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The Evolution of the Eye, Demystified

Otangelo Grasso

evolution

How did the eye evolve? Michael Behe in 2006 and Jonathan Wells in 2017 wrote about the irreducible complexity of the light-sensing cascade that makes vision possible. Yet Darwinists persist in asserting that this wondrous organ emerged, without guidance or direction, from a presumed ancestral eyespot. 

This is an update on that important subject. I wish to emphasize the irreducible complexity of the visual cycle, on top of the sheer anatomical complexity of the human eye with its over two million working parts, second only to the human brain in complexity.  

Function and Phototaxis

Eyespots only perform a function when embedded in an interdependent system such as the one devoted to locomotion in the green algae Chlamydomonas. Phototaxis is a movement that occurs when a whole organism moves either closer to, or away from a light source, such as the sun. It is essential, for example, for green algae, which can move towards light to perform photosynthesis, capturing light and transforming it into chemical energy. Yet green algae also move away from the light to protect themselves against an intense source of illumination. Eyespots are the simplest eyes found in nature. They are composed of rhodopsins, which are light-sensitive proteins, and orange-red colored pigment granules, which have their color by selectively absorbing or reflecting light. The color spectrum, which is reflected, is the one that becomes visible to our eyes.

The pigment spot reduces the illumination from one direction or changes the wavelength of the incident light falling on the photoreceptor. It thus allows the organism to move in the direction of the light or away from it.

As an interdependent system, this visual system requires certain essential components, including rhodopsin proteins, a pigment spot, and ion flux. If one part is missing, the organism cannot move by phototaxis. Natural selection will not select any intermediate evolutionary step, since the system, with any of the required elements missing, would confer no function, and thus no survival advantage. 

Anything but Simple

While proponents of unguided evolution characterize the light-sensitive spot of some ancestral creatures as simple, it is anything but that. As a 2015 article in Frontiers in Plant Science notes, eyespots have a “high ultrastructural complexity.” Of course, this may be said, all the more so, of more advanced eyes. Consider some of the details. In forms ranging from the “simplest,” most rudimentary eye, such as eyespots in unicellular organisms, e.g. Chlamydomonas, to complex vertebrate eyes, such as our own camera eyes, rhodopsin proteins capture the light and are the first and central players in a complex chain of biochemical events. There is no vision without rhodopsin proteins. Unless rhodopsin transforms light into a signal, and that signal is used by a signal transduction pathway to promote phototaxis, neither rhodopsins nor eyespots would have a function on their own. 

Rhodopsins themselves are complex. They are composed of two parts: opsin proteins, which are made of seven α-helices forming a circle, and retinal, which is a light-absorbing chromophore. Retinal is covalently linked to the opsins and horizontally positioned in the pocket inside the opsin tunnel. When a single photon hits retinal, a small conformational change is triggered in the opsin, and that triggers a cascade of several chemical reactions and biochemical transformations, ultimatively leading to sight. A 2016 article in Nature Communications observed that “rhodopsin functions as a molecular off–on switch; it is designed to be fully inactive in the dark and to rapidly convert to a fully active structure in the light.”

As a general note, functional molecules, such as those within the catalytic sites of enzymes (in our case, retinal cofactors), require high specificity in their form and are thus well conserved (unchanged, or non-evolved ) across organisms. That is because mutations within these sites usually do not confer any advantage.

Recruiting and Co-Option

In seeking to explain how biological novelties arise, evolutionists often point to the recruiting and co-option of extant building blocks. In such a scenario, the building blocks are incorporated into new systems by natural selection of new functions. Rhodopsin would have to undergo evolution by recruiting retinal cofactors, which it would have to find fully formed and functional, finely tuned and just the right size to fit the binding pocket of opsin, a molecule obtained by a complex multistep biosynthesis pathway starting with carotenoid organic pigments from fruits, flowers, trees, or vegetables. It would require elaborate import mechanisms from the outside into the eyespot and the information on how to insert it in the opsin binding pocket to form rhodopsin and attach it at the right place. 

