Uatu is a member of a race of extraterrestrials known as the Watchers who after billions of years of existence had created a utopian society on their homeworld and lived nearly immortal lives. Uatu was the son of Ikor, a kindhearted individual who sought to share the wealth of his peoples knowledge to other races in the universe that they viewed as less fortunate. Uatu and other members of their race agreed that this should be so, the only dissenter being Emmu who warned that other races were not as altruistic as their and could abuse whatever knowledge they gave. Emmu was outvoted and they all agreed to share their knowledge with the Prosilican race. Utilizing a means of space travel by transforming themselves into energy forms, Uatu and his people visited the Prosilians and shared with them their knowledge of atomic energy. After teaching the Prosilians what they knew, they left that world to explore elsewhere They did not return to the Prosilican homeworld until many years later only to discover that the Proscilians used the knowledge of atomic energy to go to war and wiped out their civilization. Some Proscilian survivors found Ikor, Uatu, and the others and cursed them for giving them the knowledge to destroy their world. Horrified by what he saw, Ikor vowed that his people would ever watch other worlds but never interfere as a penance for their horrible crime. Uatu was tried to convince his father that his deeds were noble, despite the outcome, but the words fell on deaf ears.
After the Prosilicans incident, Uatu asked his father if their vow of non-interference would be a hindrance to races that were in serious danger. Ikor advised his son that no matter what, they should never interfere in the affairs of another alien world ever again. Now calling themselves the Watchers, they travelled the universe viewing everything they encountered. Thousands of years later, Uatu came across a situation that tested the vow of his people when he witnessed the Yrds disposing atomic waste by jettisoning it into space, potentially threatening some other distant world with destruction. Uatu watched the mass of atomic waste for decades when one day he witnessed a super-nova jettison a rogue planet into space. Uatu witnessed as the planet struck the cloud of atomic waste, destroying both. This reinforced in Uatu's mind that his father was right, that they should leave things to fate. Eventually the Watchers assigned themselves onto the moons of various inhabited worlds to observe activities there. Uatu was assigned to the planet Earth and constructed a citadel on the Blue Area of Earth's moon. Not only did Uatu watch history on Earth, but also took an interest in viewing the his chosen world in alternate universes. His initial intentions was also to find a reality where his people did not make their grave error, however to date Uatu has yet to find such a reality.
Available records show that the Uatu -- or simply the Watcher as he became known -- apparently watched Earth for centuries uninterrupted and undiscovered. This changed in the 1950s when his citadel was compromised by scientists Howard Stark and Nathaniel Richards. However, their release was negotiated by the costumed hero known as the Destroyer and his V-Battalion. In 1962 the Watcher met Blue Marvel, a supremely powerful African-American hero struggling to find a place in America, which at the time was in the midst of a Civil Rights movement. After winning a Presidential Medal of Freedom the Blue Marvel went to the moon to mull over his situation, growing angry and striking the ground. The commotion brought out the Watcher who tried to impart his wisdom over living creatures desire to kill their own brothers despite the color of their skin. Their conversation was cut short when the Blue Marvel acted to stop an incoming alien invasion . The Watcher later observed the destruction and rebuilding of the planet Xandar after it was blown apart by Luphomoid invaders.
After a number of decades of silent observation, the Watcher detected a temporal disturbance and drew it to his citadel. It turned out to be a human scientist named Cassandra Locke a time traveller from the 22nd Century of Earth-700. This was on the eve when the group of heroes known as the First Line were engaged in preventing an invasion of Earth by the Skrulls. When the Watcher refused to interfere, Cassandra teleported herself aboard one of the invasion fleets to try and help. The Watcher stood by as the First Line sacrificed their lives to stop the invasion, while Cassandra went further back in time to learn what she could. The Watcher was later visited by training mystic Stephen Strange, telling him what he had witnessed. Uatu then revealed to Strange three people he thought would some day usher in a new age of heroes: Reed Richards, Bruce Banner and Henry Pym. 
Revealing Himself to the Fantastic FourMany years later a new group of heroes, known as the Fantastic Four were born when Cosmic Rays mutated them into super-humans. Incidentally their leader was Reed Richards -- aka Mr. Fantastic. One of their early adventures took them exploring the Blue Area of the Moon. Upon arrival on the moon they were attacked by similarly cosmic ray enhanced Russian scientist the Red Ghost and his Super-Apes. A battle broke out a between both sides prompting the Watcher to reveal himself to the Red Ghost and the Fantastic Four's Thing. Uatu revealed his life's mission and allowed the two factions continue their battle on the Moon's surface. When the Fantastic Four were the victors, the Watcher revealed himself once more and encouraged them to keep seeking out the stars before deciding to leave to avoid being interrupted once again. The Fantastic Four returned to Earth, while the Red Ghost and his Super-Apes were left stranded on the moon.  Thus was the first of many interactions between the Watcher of Earth and the Fantastic Four.
Sometime after this, the Watcher came to warn the Fantastic Four of the danger of the Molecule Man, a man who could control inorganic molecules. When the Watcher was satisfied that the Fantastic Four had defeated their foe, he then stepped in and captured the Molecule Man, telling them that the foe would not threaten their world again before undoing the damage done by the Molecule Man and returning to his silent observation of the Earth.
Observing Other Worlds
On his absence from Earth the Watcher began watching various other worlds and alternate universes that could potentially pose a threat to the Earth he was assigned to watch. He witnessed as the alien Sneepers began waiting for the human race to annihilate themselves before launching a take over of the planet. He also observed various alternate realities where the human race advanced many years. Returning to the moon the Watcher was observing the Earth one day when he was visited by an alien named Xakku who sought to steal Earth's sun but wanted to ensure that the Watcher would not interfere. After confirming this, Xakku explained his actual plot. While the Watcher was sworn not to interfere, he did keep Xakku detained by asking him questions about his homeworld long enough for the alien's ship to sink in the bog it was landed on stranded Xakku on the moon and leaving him for dead. When Xakku begged for help, the Watcher reminded him that he could not interfere. When the Red Ghost stranded the Fantastic Four on the moon for revenge over his previous defeat, the Watcher allowed the Fantastic Four to seek refuge in his citadel while he was away from the planet, but asked them to defend his devices from the Red Ghost. When the Fantastic Four defeated the Red Ghost by knocking him through a portal the Watcher thanked them by returning the Fantastic Four to Earth so he could continue observing other worlds. 
Uatu next observed a world of barbarians where two tribes were fighting over the beautiful Queen Kalthea. The Watcher instantly fell in love with the beautiful blonde but refused to interfere until one of the barbarians threatened her life. Appearing between the queen and the barbarian, Uatu frightened the barbarian away. Kalthea was so grateful to the Watcher she offered to rule her world as her consort. Realizing that he could not possibly get involved in such a way due to his oath, the Watcher refused and then began acting arrogant so that Queen Kalthea would reject him. The Watcher then returned to his citadel on Earth's moon returning to his silent observation of the Earth. The Watcher was later visited by a ship full of space pirate that were seeking to loot a distant planet. After confirming that the Watcher was sworn not to interfere with other races, they began constructing a massive cannon to fire at the alien world. Before they could attack the Watcher suddenly destroyed their weapon and banished the pirates to a distant world -- revealing that the world that they chose to attack was the homeworld of the Watchers, the only world he could step in to assist. 
Renewed Focus on Earth
The Watcher then began focusing his observations back to the Earth, alternating between living on his citadel on the moon or on the Watcher homeworld. Uatu observed how the Avengers prevented the Earth from becoming a battleground between the Kallusian and Yirbek races. Detecting the time traveller Kang the Conqueror was trying to conquer the 6th Century, the Watcher enlisted the aid of the Fantastic Four's Thing and Human Torch, sending them back in time to stop Kang and preserve history. When the Fantastic Four's Reed Richards and Susan Storm were to be married, their old foe Dr. Doom used an Emotion Changer device to force an army of super-villains to crash the wedding. While the super-heroes in attendance fought off the attackers the Watcher appeared before Reed Richards offering his aid. Transporting Reed to his citadel, the Watcher allowed Reed to choose a weapon of his choice to repel the invaders. Reed selected a Time Displacer that banished all the villains to where they came and erased their memories of the attack. Thanks to Uatu's assistance, the wedding continued without further interruption. As a wedding gift, Uatu allowed Reed and Sue to have their honeymoon at his citadel.While the Watcher typically kept his vow of non-interference this all changed when he discovered that the Silver Surfer, herald of Galactus was coming to scout the planet Earth as a potential meal for his master. Using sophisticated devices, the Watcher attempted to mask the Earth from detection using holograms of fire and debris, causing a worldwide panic. The Watcher also warned the Fantastic Four of the coming danger, and they battled the Surfer. However, despite his best efforts the Watcher was unable to prevent Galactus from reaching Earth. When Uatu was unable to convince Galactus to spare the Earth, he boosted the Human Torch's powers and guided him through hyperspace to the Worldship of Galactus to recover the Ultimate Nullifier, the only weapon known to be powerful enough to destroy an entire universe and the one item that Galactus feared. When the Torch succeeded in his mission the Watcher guided him back to Earth where Mr. Fantastic used the Ultimate Nullifier to force Galactus to surrender. After banishing the Surfer into exile for helping the Fantastic Four, Galactus left the Earth to seek out a new world to consume. With the threat passed, the Watcher returned to his citadel on the Moon.
When the gamma-ray mutated genius the Leader became aware of the existence of the Watcher and his world and the technological bounties there, he sent the Hulk to the Watcher's homeworld to recover the so-called Ultimate Machine. Sworn to not interfere, the Watcher allowed the Hulk to collect what he sought but the gamma spawned brute was attacked by the Amphibian an alien also seeking out the power of the Ultimate Machine. When it was clear that the Hulk was the victor of the battle, Uatu teleported the Amphibian away so none of his devices or exhibits could get destroyed. The Watcher then allowed the Hulk to take the Ultimate Machine and leave. When the Leader dawned the Ultimate Machine it's combined knowledge proved too much for even his enhanced brain and he was seemingly slain by the device. When the Hulk later abandoned the Ultimate Machine, the Watcher recovered the Ultimate Machine before its power could be abused further. Sometime after this, the Fantastic Four's foe Dr. Doom had stolen the Power Cosmic from the Silver Surfer. In order to avoid the temptation of interfering the Watcher left the universe. Even without the Watcher's assistance the Fantastic Four were still able to defeat. The Watcher had begun setting up space traps that would allow him to examine meteors by this time. When signalled that one of these traps had caught something the Watcher was mortified that he caught the slumbering form of the artificial being known as Him. Not wishing to interfere further in Him's life, the Watcher freed the slumbering cocoon and placed it in a NASA satellite and sent it back to Earth. 
Later when the Silver Surfer decided to wage war against the human race in order to unite the planet against the common foe, the Watcher warned the Fantastic Four of the danger, allowing the group to track down the Surfer and convince him of the errors of his ways. The Watcher also observed his realities Avengers go up against their Earth-689 counterparts who were being used as pawns of the Scarlet Centurion. After the Avengers of his world succeeded the Watcher informed the Avengers of the connection between the Scarlet Centurion, Kang the Conqueror and Rama-Tut.
- ↑ 1.0 1.1 1.2 Original Sin #0
- ↑ 2.0 2.1 Tales of Suspense #53
- ↑ 3.0 3.1 Tales of Suspense #54
- ↑ Tales of Suspense #53 states that they went to witness a multidimensional eclipse, while Original Sin #0 has stated that they went to explore the universe.
- ↑ 5.0 5.1 5.2 Fantastic Four #13
- ↑ What If? #1
- ↑ Citizen V and the V-Battalion #2
- ↑ Adam: Legend of the Blue Marvel #3
- ↑ Fantastic Four #204
- ↑ Marvel: The Lost Generation #12
- ↑ Fantastic Four #1
- ↑ Fantastic Four #20
- ↑ Tales of Suspense #49
- ↑ Tales of Suspense #50
- ↑ Tales of Suspense #51
- ↑ Tales of Suspense #52
- ↑ Tales of Suspense #55
- ↑ Fantastic Four #29
- ↑ Tales of Suspense #56
- ↑ Tales of Suspense #57
- ↑ Avengers #14
- ↑ Strange Tales #134
- ↑ Fantastic Four Annual #3
- ↑ Marvel: Heroes & Legends #1
- ↑ Fantastic Four: The Wedding Special #1
- ↑ 26.0 26.1 Fantastic Four #49
- ↑ Fantastic Four #48
- ↑ Fantastic Four #50
- ↑ Tales to Astonish #73
- ↑ Tales to Astonish #74
- ↑ Tales to Astonish #75
- ↑ Fantastic Four #60
- ↑ Thor #165
- ↑ Fantastic Four #72
- ↑ Avengers Annual #2