In a previous post, I discuss trilobites, the most common type of arthropod fossil on the planet Earth. Trilobites were marine arthropods that lived in the oceans in a period that lasted from over 500 million years ago until the end of the Permian extinction about 250 million years ago. The trilobites were armored predators. The armor, which incorporates calcium, does not easily degrade even after the death of the trilobite and preserves the exoskeleton of trilobites in the fossil record.
The “predatory lifestyle” and the evolution of vision has placed trilobites in the center of a theory, “The Light Switch Theory” to explain the rapid evolution of external body forms that occurred in multiple groups of organisms during the “Cambrian Explosion” between 525 and 515 million years ago. During this period of life on earth, all major groups of animals (phyla) evolved marked changes in external form. The environmental conditions that promoted large and various changes across many phyla has been a topic of speculation for evolutionary biologists.
Andrew Parker has promoted his “Light Switch Theory” described in his book, “In The Blink Of An Eye: How Vision Sparked The Big Bang Of Evolution” and updates in a more recent article, “On the Origin of Optics”, in the March 2011 issue of Optics and Laser Technology. The Light Switch Theory puts the development of vision and compound eyes by predatory trilobites as the key event that led to rapid changes in external structures.
According to the Light Switch Theory, prior to 521 million years ago animals lacked vision, the kind that can form images as we experience vision today. Animals could detect relative light and dark but could not visualize the world around them. Predators were few and less efficient, relying on sound and odor to detect prey. The evolution of a true eye that could visualize images would be a huge advantage to a predator. Once a rudimentary image vision evolved, the selection for adaptations that improved vision would be huge. Those predators best able to detect prey would grow faster and could potentially produce more offspring.
The development of a predator with vision would have sent the prey scurrying. Adaptations for defense against the new visual power would have been huge. The prey could have a variety of adaptions, precisely the types of adaptations that “exploded” during this period. Some prey evolved armor, physical defenses that limited opportunity for attack. Some evolved behavioral defenses, hiding in burrows or otherwise avoiding detection. Cryptic coloration, blending with the environment, evolved. Some species in other phyla eventually evolved their own visual detection system, to alert them to the dangers. Others strengthened their chemical defenses. Against a predator that could see, warning coloration developed as a strategy to deter predation.
The Light Switch Theory makes a number of predictions that can be tested by examining the fossil record. Evidence for the evolution of vision in trilobites and other animals are being explored. The evolution of structural colors can now be examined with new information on how the structural colors are produced and techniques for investigating them as I discussed in a previous post. The light switch theory is valuable as a new way to think about evolution during the Cambrian Explosion and organize research questions and new information.