Firstborn

"Now they were Lords of the Galaxy, and could rove at will among the stars, or sink like a subtle mist through the very interstices of space. Though they were freed at last from the tyranny of matter, they had not wholly forgotten their origin, in the warm slime of a vanished sea. And their marvellous instruments still continued to function, watching over the experiments started so many ages ago."

- 3001: Final Odyssey prologue The Firstborn are an unnamed alien race indirectly appearing in the Space Odyssey franchise. Much about them is unknown, although it is known that they are an incredibly sophisticated noncorporeal species, and one of the oldest in the known galaxy. They are essentially the reason that humans evolved into a sentient race.

Homeworld
The homeworld of the Firstborn is almost completely unknown, although it is known that the planet lies approximately 450 light-years from Earth. It would seem that in its early history the planet may have been quite similar to Earth in its equivalent time, as the Firstborn are stated to have evolved from a 'warm slime' (primordial ooze). Over the course of millions of years, the Firstborn evolved into a spacefaring civilisation and spread away from this planet into the cosmos. It is unknown what the planet may look like today, or even back then.

Biology
Little is known of Firstborn biology, especially since they have become noncorporeal entities. It has been stated that in earlier times they were indeed beings of flesh and blood like humans, although their appearance at this evolutionary stage has not been described.

History
Millions of years ago, the Firstborn evolved from a 'warm slime' on a strange, faraway planet. Several million years later they had become a spacefaring civilisation and proceeded to expand out into and explore the galaxy. They were disappointed to discover that although life was common, intelligent life rarely successfully evolved. Using their incredible technology, the Firstborn catalysed the evolution of intelligent life on many planets, including Earth. Once a potentially intelligent species was discovered, they placed a strange artifact on the planet in question that would give a slight evolutionary 'nudge' to the species, leading it towards sapience. Four million years ago, the Firstborn discovered Earth, and upon it, the ancestors of the human race. After exploring the planet, the Firstborn left on Earth one of these artefacts in the Great Rift Valley of Africa, and departed. One of these 'monoliths' was also left on Earth's moon, Luna, in order to contact the Firstborn once humans had become a spacefaring civilisation. The Firstborn continued to evolve, and eventually they found a way to integrate themselves into the fabric of space itself, and become the noncorporeal entities they are today.

Trivia

 * According to The Lost Worlds of 2001, Arthur C. Clarke considered what the creators of the Monolith to look like: "Apart from their unusual height - more than seven feet - the creatures that looked down upon the world of Pliocene were strikingly human (...) With a little plastic surgery, Clindar could have passed for a man. He was hairless...".