A shared trait common to all light-sensitive organs are opsins. Opsins belong to a family of photo-sensitive proteins and fall into nine groups, which already existed in the urbilaterian, the last common ancestor of all bilaterally symmetrical animals. Additionally, the genetic toolkit for positioning eyes is shared by all animals: The PAX6 gene controls where eyes develop in animals ranging from octopuses to mice and fruit flies. Such high-level genes are, by implication, much older than many of the structures that they control today; they must originally have served a different purpose, before they were co-opted for eye development.