Eye

An eye is a sensory organ used to perceive visual information. It works by detecting light and converting it into electro-chemical impulses in neurons. These impulses travel through neural pathways to the brain. In many higher organisms, the eye acts as a complex optical system. It collects light from the environment and regulates its intensity using a diaphragm. An adjustable assembly of lenses then focuses this light to form an image. This image is converted into electrical signals and sent via the optic nerve to the visual cortex.

There are ten fundamentally different eye layouts in nature. These are categorized into compound eyes and non-compound eyes. Compound eyes consist of multiple small visual units called facets. These are common in insects and crustaceans. Some compound eyes contain up to 28,000 sensors arranged hexagonally. This arrangement can provide a full 360° field of vision.

Because they use many small sensors, compound eyes are very sensitive to motion. They can produce a single pixelated image or multiple images per eye.

Non-compound eyes, often called simple eyes, use a single lens. These eyes focus light onto a light-sensitive layer called the retina. This type of eye is common in mammals, including humans. The retina contains specialized cells to process light. Cone cells allow for color vision, while rod cells help with low-light contrasts.

In many vertebrates, the eye is a spheroid shape filled with a transparent gel called vitreous humour. An iris often sits in front of the pupil to control light levels.

The simplest form of an eye is the pit eye, or eye-spot. These are small, sometimes comprising only 100 cells covering 100 μm. They may be set into a pit to reduce the angle of incoming light. This allows the organism to deduce the direction of the light source. Some insects also have ocelli, which are very simple eyes. Ocelli lack a lens to project a sharp image, so they mostly detect changes in light intensity. This helps animals, such as flying insects, sense which way is up.

Evolutionary history shows that eyes have changed significantly over time. The first proto-eyes evolved about 600 million years ago during the Cambrian explosion. The last common ancestor of animals already possessed the biochemical toolkit for vision. Advanced eyes have since evolved in 96% of animal species across six main phyla.

Extinct animals like trilobites had unique compound eyes made of clear calcite crystals. Most modern arthropods have much softer eyes compared to those trilobites.

Different environments have led to specialized visual adaptations. For example, some organisms living near deep-sea vents have compound eyes that see infra-red light. This helps them avoid being boiled by hot vents. The mantis shrimp possesses the world's most complex color vision system, known as hyperspectral color vision.

In terrestrial animals, a refractive cornea helps focus light. However, animals like penguins that returned to the water lose this curved cornea. They rely more on lens-based vision because the refractive index of water is similar to their internal fluids.

Vision also serves functions beyond just making images. Retinal photosensitive ganglion cells can send signals to the suprachiasmatic nuclei. This process helps organisms manage circadian adjustment, or their internal biological clock. These cells also signal the pretectal area to control the pupillary light reflex.

This shows how the eye is connected to broader biological systems that regulate life cycles and light responses.