Sun In Our Eyes

Long before there were eyes on our Earth to see it, there was light created in the midst of a star 93 million miles from us and 25,000 light years from the galactic black hole in the center of the Milky Way, the only galaxy that, more than likely, humanity, as we know ourselves now, will ever live in. 

For billions of years in billions of stars throughout the bulging universe, the constant coalescing force of gravity has been the catalyst for stars to form from cosmic dust and to be able to synthesize photons, the massless packets of electromagnetic energy emitted when tiny hydrogen atoms fuse together into slightly heavier helium atoms. Existing as both electromagnetic waves and particles of energy, the photons generated from this gravity-induced stellar fusion process inside the crucibles of immense heat and pressure in the cores of stars are what we see as light.

About 4.5 billion years ago our sun and the planets circling it formed from a dense molecular cloud made mostly of molecules of two hydrogen atoms-H2, carbon and other elements. Our Earth wound up being the third terrestrial planet from the sun in the new solar system and only 8.3 light minutes from what has become our own personal source of stellar photons.

For millions of years, on our young hot and rocky, auspiciously located planet, there was nothing alive to sense the photons of light emanating from the sun.  Early Earth consisted of only the lifeless elements found in the primordial dust that formed it and the various elements imbedded in the whizzing meteors and comets that sailed headlong into the planet before it had enough atmospheric gases to impede their progress. The teeming planet that now sustains us was lifeless regolith and water.

Future visualizers from dragon flies to whales had to wait until time and happenstance gave Earth the right combination of the fundamental elements of life such as carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur that had been blown out into space from the cores of exploding stars, to settle themselves into safe niches in early Earth’s hostile environment. After many years of mixing into numerous random combinations brought on by the churning of Earth’s surface from volcanos, and the pelting from numerous meteors and comets, plus electrical impulses from lightning, these elemental pre-cursers of life achieved an arrangement that began to form chains of molecules called amino acids, the building blocks of proteins. With the creation of amino acids, the inexorable march of evolution that led from inorganic chemical compounds to organic molecules had begun.

Over the next billion years or so, in the incubator-warm waters of early Earth, amino acids were able to evolve into molecular systems of polypeptides, proteins, ribonucleic acid, deoxyribonucleic acid, and on to primitive prokaryotes. As time passed in evolution’s unremitting procession, one protein combination led to another as countless primitive microorganisms evolved successfully or went extinct. These random mutations leading to a plethora of life forms set in motion the circumstances that eventually brought about cellular sensitivity to light.

Then around 3.5 billion years ago a branch of single-celled bacteria developed a type of light sensing photoreceptor protein. The bacteria are called cyanobacteria after their beautiful blue-green color and the photoreceptor protein it developed is cyanobacteriochrome. Along with cyanobacteriochrome, cyanobacteria also developed a green pigment called chlorophyll that absorbs sunlight and which led to the evolution of a process whereby light from the sun, carbon dioxide from Earth’s atmosphere, and water are turned into glucose which the cyanobacteria used as food. Thus, cyanobacteria became the first single-celled photoautotrophs, that is that through the process called photosynthesis, they could produce their own food. This development has turned out to be one of the most important biological processes on Earth. Cyanobacteria not only produced its own food, but also produced oxygen as a byproduct of this primitive photosynthesis. The production of oxygen, in time, would change the way aerobic life breathes and lives.

In photosynthesis, the carbon dioxide molecules are split into carbon atoms and oxygen atoms. The carbon and some of the oxygen is used, and the unused oxygen is expelled as waste.  In time, as cyanobacteria became more abundant, the oxygen given off by it spread throughout Earth’s atmosphere. Today oxygen makes up about 20% of our atmosphere and is absolutely essential to animal life. Humans, for example, breathe in about 2,000 gallons or 7,570 liters of air per day, which works out to be roughly 400 gallons or 1514.2 liters of oxygen. Without that oxygen we could not live.

As evolutionary as cyanobacteria was in its use of light with its profound photoautotrophic implications for future life on Earth, their thylakoids, the membranes where the light dependent reactions of photosynthesis take place, never developed into rudimentary eyes. Cyanobacteria could not see the beautiful world they were helping create. There were no eyes and no vision when cyanobacteria evolved.

Many years later, other autotrophs such as some species of trees and flowers developed light sensing proteins that respond to light and caused a response by elongating the cells of the stem on the side that is farthest away from the light, a phenomenon described by Charles Darwin and his son, Francis in their 1880 book The Power of Movement in Plants. This elongation on the shady side of the stem, causes the plant to bend in the opposite direction toward the sunny side. Sunflowers, for example, lean east in the morning and follow the sunlight toward the west during the day, then at night reposition themselves eastward again.

 However, both early plant-like autotrophs and animal-like heterotrophs remained blind and barely multicellular for many millions of Earth’s formative years. In time, however, heterotrophic animals would branch off in a different direction, and evolve into a creature that needed sight to survive.

Heterotrophs faced a problem in that they, unlike autotrophs, could not make their own food from combining sunlight, atmospheric elements, and the nutrients in soil. Most of them needed the means to hunt, to find food sources outside themselves, and sight would enable them to be better predators or to avoid becoming prey to their fellow food seekers. In spite of the dire importance of sight to early heterotrophs on our photon-saturated Earth, the complexity of the animal eye evolved very slowly.

It was not until some 700 million years ago, a relatively short time when considering the 4.5-billion- year age of the Earth, that there arose the right combination of amino acids and peptides in animals to create a protein that blinked when a photon happened to land on it. This sensitive protein, which we nomenclating humans have named an opsin, gave animals the ability to carry out a type of primitive phototransduction, a complex sequence of events involving the eye’s rods, cones, and retinal ganglion cells that convert light to electrical impulses that are sent to the brain.

