Think the eye defies evolutionary theory? Think again

The compound lens of the insect eye

Win for Darwin

When Darwin proposed his theory of evolution by natural selection, he recognized at the time that the eye might be a problem. In fact, he even said it was “absurd” to think that the complex human eye could have evolved as a result of random mutations and natural selection. Although evolution remains a fact, and natural selection remains a theory, the human eye now has some solid evolutionary precedence. A group of scientists that has established a primitive marine worm, Platynereis dumerilii, as a developmental biology model has found that it provides the key to the evolution of the human—and insect—eye.

Multiple events in eye evolution, or only one?

The divide over the eye occurred because the insects have the familiar compound-lens—think how fly eyesight is depicted—and vertebrates have a single lens. Additionally, insects use rhabdomeric photoreceptors, and vertebrates have a type known as ciliary receptors. The rhabdomeric receptors increase surface area in the manner of our small intestine—by having finger-like extensions of the cell. The ciliary cells have a hairy appearance because of cilia that pop outward from the cell. A burning question in evolutionary biology was how these two very different kinds of eyes with different types of photoreceptors evolved. Were there multiple events of eye evolution, or just one?

Just once?

P. dumerilii work indicates a single evolutionary event, although the usual scientific caveats in the absence of an eyewitness still apply. This little polychaete worm, a living fossil, hasn’t changed in about 600 million years, and part of its prototypical insect brain responds to light. In this system is a complex of cells that forms three pairs of eyes and has two types of photoreceptor cells. Yep, those two types are the ciliary and the rhabdomeric. This little marine worm has both kinds of receptors, using the rhabdomeric receptors in its little eyes and the ciliary receptors in its brain. Researchers speculate that the light receptors in the brain serve to regulate the animal’s circadian rhythm.

How could the existence of these two types of receptors simultaneously lead to the evolution of two very different kinds of eyes? An ancestral form could have had duplicate copies of one or both genes present. Ultimately, if the second copy of the rhabdomeric receptor gene were recruited to an eye-like structure, evolution continued down the insect path. But, if the second copy of a ciliary cell’s photoreceiving gene were co-opted for another function, and the cells were ultimately recruited from the brain for use in the eye, then evolution marched in the vertebrate direction.

All of the above is completely speculation, although this worm’s light-sensitive molecule, or opsin, is very much like the opsin our own rods and cones make, and the molecular biology strongly indicates a relationship. It doesn’t completely rule out multiple eye-evolution events, but it certainly provides some nice evidence for a common eye ancestor for insects and vertebrates.

Note: This work appeared in 2004 and got a detailed writeup at Pharyngula.

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