Evolution Diary

PZ Myers wrote an excellent article on the evolution of the eye. Nice read.

 How different are animal eyes? In the metazoans, about a third of all phyla don’t have any eyes at all, although light sensitive molecules are ubiquitous (for example, in sea urchin tube feet), and can be found in bacteria and, obviously, plants.

Another third of the metazoan phyla have light sensitive organs, specialized epithelial patches that respond to changes in light levels. They may have some morphological specializations, but they don’t focus an image in any way, and can’t resolve patterns of light.

The rest have distinct, image-forming eyes that focus light on an array of light-sensitive cells, and can detect patterns of light and shadow that are used to perceive a picture of the world around them. For us motile animals, eyes seem to be part of the recipe for success—the six animal phyla (cnidaria, mollusca, annelida, onychophora, arthropoda, and chordata) that contain 96% of all animal species also primitively evolved eyes.

These animal eyes fall into two major categories, with further subdivisions. Chambered eyes have a single optical element—a slit, a lens, a mirror—and focus an image on a 2-dimensional array of photoreceptors, the retina. Compound eyes use multiple optical elements.

Click here to read the whole article