Thus, with the ability to convert responses to photons into electrical nerve impulses that send signals into the brain where they are turned into intelligible images, animal sight had its humble electrochemical genesis.

Photoreceptors in animals were weak for thousands of years, little more than a few sensitive cells making up primitive ocelli or eye spots in a few invertebrates such as ancient jellyfish and mollusks that could only detect dim light from dark.

Life on Earth, however, has never been satisfied with the inelegant; the essence of evolution is complexity and adaptability. So it was that in time the progression of evolution with its propensity to perpetuate systems that work over those that do not, began to fashion a better response to light than mere fuzziness.

Gradually in heterotrophic animals with their chemical- electrical progression of specialized retinal cells called rods for seeing dim light and cones for seeing color, as well as the iris at the front of the eye that controls the size of the pupil and the amount of light getting into the eye, eyes became more sensitive and rounded and the ability to see became more refined.

In the next few million years, especially during the Cambrian period around 530 million years ago, life branched out in many directions creating a vast and wonderful ecosystem with millions of species interacting in what became a grand and elegant network of aquatic and terrestrial plants and animals. As heterotrophic hunting and foraging behavior evolved from species to species, improvements and variations in the eye-to-brain mechanism of vision enhanced the abilities of eyes to perceive light and movement. Mere eyespots enlarged and the photoreceptor cells developed greater sensitivity to the gentle touch of the massless photons that the sun sent their way. What early animals had seen as a shapeless shadow, their descendants were beginning to detect as predator, prey, or mate. By the time some amphibious animals moved onto land from water, about 500 million years ago, the eye was well developed for life in their new habitat.

As each species of animal adapted to the environment it found itself in, their eyes developed specific characteristics that enabled them to find food and avoid danger. Through the process of natural selection, propagating what worked and eliminating what did not, insects, crabs, reptiles, scallops, fish, birds, and mammals over time acquired the eyes that suited the life they lived. Whether nocturnal, diurnal, in water, the branches of trees, soaring high in the sky, standing tall, or scurrying along the ground, eyes showed animals the way to go.

Some animals, however, evolved to not need excellent vision because they use other means to perceive what is around them. Bats and dolphins, for example, have eyes and can see, but use echolocation to determine what is in front of them. They send out pulses of sound and then interpret the signals that are reflected back to them. Horses, cows, and other species of animals have their eyes set on the sides of their heads rather than straight ahead. This gives them great peripheral vision, but they not as good as seeing things directly in front of them as animals with binocular vision, that is, with both eyes in the front of their head. Like our primate ancestors, we inherited binocular vision which gives us a full 190-degree field of vision.

We humans spend time in the sun absorbing photons that titillate the photopsin and rhodopsin proteins in the retinas in the back of our round, lensed eyes. The sun emits a wide range of frequencies from radio waves to gamma rays, yet the human eye is only able to respond to a range of frequencies in the middle of the electromagnetic spectrum, those from red with wavelengths ranging from 740 nanometers to 625 nm, to violet with wavelengths 450 nm to 380 nm. Frequencies below or above this range are constantly around us but invisible to us.

Photoreceptors shaped like tiny rods respond to low light and help us make out shapes at night. Nocturnal animals such as cats and other animals that hunt at night have more rods than humans and have better night vision. Receptors that are more pointed at one end are called cones and are responsible for seeing colors. Although, like cats, humans have more rods than cones, we have enough cones to have better color vision than most other mammals.

Humans are trichromatic, meaning there are three types of cones in our retinas that sense a range of frequencies in the visible spectrum. Cones that pick up low frequencies, or the longest wavelengths, enable us to see colors in the red to yellow range, another set of cones respond to medium wavelengths and allow us to see colors in the yellow – green – to light blue spectrum. Cones that respond to the highest frequencies or shortest wavelengths see colors in the blue to violet range.

Studies show that it takes only about 5 photons hitting the human eye for the photoreceptors to respond and send a signal to the brain. But that would probably be a pretty fuzzy image. The more photons, the brighter the image. But too much stimulation, such as staring straight at the sun, can overload and damage the photoreceptors. Sunglasses on a sunny day help us find a good balance.

All of these responses to photons trigger electrical signals that are sent into the brain, first through the lateral geniculate nucleus in the thalamus, the brain’s ‘relay station’ and then on to the occipital lobe in the back of the brain where the stimuli from the three types of cones are blended and organized into colorful images that we see. The images are then sent to other parts of the brain such as the hippocampus where they are stored as memory. Thus, we are able to remember things we have seen such as people’s faces, or a beautiful sunset.

Humans have relatively good visual acuity compared to other mammals. We are generally better at seeing small details than dogs, cats, and horses, for example. The best visual acuity, however, is found in the 10,000 species of birds on Earth, especially birds of prey, such as eagles and hawks. With eyesight up to eight times more powerful than humans, they are able to see small animals from high in the sky. Birds also have the ability to see in the ultraviolet range, which means they can see colors beyond the frequency of violet where our vision stops. Our amazing eyes have come through the same processes of natural selection as other organs of our marvelously complex bodies. The result is that most of us are able to see the entire rainbow spectrum that Isaac Newton saw emanating from his sunlit prism from blood red to chlorophyll green to sky blue. Without our eyes we would never have met our old friend Roy G Biv with his red, orange, yellow, green, blue, indigo, and violet packets of photons that make our Earth such a pleasantly colorful place to live.