Well, at their most basic, the eyes of animals have three jobs, according to biologists:
- the detection of light
- the detection of shadows
- the transmission of this information about light and dark to motor structures (because it’s one thing to be able to tell where a shadow’s coming from, quite another to be able to move away from it if you think it’s the shadow of a big, bad beast)
In the simplest kind of eye in the animal kingdom, found most often on the various tiny marine creatures we collectively call plankton, the motor structures that act on visual information aren’t muscle cells, like what you’d find in humans and other vertebrates, but cells lined with tiny hairs called cilia, which move the itty bitty organisms through their watery environments like the oars on a ship. The eye that provides these ciliated cells with info is similarly simple (alliteration away!); it’s made up of only two cells, one that receives the light, and another that processes shadows with the help of pigment.
Animal eyes range wildly in complexity, from the basic cup-shaped light and shadow detectors on the top sides of flatworms to the camera-like instruments of focus that we humans tend to roll every time we hear our bosses announce another brilliant idea for efficiency in the workplace.
There are sorts of stops in between, too. Squids and octopuses, for example, possess eyes that have lenses, as human eyes do, but lack the cells called cones that provide the human eye with the ability to perceive color. They can adjust their lenses to focus on objects near or far away but can only visualize those objects in terms of light and dark (though research suggests that octopuses have ways of reacting to color in their environments that have no parallels in human anatomy, which makes them really, really neat).
When it comes to human eyes, it’s been proposed that eyes have other functions besides the three listed above. Humans eyes not only receive information; they apparently also communicate it.
Unlike our relatives the great apes, humans have eyes that are partially white. This white part of our eyes is a structure called the sclera. The primate eye has a sclera, too, but it’s dark in color, frequently brown.
In 2006, anthropologists at the Max Planck Institute for Evolutionary Anthropology ran an experiment in which they had both great apes and human babies watch while researchers looked in one direction, then another, either by moving their heads or moving their eyes. The anthropologists found that the apes were more likely to follow the researchers’ gaze when the researchers moved their heads, while the human tots were more likely to follow when the researchers moved their eyes.
What this suggested is that our eyes evolved as a way of helping us cooperate on certain tasks. With our irises more visible against a white background, the researchers proposed, it would be easier to see where we were looking, so that others could see what caught our attention, too.
And let’s face it: as all the selfies on the Internet demonstrate, we spend a lot of time looking at human faces. In fact, it’s been suggested that eye contact is essential for creating a bond between a human infant and a caregiver, and that human babies spend twice as long staring into their caretakers’ eyes as primate newborns do. Is this behavior influenced by the evolution of the human eye? As always in science, research continues, but there’s a strong possibility that the eyes in this case really do have it.