Aliens Timeline and Summary

Millions of years ago, the aliens attain space flight.
They go around the galaxy looking for species who might develop intelligence, and giving them a shove or two up the evolutionary ladder.
They come to earth and nudge the man-apes. They also leave a sentinel on the moon, and a Star Gate on Japetus out by Saturn (they create Saturn's rings too, while they're passing through).
Te aliens go away, and learn to put their brains first into ships, then into lattices of light, allowing them to become immortal.
When David Bowman pops through the Star Gate, the aliens turn him into a super Space Baby.
Why? Who knows? They're aliens.