Star Trek: Voyager has a specific answer to this question.

EDIT: To anyone unfamiliar with Star Trek, this episode of Voyager is “Distant Origin” (season 3 episode 23). In the episode, a species named Voth, encounter the remains of a Provisional Ensign Hogan who had died back in “Basics, Part II” (season 3 episode 1). Among a slim minority of the Voth science community is what is known as the Distant Origin Theory which suggest the species came from a far away place, which goes against The Doctrine, a narrow minded belief that Voth have always lived in the part of the galaxy they are currently located. During the episode it is determined that Voth and Humans share related DNA, which is later hypothesized to mean dinosaurs on Earth evolved to a point which they left the planet to live elsewhere.