bounces off enemies, mates, and shelter. The box jellyfish uses it to find safe pastures. You use it to survey landscapes, interpret facial expressions, and read these words. The variety of tasks that eyes perform is limited only by the fecundity of nature. They represent a collision between the constancy of physics and the messiness of biology. To understand how eyes evolved, scientists need to do more than examine their structures. They need to do what Nilsson did with the box jellyfish: understand how animals use their eyes.
Around 540 million years ago, the ancestors of most modern animal groups suddenly appeared on the scene, in an outburst of speciation known as the Cambrian explosion. Many of these pioneering creatures left fossils behind. Some are so well preserved that scientists have been able to use scanning electron microscope images to piece together their inner anatomy, eyes included, and reconstruct their owners’ view of the world.
“I was amazed,” says Brigitte Schoenemann from the University of Cologne. “We can even calculate how many photons they would have captured.”
But these eyes were already complex, and there are no traces of their simpler precursors. The fossil record tells us nothing about how sightless animals first came to see the world. This mystery flustered Charles Darwin. “To suppose that the eye, with all its inimitable contrivances ... could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree,” he wrote in Origin of Species.
Creationists like to end the quotation there, with the great man doubting his own theory. But in the very next sentence, Darwin solves his own dilemma: “Yet reason tells me, that if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist … then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.”
The gradations he spoke of can be shown to exist. Living animals illustrate every possible intermediate between the primitive light-sensitive patches on an earthworm and the supersharp camera eyes of eagles. Nilsson has even shown that the former can evolve into the latter in a surprisingly short amount of time.
He created a simulation that starts with a small, flat patch of pigmented light-sensitive cells. With each yearlong generation, it becomes a little thicker. It slowly curves from a sheet into a cup. It gains a crude lens, which gradually improves. Even under the most pessimistic conditions, with the eye improving by just 0.005 percent each generation, it takes just 364,000 years for the simple sheet to become a fully functioning camera-like organ. As far as evolution goes, that’s a blink of an eye.
But simple eyes should not be seen as just stepping-stones along a path toward greater complexity. Those that exist today are tailored to the needs of their users. A sea star’s eyes—one on the tip of each arm—can’t see color, fine detail, or fast-moving objects; they would send an eagle crashing into a tree. Then again, a sea star isn’t trying to spot and snag a running rabbit. It merely needs to spot coral reefs—huge, immobile chunks of landscape—so it can slowly amble home. Its eyes can do that; it has no need to evolve anything better. To stick an eagle’s eye on a sea star would be an exercise in ludicrous excess.
“Eyes didn’t evolve from poor to perfect,” Nilsson says. “They evolved from performing a few simple tasks perfectly to performing many complex tasks excellently.”
A few years ago he enshrined this concept in a model that charts eye evolution in four stages, each defined not by physical structures but by the things that they allow animals to do. The first stage involves monitoring the intensity of ambient light, to gauge the time of day or the animal’s depth in a column of water. You don’t need a true eye for this; an isolated photoreceptor will do. Hydra, a small relative of jellyfish, has no eyes, but it does have photoreceptors in its body. Todd Oakley and David Plachetzki from the University of California, Santa Barbara, showed that these receptors control hydra’s stinging cells, so that they fire more easily in darkness. Perhaps this allows the creature to react to the shadows of passing victims or to reserve its stings for nighttime, when its prey is more common.
In the second stage of Nilsson’s model, animals can tell where light is coming from, because their photoreceptors gain a shield—usually a dark pigment—that blocks light from certain directions. A receptor like this gives its owner a one-pixel sense of the world—not enough to qualify as true vision but enough to move toward a source of light or swim away from it into a shadowy refuge. That’s exactly what many marine larvae do.
