The Firstborn

The Firstborn are the monolith aliens who have left these 1:4:9 black crystalline slabs throughout many star systems. They are described in Arthur C. Clarke’s 3001: The Final Odyssey.

3001: The Final Odyssey
The Firstborn possess the power of deep space travel. Millions of years ago, they too felt the awe, and the wonder, and the loneliness of outerspace. Though not remotely human, they were flesh and blood, and they sought for fellowship among the stars.

Earth pre-dawn
The Firstborn travelled the stars for a thousand years before finding the Solar System. When they arrived to Earth, the great dinosaurs had long since passed away. Their explorers saw a world swarming with life, and for years they studied, collected, catalogued. Afterwards, they began to modify and tinker with the destiny of many species, on land and in the seas. Once satisified, they set out once more into the abyss, knowing that they would never come this way again. There wasn’t any need: as “the servants they had left behind would do the rest” — thus the monoliths.

Lords of the Galaxy
Once machine-entities, they had learned to store knowledge in the structure of space itself, and to preserve their thoughts for eternity in frozen lattices of light. Subsequently, into pure energy, they transformed themselves; and on a thousand worlds, the empty shells (their bodies) they had discarded—having twitched for a while in a mindless dance of death, then crumbled into dust.

Roving, at will, among the stars, through the very interstices of space, they became Lords of the Galaxy. Even though freed from “the tyranny of matter”, they had not wholly forgotten their origin—in the warm slime of a vanished sea. Their marvellous instruments (the monoliths) still continued to function, watching over the experiments they started many ages ago.

They had not always been obedient to the mandates of their creators; like all material things, they were not immune to the “corruption of Time” and its patient, “unsleeping servant, Entropy”. And sometimes—they sought goals of their own.

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...".