On the Origin of Eyes
The evolution of the eye progressed to incorporate an eyespot that contained many cells functioning as photoreceptors. This eyespot depressed into a cup that allowed the animal to determine light brightness. As the eye pit became deeper, the animal was increasingly able to gauge the direction of the light's source. These 'pit eyes' have been found in ancient snails from the Cambrian period and are still found in several planarians alive today.
The earliest evidence of the eye dates back to about 540 million years ago, during the lower Cambrian period. This was a pivotal time period in the evolutionary process, and has been dubbed the “Cambrian explosion,” as the rate of evolution increased dramatically during this time. Scientists have determined that this first eye acted as a simple light sensor. These very first eyes are referred to as “eyespots,” and were found in unicellular organisms. These “spots” could differentiate between light and dark, but could not distinguish shapes or the origin of the light which it took in. This allowed these organisms to distinguish day from night.
From the pit eye, it is thought that transparent cells grew over the eye pit to prevent infection and contamination. The contents of the eye pit could now slowly change and become specialized. The transparent humor arose, which allowed for the filtering of colors, blocking of UV light, the ability to work in and out of water, and allowed for a higher refractive index.
Now that the eyeball was present, lenses began to evolve. These have been found to evolve independently in several different phyla. The evolution of the camera-type eye most likely occurred when the transparent cells split into two layers with a liquid behind the layer. The liquid functioned as a nutrition source for the cells, which allowed for more mechanical protection and allowed the cells to thicken. This supposed path has not been discovered in fossils, and it is not likely to be discovered as fossilization does not preserve soft tissue. Lenses continued to change and evolve based on predators' need for more powerful vision, or the preys' need a larger range of vision. Therefore, the eye grew larger and the lens became more curved, until it began to resemble the type of eyes found in humans and other vertebrates. This evolution from an eyespot to a complex eye with a lens occurred very quickly in terms of evolution, as it happened in just over 100 million years.
Today, the eye has many different parts; each with its own specific function. The aqueous humor is a water fluid that is located between the cornea and the iris. It has a refractive index and is regularly replenished by the ciliary body. The ciliary body is a muscle tissue that is connected to ligaments. The ligaments are attached to the eye's lens, and the tension on the ligaments will change the power of the lens. The cornea is made of transparent cells, and provides most of the focusing power within the eye. The iris is located in front of the ciliary body, and is made up of muscle and pigmented tissue. The hole in the iris, known as the pupil, controls how much light enters the eye. The optic nerve serves to connect the retina to the spot at the middle of the brain, and is the first connection of the visual system to the brain. The retina is made up of sensory, transparent, and nervous tissue. The retina contains two types of cells: rods and cones. Cones differentiate color and fine detail, and rods are required for peripheral and night vision. All the nerve fibers come together at the back of the retina and form the optic nerve.
Today there are 10 different types of eyes found in animals, which can be categorized into non-compound eyes and compound eyes. Non-compound eyes are those that are found in humans, or versions present during the evolution of the eye. More than 85% of all animals have a form of non-compound eye. These eyes have a better image resolution, as the lenses are much larger than in compound eyes. Compound eyes are typically found in arthropods and other insects. These eyes have a very large view angle and can detect very fast movement; even the polarization of light, in some cases. There are two types of compound eyes: apposition eyes, which are the most common and are found in all arthropods; and superposition eyes, which are typically used by nocturnal insects and are found in mayflies, crayfish, shrimp, lobsters and prawns. Apposition eyes gather many images in each eye, and then all the images are combined in the brain. The superposition eye's different lenses take light in at an angle and then reflect it to the other side at the same light angle.
The eye is an organ of intrigue to many. To learn more about the evolution of the eye, consult The following sources: