2001 A Space Odyssey raised a similar question.
In the final book of the “Odyssey” series (3001) the aliens were highly advanced, no longer needing physical form. The monoliths were their only physical contact with normal space. They were “elder glactic inhabitants” which cataloged, studied, enhanced and judged which species were “acceptable” for further development.
From Wikipedia:
http://en.wikipedia.org/wiki/3001:_The_Final_Odyssey
The book begins with a brief prologue describing the aliens who created the Monoliths. They apparently evolved from “warm slime”, and over the course of millions of years, turned into a space-faring species. As they explored the galaxy, they saw that few intelligent species ever successfully evolved. Therefore, they travelled the universe and catalyzed the evolution of intelligent species wherever they went, including Earth, by increasing the evolving species’ odds of survival. Upon reaching Earth, they performed experiments on many species to encourage the development of intelligence. Then they left, leaving the monoliths behind. After visiting Earth, the aliens continued to evolve, to the point where they eventually found a way to integrate themselves into the fabric of space, and became non-corporeal beings. Meanwhile, back in the Solar system, the Monoliths continued to watch over humanity. However, the Monoliths were capable of degenerating and acting independently of their original programming.