Sometime around Earth year 1960, the life prolongation project annihilated the entire adult population of Earth Two, leaving only children, who were feral and had survived without adult care for over three hundred years until the USS Enterprise found them in 2266.
The children scrounged for food where they could. They knew the area where they lived extremely well, and were fond of games they called foolies. These games included taunting a landing party from the Enterprise. The children occasionally became more violent, especially when they felt threatened. The events of the plague had instilled in them a fear and distrust of grups; it took a great deal of effort for James T. Kirk to convince them that he and his people meant them no harm.
These children were infected with the life prolongation project, just as the adults had been. In the children, the complex functioned as intended, greatly increasing their lifespans. But when they reached puberty, the complex entered its fatal form, and death followed rapidly.
When the Enterprise left, the children remained on their world, in the care of a medical team. Additional specialists were dispatched from elsewhere in the Federation to care for them. If the Enterprise hadn't found them when it did, diminishing food supplies would have meant starvation for most of the children within a few months time. (TOS: "Miri")
The idea of the Onlies was thought up by "Miri" writer Adrian Spies. "When I went to see Gene I offered him the idea of a bunch of kids in this place where they are permanently young, but are really very old," recalled Spies. "If he was to tell you that he came up with the idea, I wouldn't say no, but it's not my memory. I do remember that he said, 'You have to develop a language for these people.' I said, 'What the hell are you talking about?' He said, 'The kids would talk differently.'" In the same conversation, Roddenberry contributed to the Onlies' language by suggesting the word "grups", which Spies appreciated. (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 31)
In the first draft script of "Miri", the Onlies were akin to the lost boys from Peter Pan. Also, their society was filled with ritualism; when it was discovered that Miri's childhood was nearly over, they individually proclaimed they could no longer regard her as their playmate. (The Star Trek Compendium, 4th ed., p. 42)
In the final draft script of "Miri", the group name "Onlies" was consistently formatted with a small initial letter. Also, whereas the final version of the episode does not use the singular term "Only" at all (consistently pluralizing it), it was used several times in dialogue. Miri first used the word when, in an attempt to explain how Onlies developed into grups, she stated, "When things happen to an only..." She later told Kirk that "the best thing about being an only" was that "nobody expects you to understand... you don't have to." In other ultimately omitted dialogue from the teleplay, Miri stated, "The Onlies mark down all the hots and colds at a secret place.... It's not much fun.... But it wasn't like that!... We just went on – being Onlies [....] We just had – foolies [....] If we were hungry, we just took something [....] But mostly – mostly it was fun, foolies.... After all, with no grups to say no, what else would Onlies do, anyway?" In addition to "hots" and "colds", another scripted but ultimately unused Onlies slang term was "mommies", meaning "can openers"; the Onlies were said to have lots of cans and "mommies".
When Miri's gang of Onlies first appeared in the final draft script, they were described thus; "There are several of them. All sizes and shapes, from toddlers up to about twelve. They have lived without restraint, taking things from empty shelves and empty houses as they need them. We see that in their outlandish clothes – whatever struck their fancy, some of the boys in tuxedos, some in military uniforms of a sort, some in snazzy sports clothes. The girls, the same way, but being girls, most are in some form of 'party' dress [....] None of these kids are evil [....] The older ones take care of the younger ones. A sort of paternal thing."
The Onlies were the focus of Judy Klass' novel The Cry of the Onlies, in which it was shown that the chief doctor who was sent to gradually bring the Onlies into adulthood and educate them was instead exploiting and abusing them. The Federation was testing a prototype spaceship on the planet, and Jahn stole it and began rampaging across the galaxy after Miri's death. Kirk and company straightened out the situation and arranged for Flint to become the Onlies' new mentor and guardian.
Another take on the Onlies was in issue #3 of Marvel Comics' Star Trek: Untold Voyages, titled "Past Imperfect", which showed Jahn marauding after Miri's death, but this time he kidnapped Dr. McCoy's daughter, Joanna, because McCoy's cure had mutated into an even worse disease.