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Animal Eyes and the Darwinian Theory of the Evolution of the Human

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Animal Eyes and the Darwinian Theory of the Evolution of the Human Animal Eyes We can learn a lot from the wonder of, and the wonder in, animal eyes. Aldo Leopold a pioneer in the conservation movement did. He wrote in Thinking like a Mountain, “We reached the old wolf in time to watch a fierce green fire dying in her eyes. I realized then, and have known ever since, that there was something new to me in those eyes – something known only to her and to the mountain. I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters’ paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view.” For Aldo Leopold, the green fire in the wolf’s eyes symbolized a new way of seeing our place in the world, and with his new insight, he provided a new ethical perspective for the environmental movement. http://vimeo.com/8669977 Light contains information about the environment, and animals without eyes can make use of the information provided by environmental light without forming an image. Euglena, a single-celled organism that did not fit nicely into Carl Linnaeus’ two kingdom system of classification, quite clearly responds to light. Its plant-like nature responds to light by photosynthesizing and its animal- like nature responds to light by moving to and staying in the light. Light causes an increase in the swimming speed, a response known as 165 photokinesis. Light also causes another response in Euglena, known as an accumulation response (phototaxis). The light sensitive Euglena cells sense the direction of light, swim quickly towards the light, and stay in the light. The Euglena cell can be described as a light meter, not unlike the one that we used to test the inverse square law. Jerome Wolken (1995) describes Euglena as a photo- neurosensory cell. Demonstration: See how the Euglena cells/organisms accumulate in the light. Light-induced movements in Euglena have been studied by Jerome Wolken, the father of Jonathan Wolken, a founding member of the Pilobolus Dance Company, who performs Shadowland. The dance company was named after Pilobolus, a light-sensitive fungus growing in Wolken’s lab that shoots its spores towards the light. Here is a drawing of Pilobolus on the night of November 3, 1911, showing its ability to sense and grow towards the direction of light. 166 The single-celled sporangiophore of Phycomyces, a fungus similar to Pilobolus acts like a light meter/converging lens that, when in air, focuses the light on the back side (A). When Phycomyces is grown in a medium that has the same refractive index as the cell, the light is not focused (C) and the sporangiophore does not bend (Castle, 1933)! Earthworms are also capable of sensing light without eyes as reported by Charles Darwin (1881) in his book, The Formation of Vegetable Mould, through the Action of Worms, with Observations of their Habits. Earthworms have light- sensitive photoreceptor cells in and under the skin throughout their body although they are concentrated in the anterior portion (Hess, 1925). The earthworms are photophobic and move away from the light and towards the dark. At dawn, they crawl into their dark holes and stay there until dusk. I wonder if the birds know their schedule. 167 Some animals have light meter-like “eyes” or ocelli, where photoreceptive cells that face the surface are arranged in pits or cup-like patches. Pit or cup-like eyes are found in the mollusc, Patella vulgata, which is a limpet that tenaciously attaches to rocks in the intertidal zone shown here on the Hugh Miller Trail near the Village of Cromarty Scotland. In the living fossil, Pleurotomaria, the small and inconspicuous eyes of this mollusc are more sunken. The photoreceptors face into the cavity that forms the optic cup and the ganglia are behind them. The cavity is open and sea water fills the upper portion of the cavity and a vitreous material fills the bottom of the optic cup. The geometry almost approaches that of a pinhole camera; however, with such a wide aperture relative to the image distance, any object would probably appear as a blob. 168 The geometry of the eye of the iridescent mollusc, Haliotis or abalone is more like a pinhole camera. The smaller aperture and more spherical retina would result in a relatively sharp image and relatively good visual acuity. The tradeoff of the smaller aperture is that the image will be dimmer. The geometry of the eye of the cephalopod mollusc, Nautilus, another living fossil, is even more like a pinhole camera. The smaller aperture and more spherical retina would result in a relatively sharp image and relatively good visual acuity. The tradeoff of the smaller aperture is that the image will be dimmer. 169 The pinhole camera eye of Nautilus, trades off dimness for visual acuity. Dimness can be overcome by the addition of a converging lens to make a camera- like eye, just as the camera obscura was improved by adding a converging lens. The gelatinous lenses of Murex and Helix that I will describe below do not have the refracting or dioptric power to produce a focused image on the retina. Murex is a mollusc that lives in the intertidal zone. We will talk about Murex later in the semester when I talk about the dyes Tyrian (royal) purple and Tekhelet. Murex has a camera-like eye where the optic cup-like retina is filled with a gelatinous lens that captures a lot of light compared with a pinhole but it probably underfocuses and is not very effective in image formation. Murex does have a cornea that interfaces with salt water (n= 1.33-1.34) at times and air (n = 1) at other times, but because the cornea is so flat is does not have much refractive or dioptric power and does not participate much in image formation. 170 Other molluscs, such as land snail, Helix, have an eye like that of Murex that captures a lot of light but is also probably not very effective in in image formation. Going back to aquatic animals, some predatory animals, such as squid and octopus that live in the sea where light may be limiting have eyes with the ability to both capture light and to produce a focused image on the retina. A lens that has the ability of accommodate mitigates the tradeoff between brightness and visual acuity, although it does introduce aberrations. The eye of a squid or an octopus is similar to the human eye in that it has a cornea and a crystalline lens to refract light, an iris that surrounds a pupil that opens in dim light, a retina and an optic nerve. The visual system is also similar 171 in that a large part of the brain is involved in visual processing. The octopus and squid eyes differ in that the human eye accommodates by changing the shape of the crystalline lens, becoming more convex when focusing nearby objects whereas the octopus eye, like a camera, accommodates by moving the crystalline lens farther from the retina when focusing nearby objects. There is also a difference in the organization of the retina in the human and octopus eye. In the human retina, except at the fovea, the photoreceptor cells are at the far side of the incoming light and the neural cells are on the near side. This by necessity results in a blind spot. In the octopus, the photoreceptor cells are at the near side of the incoming light and the neural cells are on the far side. Consequently, there is no blind spot. The giant or colossal squid is about 4.2 meters long and lives 1000 meters beneath the sea. The large soccer ball-sized eye, orange-sized lens, and its 8-9 cm in diameter pupil, results in a large light gathering capacity that helps the giant squid see in deep waters. Humans can only see in waters 500-600 meters down. The giant squid eye also has photophores that contain bioluminescent bacteria 172 help the giant squid to see in deep waters. http://squid.tepapa.govt.nz/anatomy/article/the-eye-of-the-colossal-squid The levels of complexity of the molluscan eye from a light-meter eye, through a pinhole camera eye to a camera eye are summarized in the figure below. Potential evolutionary relationships between these organisms are given below: 173 Let’s look at the eyes of chordates, including hagfish, lampreys and other vertebrates, the ancestors of which existed at the time of the Cambrian explosion about 543 million years ago. Chordates are bilaterally symmetrical and possess a notochord, which is a support structure that develops into the backbone in vertebrates. I am considering time to be real and unidirectional. This is not a common belief nowadays. To quote an anonymous reviewer of one of my recent papers (2/7/15): “I’m worried that the author is addressing a non-problem. Even if time’s arrow is not real, there still may be local time asymmetries – in the same way as organisms constitute local violations of the law of increasing entropy.” In In the Blink of an Eye: How Vision Sparked the Big Bang of Evolution and Seven Deadly Colours: The Genius of Nature’s Palette and How it Eluded Darwin, Andrew Parker suggests that the rapid diversification of animals that took place in the Cambrian was a result to the evolution of eyes which led to intense predation and the ability to escape it.
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