The Ba'ku civilization was founded by refugees from a civilization that had developed technologically-advanced weapons of mass destruction in the 21st century. A small group broke away from that civilization and settled on their new planet where the metaphasic radiation of the planet's rings kept them eternally youthful. It was on that planet that they rejected the technological lifestyle of their forefathers and chose to live a simple life.
Some younger members of the society, however, chose to reject the simple lifestyle and wanted to explore the stars. They were rebuffed by the other Ba'ku and exiled from the planet. These exiles came to be known as the Son'a. In 2375, the Son'a, led by Adhar Ru'afo, and the Federation began a joint mission to covertly relocate the Ba'ku people to enable the Son'a to harvest the metaphasic radiation surrounding the planet. They were foiled, however, by the crew of the USS Enterprise-E, and the Son'a were exposed as former Ba'ku seeking revenge. When their plans fell apart, several members of the Son'a chose to return to the Ba'ku. (Star Trek: Insurrection)