Evolution of Vision

Introduction:

Image: Diagram of a human eye, from National Eye Institute, National Institutes of Health.

The human eye is an impressive piece of hardware that captures light as it scatters off and through objects and focuses it into a coherent image. As light haphazardly bounces into the eye, it first passed through the transparent cornea. After the cornea, light then passes through the pupil (the open hole in the center of the ring-like muscle called the iris). The aperture of the iris can be finely adjusted to collect the perfect amount of light which then enters the lens. As like passes through the lens, it is refracted (or bent) to project a sharp image onto a mat of sensitive receptors in the back of the eye.

But this style of eye (called a "camera eye") is only one solution to the engineering problem of sight. In fact, “evolution has exploited nearly every optical principle known to physics, and produced eyes of many different designs from camera-type eyes, to compound eyes, and eyes that use mirrors” (Land and Nilsson, Animal Eyes, p.1). In this section, we will outline the evolutionary history of eye anatomy and function.

Evolutionary oddball

While most camera-type eyes follow a similar architectural format, there are certainly a few outliers. The barreleye fish (Macopinna microstoma) lives in the deep sea at the point which almost no light penetrates. It evolved super-sensitive, tube-shaped eyes positioned within a transparent head. Recently, scientists discovered that the fish can rotate the "tubes," allowing it to look directly upward to look-out for predators or forward in search of prey (Robison & Reisenbichler 2008).

The first eyes:

The first ‘eyes’ seem to have evolved by 530 MYA during the Cambrian explosion--a time of rapid diversification of bizarre creatures that represent the ancestors of modern animal groups. It is likely that these first ‘eyes’ were little more than single-cell photoreceptors that could sense no more than absences or presence of light. Though the first eyes were very basic, these simple sensors provided the evolutionary seeds for complex eyes.

Animal vision:

All organisms sense the world through receptors that transfer an input from the environment (e.g. light, chemicals, pressure, heat) into a neural impulse. All animals that sense light, from bacteria, to plants and animals, do so through photoreceptors--cells that contain a light-reactive pigment (rhodopsin, in animals).

In the simplest form, photoreceptive cells absorb a light photon and then initiate a signal to tell the animal that it is light. Over evolutionary time, photoreceptive cells accumulated and animal brains developed the ability to process complex signals from many receptors at once. Clusters of cells allowed for more complex image formation than single receptors. For some animal lineages, the clusters grew larger and even more complex. The retina of the human eye is one result of this process--it contains a multitude of receptors packed tightly side-by-side. Each receptor sends a signal to the brain and our brains then interpret all of these signals into our perception of sight.

While we humans put all of our light-receptive energies into our eyes, some animals retained simple proto-eyes in other parts of the body. In fact, simple photoreceptive cells present throughout the body of many animals, including some vertebrates, allow them to sense light through their skin, which can be useful for sensing day versus night, the shadow of an approaching predator, or ensuring that all body parts are safely pulled into a hiding place out of the sun. Other animals retained more complex proto-eyes on their heads (see box below).

Median Eyes

Image: Fig. S1. (From Foa et al. 2009) Dorsal surface of the head of a ruin lizard. (A) The solid arrow indicates the parietal eye. (B) lizard, with parietal eye and parietal scale painted black.

Some animals, like frogs and lizards have evolved more refined light sensing organs in addition to primary eyes, called median or pineal eyes located throughout the body. While median eyes cannot form images, they allow the animal to sense the quality and orientation of radiation from the sun even when the animal’s eyes are closed.

In fact, a team of researchers (Foà et al. 2009) showed that lizards could navigate a maze using only the parietal eye, but failed when the parietal eye was covered.

References:

Foà, A., Basaglia, F., Beltrami, G., Carnacina, M., Moretto, E., and Bertolucci, C. (2009). Orientation of lizards in a Morris water-maze: roles of the sun compass and the parietal eye. J. Exp. Biol. 212, 2918–2924.

Gehring, W. J. (2005). New perspectives on eye development and the evolution of eyes and photoreceptors. J. Hered. 96, 171–184.

Land, M. F., and Nilsson, D.-E. (2012). Animal Eyes. Oxford University Press.

Robison, B. H., and Reisenbichler, K. R. (2008). Macropinna microstoma and the Paradox of Its Tubular Eyes. Copeia 2008, 780–784.