How Did We Evolve Eyes to See?
Only technology so sophisticated as spectrographs, interferometers, and the Hubble Space Telescope could bring us to our current understanding of light as essentially a wave of energy moving through space. No one in the ancient world could realize that light is a type of electromagnetic radiation existing in waves, oscillations, and diffractions. But they could relate to it in metaphorical and intuitive ways that astound me.
Looking at the Bible:
Light was associated with good deeds (Matthew 5:16).
Light overpowers darkness (John 1:5).
Light is associated with God (John 8:12), with salvation (Psalm 27:1), with healthy eyes (Luke 11:34-35), with the dead rising (Ephesians 5:14), with clarity (Psalm 119:130), and with wisdom (Ecclesiastes 2:13), as folly is the companion of darkness. It is not surprising to me that the ancients, unaided by spectrographs, would dichotomize good and evil based on terminology about a force of nature that eludes us as to how it meets our greatest needs. It is not surprising to me that the authors of the Bible, based on casual observations of the world, would associate light with clarity, truth, and goodness as they regard Jesus as the ontological source of truth. Everyday experience teaches us that we feel moments of truth when nature is clarified for us in fulfillment of the basic need of understanding (remember Maslow's hierarchy of needs). Those moments of truth are so powerful; it feels God-like or almost equivalent to a light shining upon you. I suspect that these powerful experiences might cause us to lose control of these sacred metaphors and add too much literalness to these texts.
Even the authors of the Qu'ran knew through casual observation that light was desirable and necessary:
"Allah is the light of the heavens and the Earth; a likeness of His light is as a niche in which is a lamp, the lamp is in a glass, (and) the glass is as it were a brightly shining star, lit from a blessed olive-tree, neither eastern nor western, the oil whereof almost gives light though fire touch it not – light upon light – Allah guides to His light whom He pleases, and Allah sets forth parables for men, and Allah is Cognizant of all things. (al-Nur, 24/35)" [1]
The atomists discussed the properties of light with terms that were much less religious. For Democritus (460 BC - 370 BC), the air between your eye and the object you see is impacted by the object and your seeing eye. Data about the object is conveyed to the eye via this compressed air. Epicurus (341 BC - 270 BC) believed in a seamless stream of atoms from the object into the eye. Plato (428 BC - 328 BC) believed that the eyes radiate rays of light that hit an object and allow us to perceive color, shape, and size.
This was foundational to the extramission theory (now obsolete) that persuaded humanity for almost 1,000 years.
In 300 BC, Euclid incorporated geometry into his belief in extramission when talking about rectilinear rays radiating from the eyes, illuminating whichever objects they fall upon, and determining their size according to the angle used [2].
The Modern Understanding of Light Properties
They were unaware of light's existence as a stream of photons, which are packets of energy with zero mass and rest energy, in constant motion as basic and discrete units. Photons are stable, with no electric charge, and have a frequency-dependent momentum. The concept originated with Albert Einstein in the 20th Century [3] [4]. The emanation of the light rays and energy rays are independent of human eyes.
They were unaware of the spectrum of electromagnetic radiation with low energy radio waves and high energy gamma rays. Shortening the wavelength heightens the energy, and elongating the wavelength decreases the energy (John McMurry, et al., 2017, pgs 66-67 [10].
The spectrum of light that is visible to the naked eye is the violet light (400 nanometers) and the red light (700 nanometers), while the radiation that falls below 400 nanometers (ultra violet) and rises above 700 nanometers (infrared) are invisible to the naked eye [3].
How Did We Evolve to See Light
Attempting to discredit Darwin, and to make it seem that he doubted the validity of natural selection, people have misquoted him:
"To suppose that the eye, with all its inimitable contrivances.... could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree." [5]
They have ignored his next words that can be read in his book titled On The Origin of Species:
"Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist.... and if such variations should be useful to any animal.... then the difficulty of believing that a perfect and complex eye could be formed by natural selection, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself first originated; but.....as some of the lowest organisms.....are known to be sensitive to light, it does not seem impossible that certain elements...should become aggregated and developed into nerves endowed with this special sensibility." [5]
It has been believed that the origin of eyes goes back to the time of Euglena, 500 - 550 million years ago.
The photoreceptor molecules (converters of light into biological signals) of eukaryotic protists, prokaryotes, and cyanobacteria share commonalities with the light-sensitive, pigment-forming opsin proteins of mammals and the information-gathering retinal neurons (called ganglion cells) [5]. These eukaryotic protists are basic single-celled organisms. The prokaryotes are pill-shaped bacteria with no nuclear envelope and the cyanobacteria are photosynthesizing, bacteria-related microorganisms that are important to Euglena.
Euglena should not be discussed without Chlamydomonas here. These two single-celled microorganisms have an orange, light-sensitive pigment spot, which scientists have called an eyespot, with carotenoid that provides the color and protects the photoreceptor from detrimental light. The pigment spot provides protection by changing the light's wavelength, contacting the photoreceptor, and allowing the microorganism's free movement either away from or toward a light source [5].