In stage three, the shielded photoreceptors cluster into groups, each pointing in a slightly different direction. Now their owners can integrate information about light coming in from different directions, producing an image of their world. They can see scenes, blurry and grainy though they may be. This marks the point when light detection becomes vision proper and when bundles of photoreceptors become bona fide eyes. Animals with stage-three eyes can find suitable homes, as sea stars do, or avoid obstacles, as box jellyfish do.
Stage four is where the evolution of eyes—and their owners—really takes off. With the addition of lenses for focusing light, vision becomes sharp and detailed. “When you get to stage four, the list of tasks has no end,” says Nilsson. This flexibility might have been one of the sparks that ignited the Cambrian explosion. Suddenly the rivalries between predators and prey, previously limited to sniffing, tasting, and feeling at close quarters, could play out over distance. An arms race began, and animals responded by ballooning in size, becoming more mobile, and evolving defensive shells, spines, and armor.
As they evolved, so did their eyes. All the basic visual structures that exist today were present during the Cambrian, but they have been elaborated in an extraordinary variety of ways—again for specialized tasks. The male mayfly looks like it has a huge compound eye glued on top of another smaller one, devoted to scanning the skies for silhouettes of flying females. The aptly named four-eyed fish has divided its two camera eyes in two, so one half sits above the water’s surface and examines the sky while the other looks out for threats and prey below. The human eye is reasonably fast, adept at detecting contrast, and surpassed in resolution only by birds of prey—a good all-around eye for the most versatile animal of all.
Far from being an obstacle to the theory of natural selection, the evolution of the complex eye is one of its most splendid exemplars. “There is grandeur in this view of life,” wrote Darwin at the end of his great work. It was his stage-four eyes that allowed him to see that splendor.
Nilsson’s model shines fresh light on an old debate: whether eyes evolved once or many times. The legendary German evolutionary biologist Ernst Mayr claimed that eyes had between 40 and 65 independent origins, because they came in so many distinct shapes and forms. The late Walter Gehring, a Swiss developmental biologist, argued that eyes evolved just once, after he discovered that the same master gene—called Pax6—controls eye development in virtually every creature with eyes.
Both men were right. True stage-three eyes did indeed evolve from their simpler stage-two precursors on several occasions; box jellyfish, for instance, developed theirs independently of mollusks, vertebrates, and arthropods. But the eyes of all those organisms are elaborations of the same basic stage-one light detectors.
We know this because all eyes are constructed from the same building blocks. Nothing that sees does so without proteins called opsins—the molecular basis of all eyes. Opsins work by embracing a chromophore, a molecule that can absorb the energy of an incoming photon. The energy rapidly snaps the chromophore into a different shape, forcing its opsin partner to likewise contort. This transformation sets off a series of chemical reactions that ends with an electrical signal. Think of the chromophore as a car key and the opsin molecule as the ignition switch. They turn, and the engine of sight whirs to life.
There are thousands of different opsins, but they are all related. A few years ago, Megan Porter, now at the University of Hawaii at Manoa, compared the sequences of almost 900 genes, coding for opsin proteins from across the animal kingdom, and confirmed that they all share a single ancestor. They arose once and then diversified into a massive family tree. Porter draws it as a circle, with branches radiating outward from a single point. It looks like a giant eye.
The mother of all opsins didn’t arise from nothing. Evolution jury-rigged the first opsins out of proteins that functioned more as clocks than as light sensors. These ancestral proteins held on to melatonin, a hormone that controls the 24-hour body clocks of many organisms. Melatonin is destroyed by light, so its absence can signal the first rays of dawn—but only once. Any creature that senses daybreak with melatonin has to continually make more of the stuff.
In contrast, the chromophores coupled to opsins don’t pose that problem. They merely change shape when they absorb light, and they can easily change back. So when melatonin-binding proteins mutated, they suddenly became reusable light sensors. Those were the first opsins. They were so efficient that evolution never came up with a better alternative; it just created variations on a theme.
