The Eyes Have It

This woman's eye is a complex sensory organ

The eye is something that we humans take advantage of. Vision is the primary sense we use to take in our environment, and as such we often don't think about how complicated our eyes are, or how easy it can be to damage them. In fact, the eye is so very complex that it is often used as an argument against the theory of evolution. However, on close examination the evolution of the eye can be easily explained through natural selection.

Compound eyes are very different

Scallops have many simple eyes

The human eye is a type of eye known as a "camera eye." This terminology is used due to the resemblance of said eye to a camera, including the fact that the image is flipped upside-down before reaching the "receiver" (film for a camera, the retina for an eye). However, there are many different types of eye out there, many of which are significantly more simplistic. For example, there are the compound eyes of insects and the simple eyes of flatworms.

The steps of camera eye evolution

Through a series of stages, a simple eye can in fact slowly evolve into a complex eye. Scientists have put forth a theory that includes what steps were most likely involved in the evolutionary process. Interestingly enough, examples of all the steps can be found in modern animals. The most simple form of eye, nothing more than a light sensitive spot, would be the starting point. Many single celled organisms have these simple light sensing eyespots. If a multicellular eyespot became indented, this would allow for slightly more sophisticated vision. Now, some sense of direction of the light can be determined due to the fact that the cup will cast shadows onto itself. This cup eye is also able to sense some movement, but that's about it. Scallops have many tiny blue eyes that are of this type.

Following the development of the cup eye, the development of the cup into a pit with a small opening would allow more clarity of the image. This smaller opening of this pinhole eye allows the light to focus better onto the light sensing cells. However, this would still only allow for very limited vision. The nautilus has an eye that is of this type. If a covering developed over the opening to the pit, this would allow for more focusing power. Hardening of this covering into a distinct lens would clarify it even more. This sort of structure is what is seen in eyes like our own. Darwin himself theorized that just such a stepwise process must have been involved in the origins of the camera eye.

Interestingly enough, the camera eye evolved on at least three separate occasions. Similar, yet still quite distinct camera eyes are seen in vertebrates like ourselves, cephalopods such as the octopus, and, believe it or not, box jellyfishes. One interesting point of comparison comes up when people claim that the human eye is perfect. Though incredibly complex, the eye is, in fact, not perfect. Some aspects of the eye actually get in the way of the ability to see. For example, due to the structure of the nerves and receptors in the eye, there is a blind spot near the center of where light focuses on the back of the retina. In addition, the nerves that transmit signals from the retina to the brain actually sit in front of the retina so that light must make it past the nerves before hitting the retina itself. In contrast, in cephalopods such as squid, the nerves are behind the retina.

Here are two interesting videos on eye evolution, including one narrated by the ever-wonderful David Attenborough.

Source is Evolutionary Analysis. Images are from Wikimedia Commons and are either under Creative Commons licenses or are copyright free: one, two, three, four. Videos are from YouTube.com: one, two.