Part 4 – The Human Eye
In Parts 2 and 3 we investigated the retinal cells of the eye and one of the many chemical sequences required in human vision. There is also a vast assortment of other proteins that are integrated in human vision as the eye is far more than just the retina. Our sight today is dependent upon a fascinating coordination of cells organized in the structural organ of the human eye.
The human eye has a cornea and lens. The human body is composed of a great number of types of tissues, and no others are like the cornea and lens. These two tissues have an unusual cell composition and structure which enables them to function as "biological glass." This article from the National Library of Medicine describes the structure and chemistry of these unique cells, and explains how these features coordinate to yield the result of transparency: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3061108/.
The cornea is composed of four layers of differing cell types which function together to protect the inner structures of the eye, refract light, focus light on the retina, and minimize the scattering of incoming light. The article from the National Library of Medicine at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5819093/ discusses the multilayers of the cornea with their differing structures and functions.
Muscles in the iris open and close the iris to control the amount of light entering the eye and change the lens shape to focus at different distances. Other muscles rotate the eyes and allow them to stay steady as we move; otherwise, we'd see double. The ciliary muscles attach to the lens with unique ligament fixture attachments. These ciliary muscles are composed of different types of muscle fibers which each have connections to nerve pathways from the midbrain controlling sympathetic and parasympathetic reactions. The activation of each fiber type yields a different result in the lens. This article from Science Direct reveals the vast complexity in the cell types and muscle fibers which compose this one muscle in the eye: https://www.sciencedirect.com/topics/neuroscience/ciliary-muscle
The eye is also dependent on a fluid system. A clear liquid called the aqueous humor fills the two chambers of the eye. The aqueous humor nourishes the lens and cornea with sugars, vitamins, and proteins. In addition to supplying the needed nutrients to the eye, the aqueous humor gives the eye its shape, maintains the needed eye pressure, and protects the cornea. The eye cannot function without this fluid, and this fluid is produced by the ciliary body. The ciliary body has the muscle tissue that controls the lens, and also has ridges called ciliary processes whose capillaries and epithelia make and deliver the needed fluid on the eye.
A true appreciation for what the human eye entails cannot be grasped without becoming aware of both the molecular chemistry that drives every cell and the functions within each of the anatomical components of the eye. For example, in the aqueous humor of the eye, 763 distinct proteins have been identified, 35% of which are believed to be involved in cell communication and signal transduction. (See the National Library of Medicine article at https://pubmed.ncbi.nlm.nih.gov/25933257/.) Each of our eye tissues are constructed from differing proteins, have multiple chemical pathways controlling their responses, and are dependent on an array of blood and nutrient supplies.
Development of the Human Eye
If the human eye evolved from a more primitive form of reception, beneficial generation of different types of cells (which involves different proteins, different cell organelles, different membranes with different attachment sites for different extracellular inputs and activators, and different intracellular molecular cascades of chemistries) must have occurred in many iterations. Today, each of the parts of the eye are interdependent and critically necessary for vision.
The cornea, lens, retina, and eye muscles coordinate together to form a camera-like structure which focuses light onto photoreceptor cells that respond to light penetration. The organ of the eye not only houses these components, but also supplies the nutrients, pressure, and protection each of the components require. Each component of the eye has multiple cell types and multiple interactions with the brain. Each cell type is filled with organelles which are subcellular structures that perform one or more specific jobs in the cell. Differing cell types are filled with specific organelles and particular chemical cascades suited to its function.
A detailed reading into even one of these types of eye tissue reveals that the complexity on the molecular level in structure and chemical pathways is enormous. A great number of iterations of stepwise beneficial changes is required to build the complexity and dependence within the eye that is present today.
I used to only consider the anatomical changes necessary for evolutionary step changes towards today's systems. But now, I realize that each anatomical component houses within it a series of chemical cascades that have molecular inputs and outputs that must coordinate to accomplish every function within these anatomical structures. Those chemistries drive the function of the anatomical parts. And these chemical cascades are not minimal; the aqueous humor alone containing at least 763 proteins.
Stepwise evolution of today's organ structures must take into account the generation of functional proteins, differing cell types, differing membrane types, needed protein/enzymes/regulators, the sequencing of inputs and outputs, the location and protection of anatomical parts, the supply of nutrients and removal of chemical wastes, the connection to nervous system control, and the ability for all of these pieces to operate successfully together at each stage.
In order to see another example of these interacting components, Part 5 will take a look at the human reproductive system.