Evolution: eyes are as complex as the human brain (Evolution)

by David Turell , Thursday, February 27, 2020, 05:53 (1730 days ago) @ David Turell

The evolution of the eye is highly specific in what is required:

https://evolutionnews.org/2020/02/the-evolution-of-the-eye-demystified/

"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.

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"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.

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"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.

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"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.

"the following is required:

"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.

"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.

"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.

"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,

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"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.

"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."

comment: A highly complex system requiring so many precisely specific interacting parts can only be the result of design.