Eye: Encyclopedia II - Eye - Evolution of eyes

Eye - Evolution of eyes

How a complex structure like the projecting eye could have evolved is often said to be a difficult question for the theory of evolution. Darwin famously treated the subject of eye evolution in his Origin of Species:

Despite the precision and complexity of the eye, theoretical analysis of eye evolution, developed by Dan-Erik Nilsson and Susanne Pelger (Nilsson and Pelger, 1994, Proc Biol Sci), demonstrated that a primitive optical sense organ could evolve into a complex human-like eye within a reasonable period (less than a million years) simply through small mutations and natural selection. Pro-intelligent design mathematician and Oxford professor David Berlinski [1] criticized these findings, including a criticism that the work contained no computer simulations (something assumed by a number of scientists but disclaimed by the original authors), and criticisms of the scientific establishment in general. [2] The original authors and other pro-evolution scientists subsequently challenged Berlinski's criticisms. [3]

Eyes in various animals show adaption to their requirements. For example, birds of prey have much greater visual acuity than humans and some, like diurnal birds of prey, can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution. However, the development of the eye is considered by many experts to be monophyletic; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago (Mya).

As far as the vertebrate/mollusk eye is concerned, intermediate, functioning stages have existed in nature, which is also an illustration of the many varieties and peculiarities of eye construction. In the monophyletic model, these variations are less illustrative of non-vertebrate types such as the arthropod (compound) eye, but as those eyes are simpler to begin with, there are fewer intermediate stages to find.

• Eye Spot- A simple patch of photosensitive cells, common among lower invertebrates. Can sense ambient brightness. Physically similar to receptor patches for taste and smell. Some organisms cover the spot in optically-transparent skin cells.

• Pit Eye- The patch gradually depresses into a cup, which first grants the ability to discriminate brightness in directions, then in finer and finer directions as the pit deepens. Pit eyes were seen in ancient snails, and are found in some invertebrates living today.

• "Pinhole Camera" Eye- As the pit deepens into a cup, then a chamber, the opening of the chamber achieves true imaging, for fine directional sensing and some shape sensing. Currently found in the Nautilus.

• Pinhole Camera with Protective Layer- An overgrowth of transparent cells prevents contamination and parasitic infestation. The chamber contents, now segregated, can slowly specialize into a transparent humour, for optimizations such as color filtering, higher refractive index, blocking of ultraviolet, or the ability to operate in and out of water. The layer may in certain classes be related to the moulting of the organism's shell or skin.

• Multiple Humours- The transparent cells over the aperture split into two layers, with liquid in between. The liquid originally serves as a circulatory fluid for oxygen, nutrients, wastes, and immune functions, allowing greater total thickness and higher mechanical protection. In addition, multiple interfaces between solids and liquids increase optical power, allowing wider viewing angles, greater imaging resolution, or both. Again, division of layers may have originated with the shedding of skin; intracellular fluid may infill naturally depending on layer depth. Note: this layout has not been found, nor is it expected to be found. Fossilization rarely preserves soft tissues. In case it does, the new humour would almost certainly close as the remains dessicate, or as sediment overburden forces the layers together. Then the fossilized eye would resemble the previous layout.

• Crystalline Lens- It is biologically difficult to maintain a transparent layer of cells as sizes and thicknesses gradually increase. Deposition of transparent but nonliving material eases the need for nutrient supply and waste removal. In trilobites the material was calcite; in humans the material is a mixture of proteins called crystallins. A gap between tissue layers naturally forms a biconvex shape, which is optically and mechanically ideal for substances of normal refractive index. A biconvex lens confers not only optical resolution, but aperture and low-light ability, as resolution is now decoupled from hole size (which slowly increases again, free from the circulatory constraints).

• Separate Cornea and Iris- Independently, a transparent layer and a nontransparent layer may split forward from the lens. (These may happen before or after crystal deposition, or not at all.) Separation of the forward layer again forms a humour, the aqueous humour. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes. This flap around the perimeter of the lens also masks optical imperfections, which are more common at lens edges. The need to mask lens imperfections gradually increases with lens curvature and power, overall lens and eye size, and the resolution and aperture needs of the organism, driven by hunting or survival requirements. This type is now functionally identical to the eye of most vertebrates, including humans.

• "Backward" Illumination of Retina- The retina may revert on itself, forming a double layer. The nerves and blood vessels can migrate to the middle, where they do not block light, or form a blind spot on the retina. This type is seen in squids, which live in the dim oceans. In cats, which hunt at night, the retina does not revert. Instead a second, reflective layer (the tapetum) forms behind the retina. Light which is not absorbed by the retina on the first pass may bounce back and be detected. As a predator, the cat simply accommodates blind spots with head and eye motion.

At some point, color vision develops when receptor cells develop multiple pigments. As a chemical instead of mechanical adaptation, this may happen at any of the points described above, or not at all, and the capability may disappear and reappear, as organisms become predator or prey. Similarly, night and day vision emerge when receptors differentiate into rods and cones, respectively.

At some point, a focusing mechanism develops. Some species move the lens back and forth, some stretch the lens flatter. Another mechanism regulates focusing chemically and independently of these two, by controlling growth of the eye and maintaining focal length. Note that a focusing method is not a requirement. As photographers know, focal errors increase as f-number decreases. Thus, an organism with small eyes, active in direct sunlight, may survive with no focus mechanism at all. As the species grows larger, or transitions to dimmer environs, a means of focusing could appear gradually.

The majority of the process is believed to have taken only a few million years, as the first predator to gain true imaging would have touched off an "arms race." Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.

Adapted from the Wikipedia article "Evolution of eyes", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki