I recently came across the transcript of the conference where George Lucas, Steven Speilberg, and Lawrence Kasdan sat down and hashed out the details of a little flick called Raiders of the Lost Ark. It's an amazing, 120 single-spaced pages of instructions on how to work out a story. But interestingly, right in the middle there's a reference to Erich Von Daniken.
Von Daniken wrote a book called Chariots of the Gods, published first in 1968. His basic premise is there's a single answer to all of the mysteries from humanity's prehistory. Evidently everything we can't explain--how the Egyptians built the pyramids, why the Mayans abandoned their civilization, what early artwork is all about, what's the truth in the stories of the gods...--can be solved: Aliens.
Aliens probably seeded earth with humanity, and returned at various intervals to help along the processes of evolution. Our destiny is in the stars, and getting there will involve mining the past for its wisdom. (Much debunking has occurred; see this article for particularly funny stuff)
(Ok, the relevance to Raiders comes in that Von Daniken thought that the Ark was built as a radio transimitter to god--the aliens--a point made by Belloq in the scene where he tells Indy to "sit down before you fall down.")
Also published in 1968: 2001 A Space Odyssey, by Arthur C. Clarke (the Kubrick film came out the same year). It's a story about mankind's evolution being guided by aliens.
Twelve years later, astrophysicist David Brin published his first book, Sundiver. It's a story set in the future, when humanity has successfully guided the evolution of dolphins and chimpanzees to the point of sentience. They've also been contacted by a number of alien species. Basic to the book (and its sequels) is the idea that no species has attained sentience without the aid of extraterrestrial intelliences. I haven't gotten through the whole of the Uplift series yet, but it's hinted that there is a "mythical" alien species, referred to only as The Progentiors, that got the whole thing started billions of years ago. One of the mysteries of the series so far (I'm only on the second book, Startide Rising) is who, if anyone, "uplifed" humanity.
I'm sure there are a number of other stories that play with this idea: that humanity has been tampered with in its evolutionary path by aliens. I'll seek out as many as I can. As far as I know, Von Daniken is the only one who has taken the idea seriously. His book is a riot, full of "evidence" in the form of pictures of ancient artwork. HIs basic logical argument goes as follows: "Look at this picture! Doesn't it look like an alien!" Or, "We have no idea how the Egyptians got the idea of life after death, so it must have come from aliens."
Oh, right, what's this got to do with Superman?
Well, it seems like Superman is an alien who comes to earth. Now, he doesn't seem to deliberately mess with evolution...but on the other hand, he's always trying to get Lois Lane to go out with him (as Clark Kent). And dating is all about mating.
I'm not an expert in the history of Superman comics, so I don't know if this issue has been broached before. But I do know that in Grant Morrison/Frank Quitely/Jamie Grant's All Star Superman, Superman does eventually record his genetic code and give it to Leo Quintum, super scientist. The purpose of this is ostensibly, or perhaps putatively, so that Quintum can merge Superman's genes with human genes and create a hybrid race so the world will be ok without him. A World without Superman is a recurring trope in Superman comics.
It might just have been me, but I got the impression that Superman wanted Quintum to impregnate Lois Lane with a superbaby (one, presumably, that because it was created in a lab wouldn't give Lois the problems that Larry Niven spelled out in Man of Steel, Woman of Kleenex).