Development of eye played key role in evolutionary advantage

The eye is a complex system, and its development affected differentiation of the species.

Daniele Veritti

There is no doubt: The eye is one of the most complex and perfect organs in the human body.

This is not the thought of only ophthalmologists. Charles Darwin, the father of the theory of evolution, said the eye’s complexity was so amazing that it put into question the evolution theory itself. He wrote: “To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.”

Other scientists since have spoken of the eye as having “astronomical improbability” and suggested its development triggered the acceleration of the species’ evolution and differentiation known as the Cambrian explosion. At that time, the phyla of creatures existing became separate due to a fast differentiation.

In fact, many authors agree that vision developed in the pre-Cambrian age when, between 550 million and 500 million years ago, a functional muscled eye with a lens and retina was developed in nature. It took, we could say, “the blink of an eye” on a geological scale.

First stages

The first rudimentary vision device probably consisted of a flat layer of photosensitive cells. It was most likely able to detect the presence or absence of light but gave no perception of light direction. Nilsson and Pelger created a mathematical model that proved that causal mutations occurred in a simple biological system, belonging to some monocellular organisms. These mutations led to the evolved and sophisticated camera eye of the vertebrates within 500,000 years, which is a short time on a geological scale.

Nilsson and Pelger calculated that 364,000 generations and less than 2,000 steps are necessary to pass from the flat eye, which in their model consisted of a photosensitive surface with a transparent layer on top and dark layer beneath, to an efficient eye with a spherical lens. Many intermediate eye configurations are still observable in nature, mainly in aquatic creatures.

The first step forward from the flat eye was curvature, granting the advantage to perceive the direction of light. This resulted in the cup eye. This simple evolution is so functional that it still exists in some present-day flatworms, such as the planarian species.

Then, an ulterior deepening and folding of the photoreceptor layer configured the so-called pinhole eye. This upgraded eye granted a better perception of the direction of light and the detection of basic shapes.

However, this kind of eye presents weak points, mainly a limited light income and a poor image definition from the lack of an efficient focusing system.

Next steps

The next step forward in the evolution of eye development is represented by the jelly substance placed in front of the light-sensitive cell layer of the eye, which acts as a rudimentary lens. This system still exists in sea snails. Along with the snails’ evolution, another diopter appeared in order to give better vision. It is the cornea that represents a specialization of external epithelial cells evolved into a transparent surface covering the pupil and dividing the inner eye camera from the outside.

As an improvement, a bubble of secretions was developed on the inner side of this kind of cornea, which acted as a lens and provided an undeniable advantage; this was the vesicular eye that can still be observed in several species. The absence of a ciliary muscle limits this eye’s focusing ability. Among the mussels, a curious attempt to respond to this lack of a ciliary muscle survived until today in the pecten, also known as scallop. The eye of this bivalve includes a curved reflective surface, called argentea, lying under the retina and resembling the mirrored surface of a car headlamp. The curved mirror can focus the light by reflecting it on the roughly concentric retinal surface in front of it. The scallop eye possesses a lens in the anterior segment of the eye; although this lens is able to correct spherical aberration, it cannot properly focus the light on the retina because focal distance is too long, so the argentea is needed. Something related to this peculiar reflective structure exists in many vertebrates, especially those that prey in a poorly illuminated environment. Examples include nocturnal birds of prey, cats and dogs. It is the tapetum lucidum that does not have a role in focusing. Indeed, its function is to enhance the light that hits the photoreceptors by reflecting it to allow better vision in the dark.

Successful vision hardware in fulfilling the need for focusing and light income is the vertebrates’ eye and also some crustaceans, spiders and other arthropods. Its secret seems to be in the lenses. Contemporary to an advanced optical system, the molecular substrate for color vision was also selected during evolution. What makes primates special is the perception of the red color long wavelength. The development of this ability was probably a result of the advantage that primates gained in being able to recognize protein-rich red leaves.

In conclusion, the eye’s configuration played a key role in terms of evolutionary advantage, pushing the differentiation of species throughout time. However, it must be pointed out that these simple steps of the eye’s evolution are not a reconstruction of the exact evolution of the eye. Rather, they represent samples of the attempts that evolution has provided to improve the eye structure, allowing a better interaction between the being and its specific environment.

References:

• Dawkins R. Climbing Mount Improbable. New York: W. W. Norton & Company; 1996.
• Lamb TD, Collin SP, Pugh EN Jr. Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. Nat Rev Neurosci. 2007;8(12):960-976.
• Mizzaro-Wimmer M, Salvini-Plawen L. Praktische Malakologie. Springer; 2001.
• Nilsson DE, Pelger S. A pessimistic estimate of time required for eye evolution. Proc Biol Sci. 1994;256(1345):53-58.

• Daniele Veritti, MD, can be reached at Department of Ophthalmology, University of Udine, p.le S. Maria della Misericordia, 33100 Udine, Italy 33100; +39-0432-559907; email: verittidaniele@gmail.com.
• Disclosure: No products or companies are mentioned that would require financial disclosure.