Filmation, to an extent, broke new ground with their adaptation of Isaac Asimov's Fantastic Voyage into a Saturday morning adventure series in 1968. Up until then, "origin" stories, which explained how the heroes came to be, weren't done. We wouldn't learn the secrets behind Space Ghost, for example, until a comic book miniseries published by DC nearly 40 years later. Other heroes of the era, such as Birdman & The Super Six, just appeared, no questions asked, though today would require a lengthy tale to reintroduce the characters. Hanna-Barbera's Shazzan & Frankenstein, Jr. were easily explained, as was Dino Boy.
The Combined Miniature Defense Force (CMDF) is introduced in the open of each episode of Fantastic Voyage, but the series premiere, "The Gathering of the Team", tells the story of how Jonathan Kidd, Erica Lane, & Guru joined with scientist Busby Birdwell to form the CMDF's field team. Marvin Miller (Aquaman) voices both Guru & Busby, plus some additional characters. "Gathering" first aired on Sept. 14, 1968.
Here is "The Gathering of the Team":
Writer Ken Sobol would script virtually every episode of the series, and remained with Filmation for a few more years, even doing some yeoman work on the Archie franchise.