It’s not the case, either, that advanced eyes always exist in advanced creatures and simple eyes always in simple ones. There are some microbes that consist entirely of single cells and which also double as surprisingly complex eyes. Consider the freshwater bacterium Synechocystis. Light that hits one side of its spherical cell becomes focused on the opposite side. The bacterium can sense where that light is coming from, and move in that direction. It is effectively a living lens, and its entire boundary is a retina. The warnowiids, a group of single-celled algae, also seem to be living eyes,
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