Life began approximately 3.75 billion years ago when the first replicative cell appeared as a product of a soapy, frothy membrane enclosing primitive cytoplasm. These early cells would quickly multiply, evolve, and acquire the ability to use light as an energy source. Cellular photoreception, and the first visual witness, emerged and would be an integral part of life thereafter. In time, this ability would be evolutionarily co-opted to become part of the sensory organ we call the eye.
Over unimaginable lengths of time and through intense competition, individual cells would evolve and become more complicated. These nucleated cells would merge with other cells in a collective mode to form multicellular animals. This revolutionary step occurred between 1000-600 million years ago and would fashion Animalia. These first metazoans arose approximately two billion years after life had begun and most already had, or would soon develop, eyes.
Metazoan life would expand over the ensuing 600 million years, diverging into many phyla with 96% of species being represented in Mollusca, Cnidaria, Porifera, Nematoda, Platyhelminthes, Annelida, Arthropoda, Echinodermata, and Chordata. Eyes would appear and evolve in different ways within these phyla, into at least 10-12 different forms and probably appeared individually at least 40 times. Each lineage would develop an eye that would suit its niche, and each of these eyes has a story to tell. A few examples illustrate the differences and importance of each.
Some jellyfish develop a camera-style eye shaped like our own with many similar characteristics. But, this eye is very different from, and completely unrelated to, a mammalian eye.
Then there is the question of why fish should have more visual pigments in their eyes and see more vibrant color than mammals? The reasons are not completely understood, but this ability produces better discrimination among the scintillating and flickering light of the coral reefs where fish likely began.
Some spiders have surprisingly good vision, at least at close range. They need such acuity if they are to be actively hunting instead of relying on passive web capture. To accomplish this, though, requires an engineering masterpiece to compact good optics and visual processing into such a tiny gem. To do this, jumping spiders have telescopic eyes and a scanning retina that produces an image much like the scanning optics of your television.
Over the hundreds of millions of years, spectacular eyes and visual abilities have emerged, and to see some of them, one needs only to watch birds flying overhead. Birds have the best vision on the planet and have adapted to almost all environments using this prowess. The magnificent visual skills of birds are on display anytime we see an Osprey catch a fish, or a swallow scoop up a mosquito; these behaviors are so commonplace that we almost don’t notice them, yet they represent the ultimate in visual perfection.
This is the story of how photoreception started and how eyes evolved. Along the way, the wondrous differences and unusual adaptations that have existed, and continue to evolve, will be explored.
Ivan Schwab takes you on a tour of how evolution produced such superb adaptations, and explains how these work by describing the ecology of pivotal animals.