Spending time in nature, we often observe the animal in its entirety. We appreciate its structure, its behaviour, its patterns and uniqueness. What we sometimes miss and which sometimes deserves more appreciation, are the little pieces making these animals what they are.
Today I want to take the time to appreciate the eye. A fascinating organ and one of the most complex structures in nature.
It comes in an array of forms and at varying levels of complexity; from the more “basic” 3rd eye of certain reptiles, fish and amphibians, yes some reptiles have a very basic third eye, of which the sole purpose is to monitor changes in light levels to the highly complex eye of most mammals, all the way to what is probably one of the most interesting of all eyes on the planet; that of the mantis shrimp which sees such a broad spectrum of light as to be almost unbelievable, or maybe just unimaginable since we can’t comprehend what it is to see that way.
And like any topic, we could branch off in any number of directions but I’m going to try and stick to the theme of the above title; where did we (and I’ll stick to mammals here) get them? How did such a structure arise?
To start off with, as humans with an understanding of our own nature we barely consider that vision may not have been an evolutionary inevitability. There are 4 other senses, that we can experience, that could have taken place as our primary sensory modality. Why aren’t we and all animals that rely primarily on their sense of sight not walking around echolocating, sniffing blindly after one another, or responding to the heat signatures of our surroundings?
To answer this, we’ll take a giant stride back in time, roughly 3,7 billion years, and have a look at the first unicellular organisms muddling their way through the shallow oceans. Certain of these cells had evolved to harness the energy of the sun and more specifically, a very narrow band of the full spectrum of light; that of the ultraviolet through to the infrared wavelengths that make up the visual spectrum. However, that may fall along the line of that age-old philosophical question, “does a tree falling in the forest still make a sound should there be no one around to hear it?” Where the visible spectrum is only visible because we have evolved to sense it, visually. But these organisms needed to be able to detect light so as to maximise their exposure to it. That detection mechanism came in the form of an eyespot, basically photoreceptors in the cell connected to the flagellum or movement mechanism of that cell. The flagellum would essentially propel the cell at random until the eyespot would detect light, this would trigger the flagellum to power down, maintaining its orientation, allowing the cell to photosynthesise.
And it was with this development in our evolutionary history; that leap from reliance on thermal energy generated by vents under the ocean’s surface to reliance on the sun for energy that our slow crawl to visual acuity began. Light became the first thing any organism developed a mechanism for sensing and thus light (and later sight) would play a role of ever-increasing importance for life on earth.
And before we slip down the slope of this becoming a biology lecture, let’s talk timescales, impossibilities, improbabilities. Darwin is quoted as saying that the idea of the eye having evolved is “absurd in the highest possible degree.” But at this point the man himself had not quite grasped the timescales involved in his theory; a billion years is not something that we as humans with a lifespan of +-80 years on the planet can really wrap our heads around. It’s along the same vein as trying to imagine universal distances; it’s almost unfathomable. One billion years, a thousand thousand thousand years is twelve million five hundred thousand generations of our current life span. Scale that down and you get a rough idea of how the impossible merely becomes improbable and how we can even think of dropping the prefix off of the latter.
And so, through countless micro-changes in our genes and thus in the physical expression of those genes did we “climb mount improbable” to quote Richard Dawkins and so did the eye evolve from that single eyespot of the first photosynthetic cells, through the basic cup eye of various flatworms that allow them to flee from light to hide under vegetation, to the pinhole eye of the Nautilus swimming through the oceans allowing for what must be the most basic image formation in the animal kingdom, and finally through to those outlandishly complex structures that adorn the skulls of us and our closest relatives.
The eye allows us to see the infinite beauty of the world around us. It is the reason why you, our guests, love coming to Londolozi; to see and experience these last wild places. With its fathomless complexity has come a fathomless beauty and the eye of any animal will always be that aspect of the bush that cannot fail to captivate.