In their book The Retina and Its Disorders, Joseph Besharse and Dean Bok state (p. 641) that “the chromophore-binding pocket is well defined, suggesting that the binding pocket has high specificity for the Schiff base and the β ionone ring.” The precise and correct binding of retinal to the opsin is essential to trigger the change of the shape of retinal, and thus necessary for visual sight. It must be specific and functional from the beginning.

So the following is required:  

  1. A Schiff base, which is a chemical compound where carbon and nitrogen atoms are bound together by a double bond, involving four, instead of two electrons, binding retinal to a side chain of a lysine amino acid. 
  2. A side chain of the amino acid Lys296 (lysine) where retinal covalently binds. Each of the seven transmembrane helices is composed of a specific number of amino acids. Bovine rhodopsin, for example, has 342 amino acids. The number 296 in Lys296 stands for the 296th amino acid in the chain. There is a pivotal role for the covalent bond between retinal and the lysine residue at position 296 in the activation pathway of rhodopsin. 
  3. An essential amino acid residue called “counterion.” The counterion, a negatively charged amino acid residue that stabilizes a positive charge on the retinal, is crucial for rhodopsin to receive visible light. 

Unless all of these specific points are right from the beginning, rhodopsin will not be functional. A coordinated and finely tuned interplay and precise orchestration between opsin and retinal right from the start is thus indispensible.

Origin of Correct Protein Folding

Hundreds of rhodopsins are embedded in the lipid bilayer of the membrane of Chlamydomonas, each using seven protein transmembrane domains, forming a pocket where retinal chromophores are inserted.  

The precision with which opsins must fold into their seven-transmembrane configuration is staggering, as JILA (formerly the Joint Institute for Laboratory Astrophysics) reported:  

Biophysicists at JILA have measured protein folding in more detail than ever before, revealing behavior that is surprisingly more complex than previously known….

[T]he JILA team identified 14 intermediate states — seven times as many as previously observed — in just one part of bacteriorhodopsin, a protein in microbes that converts light to chemical energy and is widely studied in research. 

“The increased complexity was stunning,” said project leader Tom Perkins, a National Institute of Standards and Technology (NIST) biophysicist… “Better instruments revealed all sorts of hidden dynamics that were obscured over the last 17 years when using conventional technology.” 

“If you miss most of the intermediate states, then you don’t really understand the system,” he said. 

Knowledge of protein folding is important because proteins must assume the correct 3-D structure to function properly. Misfolding may inactivate a protein or make it toxic. Several neurodegenerative and other diseases are attributed to incorrect folding of certain proteins. [Emphasis added.]

An article in the journal Eye (“Light and the evolution of vision”) confirms:

[E]ven as far back as the prokaryotes the complex seven transmembrane domain arrangement of opsin molecules seems to prevail without simpler photoreceptors existing concurrently. Darwin’s original puzzle over ocular evolution seems still to be with us but now at a molecular level.

As for retinal, the second essential component of rhodopsin, a paper in the journal Vision Research reports:

11-cis-Retinal is a unique molecule with a chemical design that allows optimal interaction with the opsin apoprotein in its binding pocket, and this is essential for the formation of the light-activated conformation of the receptor. 

Remarkably, all structural details in the retinal chromophore are functionally important. As another paper, this one in the journal Trends in Biochemical Sciences, finds: 

Although there is an intriguing evolutionary conservation of the key components involved in the production and recycling of chromophores, these genes have also adapted to the specific requirements of insect and vertebrate vision.

We have, so far, only scratched the surface. But we can safely say that the origin of both vision and its key player, rhodopsins, cannot be explained by the evolutionary mechanisms of random mutations and natural selection. Instead they must have existed from inception as a unified and codified system. Such an observation, I believe, is best explained by intelligent design.

Image credit: Steve Long via Unsplash.