Euglena gracilis is a self-sustaining, carbon-eating, photosynthesizing microorganism but can survive on plant material in a low-light environment. It can move toward light to nourish itself and move away from extreme illumination based on slender threadlike structures, called flagella, that propel it after light-detection is made. Neighbored with the eyespot is the photoreceptor carrying 2x10^7 rhodopsin-like molecules, while the 10^8 photons striking the protein have 500-525 nanometers of maximum absorption. As light provokes movement or positive phototaxis, the biological agents act as though they are recording electrical activity in the retina, which has been likened to an electroretinogram [5].
Chlamydomonas has thylakoid membranes, the site at which the light reactions occur, with rotund-shaped bodies abounding in carotenoid and photoreceptors in between their thin cell walls/linings. Chlamydomonas has chromophore atoms responsible for blue colors at 440 nm wavelengths. These blue-light receptors, which are intertwined with circadian rhythms, have two forms (CRY1 and CRY2) found in human ganglion cells responsible for vision adaptation and changing the diameter of your pupils [5].
Euglena may depend on rhodopsins (responsible for purplish-red colors) for movement toward light, but it also depends on enzymes catalyzing cyclic adenosine monophosphate (cAMP) for moving away from light. cAMP are important messengers for transmitting chemical and physical signals [5].
Vision and eyes evolved millions of years ago, but those exact millions are debated. We believe that the evolution began in water, where electromagnetic energy travels well [6].
Could it have been 500 - 550 million years ago during the Cambrian explosion when a spate of speciation occurred? Today, we have the technology advancements that unveil the opsin proteins foundational to all eye-carrying organisms, indicating they shared a common ancestor. The Earth's considerable environmental changes forced microorganisms to navigate in different ways. The complexity and specialization of the new eyes involved different shapes and colors [7].
Although it is believed that most phyla did not survive long ago despite the many different types that emerged onto the scene, since we can only identify 4 phyla that can form images today, and 3 of them (Mollusca, Arthropoda, and Chordata) make up 96% of species [8].
Extinct marine arthropods (Olenellus fowleri) are said to have had the first eyes known to man, according to the calcite (white or colorless mineral consisting of calcium carbonate) in their optical units which can preserve themselves without fossilization. Although this does not mean they had the first eyes ever to exist. They probably existed between 600 and 550 million years before the Cambrian explosion, indicating that eyes were developing before the Cambrian period, but we do not have pre-Cambrian trilobite/arthropod records of that kind. We classically believe that the middle or early Cambrian was the time period of the oldest Olenellus [8].
Vision is so precious that we might often assume there is no way the evolutionary development of sentient life could have diverged from it. But there could have been a divergence. It was not unavoidable. According to what we see in stromatolite fossils, cyanobacteria directly descended from the simple, primitive, replicative cells that emerged 3.7 billion years ago. The cyanobacteria led to the first chloroplasts that lived inside other organisms (endosymbiosis) and used the sun's energy to turn the planet green and generate the beta carotene vital to rhodopsins. Descendents had to adapt to a changing environment where sunlight was a chief energy source, while the short wavelengths of gamma rays and X-rays were generating cell-damaging free radicals. The ultraviolet and infrared frequencies were not accessible as fuel for cellular metabolism because protection could not be provided against their cell-damaging effects [8].
The opsin of rhodopsin is thought to have probably evolved from a G-protein coupled receptor (cell surface receptors that act like inbox messages informing cells about the presence or absence of life-sustaining nutrients). But this has not been ascertained. Archaea got its energy from the opsin-like compounds that united with retinas, operating as a proton pump for the energy source, which led to a transition of molecules from the microbial opsins to metazoan opsins that shared a common ancestor [8].
There were other additions attached to rhodopsin as a vital part of ocular evolution. Their vestiges can be found in jellies and sponges. There were genetic predispositions for making the eye as retinas combined with chromophore atoms to become responsible for the color in compounds, and opsins were structured with G-protein coupled receptors (GPCR). Opsins, or their antecedents, covalently bonded with retinas, along with a cilium or two, in the slow development into an eyespot. Light and dark could be detected in the electromagnetism-sensing cells of the eyespot in multicellular animals, such as a leech. After thousands of generations, the eyespot would improve its competition as an organ by developing a concave cup. From the eyespot we got the camera-style eye with a cornea, lens, and extraocular muscles that make up the human eye. Compare this to dragonfly eyes. A dragonfly has 30,000 identical, repeated, individual units or ommatidia layered with the light-sensing cells and spatial information that make up the compound eye [8]. Its visual acuity relies on ommatidia size and number whereas our visual acuity relies on photosensitivity and retinal density [9].
Realizing how creationists have enjoyed arguing that the eye is too intricate to have evolved, this topic is worth revisiting at the right time in the future.
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Works cited:
[2] https://link.springer.com/chapter/10.1007/978-3-319-31903-2_1#Sec2
[3] https://andor.oxinst.com/learning/view/article/what-is-light
[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4763119/#bib10
[6] https://nexoptic.com/2019/03/28/the-electromagnetic-spectrum-explained/
[7] https://www.scienceworld.ca/stories/eyes-how/
[8] https://www.nature.com/articles/eye2017226
[9] Difference in Physics in the Eyes of Human and Insect (tuitionphysics.com)