National Geographic : 2016 Feb
Seeing the light 39 contrast, a fly’s compound eye divides incom- ing light among thousands of separate units, each with its own lens and photoreceptors. Hu- man, 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 pre- dictable 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 sur- vey 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, scien- tists need to do more than examine their struc- tures. They need to do what Nilsson did with the box jellyfish: understand how animals use their eyes. Around 540 million years ago, the ances- tors of most modern animal groups suddenly appeared on the scene, in an outburst of specia- tion 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 Tune in Sunday, February 14, to National Geographic Channel’s Explorer series episode Eyes Wide Open. The pair of black dots on the head of the flatworm Dugesia doroto- cephala represent some of the simplest true eyes: unembellished pits that can sense the direction of incoming light but lack any kind of focusing lens.