The same can’t be said for other eye components. Take lenses. Almost all of them are made from proteins called crystallins, which improve their owners’ vision by focusing light onto underlying photoreceptors. But unlike opsins, with their single dynasty, crystallins are unified by name only. Yours are unrelated to those of a squid or a fly. Different animal groups have independently evolved their own brand of crystallins by co-opting proteins that had very different jobs, unrelated to vision: Some broke down alcohol; others dealt with stress. But all were stable, easy to pack, and capable of bending light—perfect for making lenses.
The weirdest lenses in nature don’t have crystallins at all. They belong to chitons—a group of marine mollusks that look like ovals adorned with armored plates. These plates are dotted with hundreds of small stage-three eyes, each with its own lens. The lenses are made of a mineral called aragonite, which the chitons assemble from calcium and carbonate molecules in seawater.
Simply put, this creature has evolved a way to sharpen its vision by looking through rocks. And when their rock lenses erode, the chitons just fabricate some new ones.
Opsins, lenses, and every other component of the eye are all testament to evolution’spatchwork tinkering. It constantly puts existing materials to new functions, and cobbles simple structures together into complex ones. But evolution has no foresight. Once it has trundled down a particular course, it can’t start from scratch again, so its works are always blighted by imperfections. Nilsson is particularly underwhelmed by compound eyes. Their structure, composed of many repeating units, sets an unforgiving ceiling on their visual resolution. If a fly wanted to see with the same resolution as a human, its eye would need to be a meter wide.
“Insects and crustaceans have become so successful despite their compound eyes, not because of them,” says Nilsson. “They would have done so much better with camera-type eyes. But evolution didn’t find that. Evolution isn’t clever.”
Eric Warrant, Nilsson’s next-door neighbor at Lund University, takes a more lenient view. “Insect eyes have a much faster temporal resolution,” he says. “Two flies will chase each other at enormous speed and see up to 300 flashes of light a second. We’re lucky to see 50.” A dragonfly’s eye gives it almost complete wraparound vision; our eyes do not. And the elephant hawk moth, which Warrant has studied intensely, has eyes so sensitive that it can still see colors by starlight. “In some ways we’re better, but in many ways, we’re worse,” Warrant says. “There’s no eye that does it all better.” Our camera eyes have their own problems. For example, our retinas are bizarrely built back to front. The photoreceptors sit behind a tangled web of neurons, which is like sticking a camera’s wires in front of its lens. The bundled nerve fibers also need to pass through a hole in the photoreceptor layer to reach the brain. That’s why we have a blind spot. There’s no benefit to these flaws; they’re just quirks of our evolutionary history.
We have evolved work-arounds. Our retinas contain long cells called Müller glia that act as optic fibers, channeling light through the morass of neurons to the underlying photoreceptors. And our brains can fill in the missing details in our blind spots. But some problems we can’t avoid. Our retinas can sometimes peel away from the underlying tissue, leading to blindness; that would never happen if the neurons sat behind the photoreceptors, anchoring them in place. This more sensible design exists in the camera eyes of octopuses and squid. An octopus doesn’t have a blind spot. It never gets a detached retina. We do, because evolution doesn’t work to a plan. It meanders mindlessly, improvising as it goes.
Sometimes it does U-turns. Eyes are as complex as their owners need them to be, and if those needs diminish, so do the eyes. Most birds and reptiles see color with four types of cone photoreceptors, each carrying an opsin that’s tuned to a different color. But mammals evolved from a nocturnal ancestor that had lost two of these cones, presumably because color vision is less important at night and because cones are most effective in bright daylight.
Most mammals are still saddled with these losses, and see the world through a limited palette. Dogs have just two cones, one tuned to blue and the other to red. But Old World primates partly reversed this loss by re-evolving a red-sensitive cone. That opened our ancestors’ eyes to a previously invisible world of reds and oranges and may have helped them discriminate between ripe and unripe fruit. Marine mammals went the other way, dispensing with the blue cone when they became aquatic. Many whales lost the red cone too. They have only rod photoreceptors—excellent for seeing in the deep ocean darkness but useless for seeing color.
