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The eye had to start somewhere. Of course, much depends on how one defines an eye. In the long course of evolution, sensory mechanisms began slowly, and many components probably were co-opted from functions distinct from those eventually used for any particular sensatory mechanism.
To be defined as an eye, a structure must be stimulated by light with the recognition of gathered spatial information. The sense of olfaction is triggered by a molecular stimulus, and may lead to a response. To be a brain, a collection of neurons must receive information in the form of electric potentials, integrate the information and contribute to a motor output.
Photoreception, in its most rudimentary form, probably began very early in life’s history. The sun provided at least one of the sources of the energy that permitted life to begin and flourish. Vitamin A, or retinal, is a molecule that was incorporated into the cell membrane of some single-celled animals first populating the hellacious earth 3.8 billion years ago. Incorporation of retinal, a proton pump and simple energy source, conferred a distinct advantage over those cells that did not have such an energy source. Did such concentrations of retinal in membranes of single cells constitute the first eye? Similarly, noxious chemicals would destroy a nascent cell membrane, and avoidance would be essential for preservation. Would the recognition and avoidance of these many noxious chemicals have come from the first “nose?” Neither negative nor even positive trophic responses toward light would seem to qualify for “real” vision as spatial resolution is at the core of any true visual system. For such spatial capabilities, there would need to be an organised system of collection and storage of sensory information. Such collection and storage probably provided the basis for the beginning of a brain. Since the potential …