Snakes evolved from lizards but have dramatically different eyes. These differences are cited widely as compelling
evidence that snakes had fossorial and nocturnal ancestors. Their eyes, however, also exhibit similarities to those of
aquatic vertebrates. We used a comparative analysis of ophthalmic data among vertebrate taxa to evaluate alternative hypotheses concerning the ecological origin of the distinctive features of the eyes of snakes. In parsimony and
phenetic analyses, eye and orbital characters retrieved groupings more consistent with ecological adaptation rather
than accepted phylogenetic relationships. Fossorial lizards and mammals cluster together, whereas snakes are
widely separated from these taxa and instead cluster with primitively aquatic vertebrates. This indicates that the
eyes of snakes most closely resemble those of aquatic vertebrates, and suggests that the early evolution of snakes
occurred in aquatic environments.