Homologies can be studied at many different levels. With respect to eye evolution, we’ve seen two extremes: many different organisms share homologous eye control genes (see tree A below), but far fewer animal lineages have structurally homologous complex eyes (see tree B below).

Whether they are searching for food, choosing a mate, or trying to avoid a predator, many animals depend on their vision for survival and reproduction. However, as you have learned in this module, different animals really do have different ways of seeing things. Complex eyes have evolved many times in different lineages, and each of those origins of complex eyes has resulted in a slightly different construction of the eye. You’ve also discovered that such analogous eyes may, in fact, be built by homologous genes.

This reflects a central idea in evolutionary theory — the diversity and unity of life. Each organism has a unique evolutionary history that sets it apart from other lineages — but it also shares a large portion of that history with other organisms. Your own eye is a case in point. It represents both the evolutionary history that makes our lineage unique — a history in which terrestrial vision is important for surviving both physically and socially. But it also represents the evolutionary history that we share with other animals — one in which the ability to sense the basic difference between light and dark has been critical.