A new look at the eye
by Stephanie Le
Imagine 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
ÒEye evolution has interested scientists ever since Darwin because the eye is such a complex organ,Ó Fernald says. Indeed the eyeÕs 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 Òfilm,Ó or retina. The cells of your retina transduce (convert) these light rays into neural signals that travel along the optic nerve to your Òdevelopment centerÓ, 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
ÒLight is so important, it has driven selection,Ó 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 -- a process known as convergent evolution.
Consider fish and cephalopods such as octopi. Their eyes evolved independently about 260 million years after the animals had separated from their last common ancestor. Fish construct their lens from a single type of tissue, whereas octopi make theirs from two types of tissue. However, because both live underwater, the resulting lenses are both 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, 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
ÒItÕs a totally new way of thinking about the visual system,Ó 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. Something to consider... FernaldÕs metaphor for vertebrate and invertebrate opsinsÕ genetic evolution: There was severe evolutionary competition. One opsin was thought to have ÒwonÓ in vertebrates, and the other ÒwonÓ in invertebrates, eventually giving rise to the eyes we see today. Genetic foraging in vertebrate and invertebrate eyes revealed that the ÒlosersÓ remained behind, each collaborating with the ÒwinnerÓ to help animals use light.