Light is, arguably, the reason we exist.
Billions of years ago, the first cells utilized light as a source of energy using photosynthesis, thus set the stage for complex multicellular life. Light, the source of all energy we consume, creates vitamin D in our skin and allows our most precious sensory organs, our eyes, to function.
Nearly every living multicellular organism has developed some light-sensing ability. The human eye harnesses light in an incredibly complex way. Light enters the eye through the cornea, is focused by the lens, and lands on the back of the eye — the retina — where special sensory cells collect information about the amount of light and its wavelengths. The optic nerve passes this information to the brain, which produces shapes, colors, and patterns. It is easy to assume the human eye has always been a perfect well-oiled machine, but to fully understand how the eye works requires looking back to how it was formed.
The first blueprints for the eye began when several light-sensing cells came together to develop an eyespot. Organisms with eyespots were able to distinguish between light and dark, giving them an evolutionary advantage over similar organisms without them. After about 35,000 generations, some of these eyespots began to form a concave shape. This cupped eye allowed spatial information to be processed by the brain, thus these organisms were able to see shapes. These simple eyes still exist today and can be seen in several worm species. Some organisms, like the nautilus, developed a cupped eye with a pinhole for additional focusing. In other cases, eyespots bulged outwards, creating a convex shape and leading to the compound eye found in many insects.
The first blueprints for the eye began when several light-sensing cells came together to develop an eyespot.
The eyes of the modern-day lamprey are primitive examples of vertebrate eyes and can help illustrate their origin. Lampreys have camera-style eyes. These eyes — found in vertebrates, cephalopods, and a few other phyla — contain a wall of light-sensitive cells and a lens that focuses the light passing through. While this is quite the leap from a cupped eye, lampreys have the best-known examples of the simplest camera-style eyes.
As some organisms ventured onto dry land, their eyes continued to evolve. Around 360 million years ago, amphibians had corneas that were clear and thick, and external eyelids began to form. At the same time, some of these organisms showed a push toward binocularity, where two eyes create one compounded image in the brain.
Mammals arose around 200 million years ago, and by then, their ocular evolutionary path had split off from those that would eventually lead to reptilian and avian eyes. From here, the mammalian path was split further, radiating into groups such as monotremes, marsupials, and early primates. The eyes of Old World monkeys, a primate, evolved three visual pigments around 40 million years ago. Most humans today have trichromatic vision. The three color-processing channels on the retina combine to visualize the spectrum of light between 750 and 400 nanometers: red to violet.
Human eyes are incredibly complex organs that have come a long way from the bundle of light-sensing cells millions of years ago.
Human eyes are incredibly complex organs that have come a long way from the bundle of light-sensing cells millions of years ago. However, the human eye is by no means the best model. Instead of three types of photoreceptor cells, mantis shrimp have between 12 and 16. Some animals can see ultraviolet or infrared light, and others can sense movement as slight as 15 degrees of rotation per hour. The process of evolution has allowed thousands of eye types to come into existence. The eyes of the animal kingdom are a testament to the importance of these sensory organs and just how much light has shaped the diversity of the world.
