The overwhelming majority of life on our planet depends on the sun for energy. Because life is so tightly linked to the sun, it is no surprise that many organisms (excluding those that live in total darkness) have evolved the ability to detect and respond to light. Plants turn their leaves toward the sun. Single-celled algae, protists, and other microbes swim toward or away from light. But it is the animals, with our image-forming eyes, that have taken light detection to the next level.
96% of animal species have eyes. The first animal eyes did little but detect light—they helped to establish day/night cycles and coordinate behavior—but more-complex eyes soon evolved. A predator who can see its prey from a distance, or a prey animal that can see the shadow of a predator approaching, has a clear survival advantage over those who can't. Even a slight improvement in image quality provides a significant survival advantage, allowing for the step-by-step evolution of increasingly complex eyes.
At its simplest, the eye incorporates three functions:
- Light detection
- Shading, in the form of dark pigment, for sensing the direction light is coming from
- Connection to motor structures, for movement in response to light
The most-basic structure that is widely accepted as an eye has just two cells: a photoreceptor that detects light, and a pigment cell that provides shading. The photoreceptor connects to ciliated cells, which engage to move the animal in response to light. The marine ragworm embryo (right) has a two-celled eye.
An eye with more photoreceptors has more power: it can detect variations in light intensity across its surface. A cup-shaped eye can better sense both the direction light is coming from and the movement of nearby objects. These improvements require only minor changes to the basic eye.
As animals evolved more-complex bodies and behaviors, the eye too became more complex. Eyes evolved connections to muscle cells rather than cells that moved by waving cilia. Neurons evolved that could process signals and coordinate behavior.
Later improvements included structures for better optics, such as lenses or mirrors that gather and focus light onto photoreceptors. Some eyes became spherical and evolved pupils that opened and closed to let in just the right amount of light for forming clear images. Muscles evolved to fine-tune focusing and to point the eye in different directions. Photoreceptors increased in number, providing more-detailed images (like adding pixels to a photograph).
Eyes most likely evolved from simple to complex through a gradual series of tiny steps. Piecing together the sequence of eye evolution is challenging, and we don't know the sequence of steps that led to every modern eye. But we do know that modern animal eyes come in many varieties, spanning a continuum from the simplest to the most complex. This demonstrates that all types of eyes are useful, and that eyes of intermediate complexity could also have formed as steps in the evolution of complex eyes.
Researchers at Lund University wanted to find out how long it might take for a complex eye to evolve. Starting with a flat, light-sensitive patch, they gradually made over 1,800 tiny improvements—forming a cup, constricting the opening, adding a lens—until they had a complex, image-forming eye. It is important to note that every tiny change these researchers made measurably improved image quality. The researchers concluded that these steps could have taken place in about 360,000 generations, or just a few hundred thousand years. 550 million years have passed since the formation of the oldest fossil eyes, enough time for complex eyes to have evolved more than 1,500 times.