Ssm Winter 2007 Biomed Visiblevolution

biology + medicine

VISIBLEVOLUTION A new look at the evolution of the eye

by Stephanie Le

I

magine finding components of a human eye in a worm. What would this tell us about eye evolution? Opponents of natural selection have frequently claimed the eye is too complex for Darwinian evolution to explain. However, current research on eyes casts new light on how they evolved. In the September 29th issue of Science, Professor Russell Fernald in Stanford’s Department of Biological Sciences reviews how genetics is helping scientists illuminate the evolution of eyes.

Envisioning the Eye

spherical to allow their eyes to focus in water. So both eyes converged on the same function along similar paths, despite having independently evolved.

Focusing on the Retina In different substances, such as water and air, light bends in distinct ways. Furthermore, light comes in different wavelengths and polarizations. All these properties have limited how eyes can work. Given these physical constraints,

“Light is so important, it has driven selection.” - Fernald

“Eye evolution has interested scientists ever since Darwin because the eye is such a complex organ,” Fernald says. Indeed the eye’s function can be overwhelming. The simplest analogy would be to compare your eye to a camera. When you look at an object, it reflects light that enters your eye and is focused by the lens in your eye onto your “film,” or retina. The cells of your retina transduce (convert) these light rays into neural signals that travel along the optic nerve to your “development center”, or brain. As complex as this pathway sounds, most animals have a way of doing it. What has puzzled scientists for decades is just how eyes have evolved to accommodate vastly different environments.

Converging to Similar Solutions “Light is so important, it has driven selection,” Fernald said. Nevertheless, despite the huge variety of environments animals live in, from air to water to underground, only eight major types of eyes exist. This is because the physics of light has constrained the evolution of optics so that vastly different species have independently developed the same solutions Credit: National Eye Institute, National Institutes of Health —a process known as convergent evolution. Consider fish and Retina cephalopods such as Lens octopi. Their eyes evolved independently about 260 million years after the animals had separated Optic from their last common nerve ancestor. Fish construct their lens from a single type of tissue, whereas octopi make theirs from two types of tissue. The human lens focuses light, reflected by However, because both objects, onto the retina. Retinal cells then transduce the light into neural signals carried live underwater, the by the optic nerve to the brain. resulting lenses are both

22 stanford scientific

it was once thought that vertebrate and invertebrate eyes evolved from a common ancestral eye. However, evidence now indicates that vertebrate and invertebrate eyes actually evolved independently. Genetic sequencing has shown that there were two closely related opsins, proteins responsible for transduction, in the last common ancestor of the vertebrates and invertebrates. One of the opsins became the foundation of the vertebrate retina, while the other became the foundation of the invertebrate retina. However, both opsins still function in both types of animals today. Vertebrate eyes have invertebrate opsins in their retina; likewise, invertebrates have vertebrate opsins in their brain. Imagine that: human proteins working in the brain of a worm!

Seeing is Believing “It’s a totally new way of thinking about the visual system,” says Fernald. Darwin himself wrote in The Origin of Species that explaining eye evolution through natural selection would be challenging. Approaching the task with genetics makes natural selection a verifiable explanation for eye evolution. While much about the eye remains to be discovered, ongoing research promises to shed light on the mysteries of vision. S STEPHANIE LE is a junior majoring in Biological Sciences. Her passions range from music to African cichlids to layout design. She also enjoys chocolate and stalking squirrels with her camera (occasionally at the same time).

To Learn More: Visit the departmental website of Dr. Russell Fernald http://www.stanford.edu/group/fernaldlab/index.shtml Read “Casting a Genetic Light on the Evolution of Eyes” http://www.sciencemag.org/cgi/content/abstract/313/5795/1914

layout design: Stephanie Le