Arretan

The Arretans are a phenomenally advanced species from the planet Arret, who explored the galaxy 600,000 years ago and colonized numerous planets. Faced with a cataclysm that ripped away the atmosphere of their world, the Arretans survived by leaving their bodies behind and transferring their minds to spherical containers, deep below the surface of their planet. In 2268, only three members of the species remained: Sargon, Thalassa and Henoch.

Biology
The Arretans were originally humanoids. Sargon has claimed that while their original bodies were similar to Humans, their minds were "infinitely greater". They're indeed capable of extraordinary psychic feats, which include reading and influencing minds, psychically torturing people, and controlling alien technology. Most notably, they can transfer their minds to the bodies of other creatures or to especially-created receptacles. At least one member of the species, Sargon, became skilled enough to transfer his consciousness to the USS Enterprise ' s computer: a feat beyond the capacity of his fellow Arretans.

Upon encountering the USS Enterprise crew in 2268, Sargon claimed that they might be descendants of the Arretans, although the records have been lost at the downfall of their civilization, so there's no way to know exactly which planets have been settled by them. While Ann Mulhall pointed out that evidence indicates that life on Earth evolved independently, the same isn't true for Vulcan, and Spock mused that Sargon's idea would "explain certain elements of Vulcan prehistory".

The idea that Vulcans are likely to descend from Arretan ancestors would explain how both species have telepathic abilities, and even the ability to transfer their minds or katra into another body or receptacle. By extension, it means that the Romulans too are descendants from the Arretans, and possibly the Mintakans as well. Going the other way, it means the Arretans themselves evolved from the DNA molecules seeded by the Ancient Humanoids.

When the mind of an Arretan is transferred to the body of another species, the host's metabolism is accelerated to dangerous levels: heart action is doubled and temperature increases to as much as 104 °F (~ 40 °C), which can quickly exhaust the body.

Culture and society
The planet Arret is an Earth-like world, described as much older than Earth. Arretans seem to have been a monogamous species, as Sargon and Thalassa are husband and wife. Eventually, the collapse of their civilization came because they became too powerful and started seeing themselves as gods. Sargon himself described the conflict to the Enterprise crew as "a struggle for such goals and the unleashing of such power that you could not comprehend".

History
Six hundred thousand years ago, the Arretans traveled in spaceships, exploring the galaxy, and colonized numerous planets. However, about half a million years ago, their planet experienced a major cataclysm and became uninhabitable, its atmosphere completely ripped away. The Arretans survived by transferring their minds into spherical receptacles. Despite the durability of the materials, these couldn't support them forever. By 2268, only three Arretan minds remained alive. They asked three members of the Enterprise crew (Captain Kirk, Spock and Dr. Ann Mulhall) to allow the Arretans to inhabit their bodies temporarily for the time it would take them to create suitable android bodies that would restore their mobility.

Appearances

 * Star Trek, s02e22, "Return to Tomorrow" (1968)