fan theory - What did the black monoliths do in 2001: A Space Odyssey?

What were the black monoliths? My initial thoughts were that they came from a more advanced species in order to assist man with advancement, shortly after it came to the primates, they discovered they could use bones as weapons.

Answer

In the book 2001: A Space Odyssey it was made explicitly clear that the monoliths were the emissary(s) of a benevolent alien species whose main aim was the advancement of less evolved species;

Night after night, the spectacle of those four plump man-apes was repeated, until it had become a source of fascinated exasperation, serving to increase Moon-Watcher's eternal, gnawing hunger. The evidence of his eyes could not have produced this effect; it needed psychological reinforcement. There were gaps in Moon-Watcher's life now that he would never remember, when the very atoms of his simple brain were being twisted into new patterns. If he survived, those patterns would become eternal, for his genes would pass them on to future generations.

It was a slow, tedious business, but the crystal monolith was patient. Neither it, nor its replicas scattered across half the globe, expected to succeed with all the scores of groups involved in the experiment. A hundred failures would not matter, when a single success could change the destiny of the world.

Latterly, the monolith on the moon served as a warning beacon that humanity had advanced to the point of having mastered space travel:

"Piecing things together after the event, we decided that the monolith was some kind of Sun-powered, or at least Sun-triggered, signaling device. The fact that it emitted its pulse immediately after sunrise, when it was exposed to daylight for the first time in three million years, could hardly be a coincidence.  

"Yet the thing had been deliberately buried - there's no doubt about that. An excavation thirty feet deep had been made, the block had been placed at the bottom of it, and the hole carefully filled.

The aliens then used the monolith (or rather a vast number of duplicate monoliths, described as "avatars") to collapse Jupiter; creating a new star for the night sky and concomitantly creating a number of new liveable habitats for humanity and the life developing on Europa.