This box jellyfish (Tripedalia cystophora) is only about half an inch across, yet it possesses 24 eyes, housed in four rhopalia. 

Four of the six eyes in each rhopalium (left) are simple photo sensors, but two have light-focusing lenses. A floating crystal weight called a statolith keeps the top lensed eye always pointing upward, scanning for mangrove canopies that signal food and shelter.

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 thesize 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.

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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.