Yeah, that whole "Life here began out there" meme was quite common in SF of the '50s, '60s,and '70s, not to mention UFO/ancient-astronaut lore of the same era, which is where Glen Larson cribbed it from in the first place. It arguably has roots in ancient mythologies positing that humans were the descendants of earlier, superior races, like the Ancient Greek belief in the Seven Ages of Man. There's probably some basis in reality for those myths as well, since there have been various times in history when fairly advanced permanent settlements, cities, and civilizations have gone into decline due to shifting climates, disease, or war and their peoples have migrated elsewhere and resumed simpler lives, their descendants retaining lore of more advanced forebears. So, like just about everything you can find in fiction, it's an idea that has roots going back throughout the history of human experience.
The "surviving nuclear holocaust" theme was also common in fiction during the Cold War, for reasons that should be self-evident. And it's natural enough to combine that with the other trope. Heck, there was a Twilight Zone
episode in the early '60s in which two astronauts who'd fled a destroyed world and settled a new one turned out be named Adam and Eve -- and that story was already a cliche even then, as stj