An ancient and mysterious people believed to have moved endangered humanoid populations to other worlds, explaining similarities among many planets' inhabitants.
The Preservers are an ancient and mysterious species known only through the traces they left behind. They are believed to have travelled the galaxy in the distant past rescuing threatened humanoid populations and resettling them on other planets, where they could continue to live. This would help explain why so many worlds visited by Starfleet resemble cultures from Earth.
In the episode The Paradise Syndrome, the Enterprise discovers a planet inhabited by the descendants of Native American peoples taken from Earth long ago. An obelisk left by the Preservers is a device designed to deflect asteroids away from the planet. James T. Kirk loses his memory and lives among the people, marrying Miramanee, while Spock works to decipher the markings on the obelisk. Spock eventually works out how to activate it in time to save the planet from an approaching asteroid.
The Preservers were one of the original series' answers to why the galaxy is full of near-human species, a question The Next Generation would later revisit with the Progenitors.