"Eyes" in the broadest sense of the word are not restricted to animals. A good place to discover this is a barnyard occupied by cows, chickens, or pigs or in roadside ponds and puddles. During the spring and summer look for water covered with a greenish scum. Then dip up a fine scum collection with a small jar or turkey baster. Using a compound microscope, examine a drop of the greenish water.

What you will find are euglenids — small single-celled organisms with several interesting features: a flexible cell covering; green chloroplasts inside the cell that harvest light to produce the euglenid’s food; a whip-like flagellum that propels the cell around, and a bright red structure called an "eyespot".

The euglenid eyespot is part of a fascinating system. The red eyespot is actually a pigment-filled shield that lies next to the base of the flagellum. Although the exact mechanism is not known, the eyespot appears to work in concert with the flagellum, allowing the euglenid to move in response to light, and to find optimal light conditions for photosynthesis. In other words, the eyespot (and parts of the flagellum) are a very primitive type of eye that evolved in ancient single-cell organisms — the ancestors of living euglenids. Do these primitive eyes have any relation to our eyes and the eyes of other animals? It’s a fascinating question; one that may be answered in part by studying the genes responsible for euglenid eyespots and for animal eyes. We also need to know more about the relation of our known single-celled ancestors, choanoflagellates, with the eyespot-equipped euglenids.

Answers to questions about the most primitive animal eyes, are being explored through studies of genes that control development. See our essay on this topic (coming soon).