The striking similarity between humans and the show’s most famous aliens, including Romulans, Vulcans, and Andorians, has been a point of contention with many fans over the years. In Star Trek: The Next Generation, the franchise writers attempted to explain this convenient lack of biological diversity using a real scientific theory called panspermia.

Proponents of this theory argue that life on Earth may have been seeded by hardy microbes—or at least, by raw ingredients such as amino acids—that traveled here via comets or asteroids. While there’s no direct evidence to back up this claim, missions such as the Rosetta orbiter have found the building blocks of life on comets, and we know tough organisms such as tardigrades can survive unprotected in space.

Scientists also believe early Earth was bombarded by meteors, which could have delivered a “starter kit” for life from elsewhere in the galaxy.

Building on the panspermia concept, Star Trek suggested that an ancient humanoid life-form intentionally seeded worlds around the Milky Way, creating new species that assumed their basic shape. While each of these species may have split off on its own evolutionary path, they are all essentially long-lost relatives. (Find out how Star Trek is right about almost everything.)