If the benefits of seeing dwindle to none, some animals lose their eyes altogether. TheMexican tetra excels at this. In the Pleistocene epoch, some of these small freshwater fish swam into several deep caves. Their eyes were of little use in the pitch blackness, so their descendants evolved into different populations of blind cavefish—pinkish-white creatures with skin covering where their eyes used to be. These degenerations occurred because eyes take a lot of energy to make and maintain. In particular, the neurons that carry signals from photoreceptor to brain must always be poised to fire—imagine drawing the string of a bow and keeping it taut for minutes, maybe hours.
This explains why animals don’t have eyes that are better than they need and why they lose eyes so readily if they no longer need them. Squandering energy on a useless sensory system is a recipe for extinction. Eyes may be assembled from old parts, plagued by ancient bugs, and prone to breaking—but they’re also exquisitely tuned to the needs of their owners. They are testament to both evolution’s endless creativity and its merciless thrift.
At the University of Maryland, Baltimore County, Tom Cronin peers into an aquarium tank, and two googly compound eyes, like muffins mounted on stalks, peer back at him. “Mr. Googles,” as Cronin affectionately calls him, is a gorgeous animal, bedecked in a kaleidoscopic coat of peach, white, green, and blood-red. He is a mantis shrimp—one of a group of crustaceans named for the quick-punching arms protruding beneath their heads, like those of praying mantis. Mr. Googles’s arms end in formidable hammers, which unfurl with such speed and force that they can shatter seashells and aquarium glass.
“He’s become a bit of a pet,” says Cronin. “He’s got a lot of charisma, and he’s very cute.”
The mantis shrimp’s eyes have three separate regions that focus on the same narrow strip of space, providing depth perception without help from the other eye. They can also see ultraviolet parts of the spectrum that are invisible to us, and polarized light that vibrates in a single plane. And while we have 3 kinds of color receptors in our retinas, Cronin discovered that mantis shrimps have 12, each tuned to a different color. “It didn’t make sense. None of it did,” he recalls.
For years scientists assumed that with all those receptors the mantis shrimp must be the undisputed champion of color discrimination, able to detect tiny differences between hues. But Hanne Thoen, at the University of Queensland, Australia, smashed that idea in 2013. She presented mantis shrimps with optic fibers displaying different colors, and rewarded them with food if they attacked one in particular. She then adjusted the colors closer together until the animals could no longer discriminate between them. They performed appallingly: They couldn’t even distinguish colors whose differences are patently obvious to our eyes.
So why all the receptors? Thoen suspects that they have everything to do with pugilistic prowess. We do a lot of visual processing in our retinas, adding and subtracting information from our cones before sending it to our brains. Perhaps the mantis shrimp instead passes the responses of all 12 of its color receptors directly up to its brain, which compares the raw data against some kind of look-up table of different colors. While the mantis shrimp is inept at discriminating between colors, such a system might make it superb at recognizing color, which in turn could help it make the quick decisions needed to launch its superlatively fast strikes.
But Cronin is unconvinced. Back in his lab, he dangles a pipette in a petri dish containing a smaller mantis shrimp—just a couple of inches long. It tracks the intruding object with its eyes, then lashes out. The blow is powerful enough to make an audible crack, like a finger snapping.
“That little guy spent a long time thinking before he whacked it. It’s not a decision they make like that,” says Cronin, snapping his own fingers. “There remains the question: What’s it all for?”
It’s the question that Dan-Eric Nilsson always asks as well. It’s not enough to know the structure of the mantis shrimp’s eyes, or the genes that are activated within them, or the neural signals that they send to the brain. Ultimately, to understand why they are the way they are, we need to know how they are used. To communicate with each other? To catch prey quickly? To better see the riot of colors in coral reefs? This is the ultimate truth of animal eyes: We can only understand their evolution when we learn to see the world through them.