Because of the success DC comics had with Alex Ross' Kingdom Come series, Marvel commissioned Ross to do the same for their universe, knowing that anything with Alex Ross' name on it would sell by the truckload. Earth X is Ross' vision of an apocalyptic future for the Marvel super heroes where everyone has some kind of mutant, super power.
Aaron Stack, aka X-51, wakes in the middle of the night in his apartment. Since he is a robot he doesn't need to sleep but he pretends to anyway to act more like his dead father (creator) by getting up in the middle of the night, opening and closing the bathroom medicine cabinet, and flushing the toilet before returning to bed, all in the name of being more human. A large black doorway appears in its bedroom and X-51 starts talking to it. Through their interaction X-51 reveals that the door has asked him if he would like to know the everything there is to know of mankind, including its destiny. On touching it he gets pulled in and taken to a dark, empty place.
Finding, itself being removed of its human disguised skin, revealing the machine underneath, X-51 makes his way across the dark, bleak landscape to find out, as the sun rises, that it was on the moon. Walking along what appears to be ruins of some civilization, we see an astronaut watching in the background with the name 'Jameson' written on its uniform.
Inside an alien house that it came upon, X-51 questions his destiny, asking if coming here to the moon was his own destiny, seeing as how no one else could just be brought to the moon and survive the experience. Inside the building he meets with Uatu, the Watcher, who tells X-51 that they are here to catalogue all of humanity. Uatu then goes on to tell X-51 of the history on the world.
Uatu tells X-51 that the Celestials came to Earth and carved out the moon from the planet, which is what caused the death of the dinosaurs. They needed the moon as an eye in the sky for Uatu to watch down from. He goes on to tell him of man's first glimpse at the Celestials, who humanity came to fear as Gods.
The Celestials molded the humans as they saw fit to but their first attempt was a failure, resulting in the Deviants, which were too aggressive, but they succeeded with their second try, creating the Eternals. These Eternals settled above and became the inspiration for myths and religions. The Deviants settled below and enslaved anyone foolish enough to place their faith with them.
The Celestials weren't the only outside influence on humanity. The Kree came to Earth to try and create an army of soldiers to help them fight their longtime nemesis, the Skrulls. Using the alien Terrigen Mists they triggered the seeds in humans that the Celestials themselves placed. These 'super soldiers' eventually rebelled against their creators and settled in the ancient city of Attilan.
Somewhere along the line the Deviants enslaved humanity but since the Celestials couldn't have that, they came back and crushed the Deviants' city, causing a shift in the tectonic plates that would then in turn cause the destruction of an entire continent destroying the city of Atlantis in the process. Uatu and X-51 then go into the definitions of good vs evil. X-51 sees evil in almost everything the Celestials do, whereas Uatu isn't concerned with what is good and what is evil, only with what is happening at the moment.
The Celestials came back a third time, as gods, to see how things were going here on earth. They came to an agreement concerning the humans, that they would no longer interfere with humanity, not knowing that it was because of them that the mutant seed was already planted within humanity.
Uatu and X-51 go on to discuss good and evil once more. Uatu says that there is no good and there is no evil, there is only man's struggle to survive and through that struggle, heroes are born. Uatu continues his story telling of the rise of the Marvel Age of heroes, how they were given life as a result of wars and hardships, and just like the heroes there arose evil men who wished to rule instead of serve. He goes on to reveal that everything that gave the great heroes like Spider-Man and Captain America their powers, only released what was already inside them, the seed that the Celestials implanted within man since from the very beginning. Soon there didn't need to be any reason to awake the buried seed, it would reveal itself on its own and these people came to be the mutants.
For what is about to happen on Earth, there must be someone to watch and catalogue it all. That is why Uatu brought X-51 there to his home on the moon. There needs to be a watcher and since Uatu has been blinded by an unrevealed attack, X-51 is asked to take his place.
X-51 wakes to find a door in his home which takes him to the surface of the moon. Walking around he discovers an alien building of some sort and enters to find Uatu, The Walker, who tells X-51 the history of mankind.
The Celestials actually started the evolution of man. First they came to Earth carving out the moon from the Earth, killing all the current life on the planet, which was during the age of the dinosaurs. Then they took people and reshaped them to their liking. The first attempt yielded the beings called Deviants, who were too aggressive, then came the Eternals. These Eternals became the first Gods.
Somewhere along the lines the celestial seed was implanted in humans, only to remain dormant. It would stay dormant until something sparked it to reveal itself, something like a bite from a radioactive spider, or a super soldier serum. Everyone already had this gene inside them but something needed to make it manifest itself. After awhile people were born with the gene already manifested without the need of any outside influence.
Uatu goes on to tell X-51 about good and evil and how humanity grows to fear and hate the super powers because of the authority they can hold over them. Uatu then asks X-51 to take his place as the Walker to transcribe all that is about to take place, for you see, Uatu is now blind and cannot perform his task on his own.
As a starting point to the overall series this was full of information that was needed to know for a better understanding of what is to come in the actual series, but this issue just had way too much information to explain to the reader before the actual story could start. Uatu must tell X-51, which is a means of telling all the readers, the history of humanity to be able to understand why what is to happen is happening in the first place. There was so much information I found that I had to go back and reread it just to get a stronger grasp of the entire history told to X-51. For that reason I gave it just above an average rating.