Earth

"I remember you tossing me into an abyss. I who was and should be king!" "So you take the world I love as recompense for your imagined slights? No, the Earth is under my protection, Loki."

- Loki and Thor

Earth is the third planet from the Sun and is one of the Nine Realms aligned by the cosmic nimbus, Yggdrasil. The various races of the Nine Realms refer to Earth as "Midgard", while the alien races living beyond the realms call it "Terra".

The Dark Ages
"Once, mankind accepted a simple truth: that they were not alone in this universe. Some worlds man believed home to their gods. Others they knew to fear. From around the cold and darkness came the Frost Giants, threatening to plunge the mortal world into a new ice age. But humanity would not face this threat alone."

- Odin

In 965 AD, a war between two alien races, the Asgardians and the Frost Giants, made its way to Earth, with the Asgardians defending the humans from the Frost Giants. The humans, in particular the peoples of Northern Europe, worshiped the Asgardians as deities, and the Asgardians would often visit Earth and teach the humans language and culture. Early Norse cultures learned much from these visits, although in time the Asgardians would withdraw back to Asgard and their memory would fall into myths and legend over the generations.

20th Century
The 1940's were a tumultuous time. Depression years and racial disharmony in Europe sparked World War II, pressing various factions into a technology race. While Nazi forces occupied much of Central Europe, a detachment calling themselves HYDRA were making gains in unconventional warfare. Their leader, Johann Schmidt, actually managed to procure the Tesseract, an Infinity Stone that the Asgardian king Odin left on Earth some time after the Frost Giant attack in Norway. In studying the Tesseract, HYDRA was able to devise new and deadly weapons for their war effort. In America, Abraham Erskine and Howard Stark were poised on verge breakthroughs that would set the stage for the world of the modern "superhero". Erskine had developed a "super soldier" serum that was to transform an ordinary man into something altogether stronger, faster, and more resilient. As a result of this experimental process, Steve Rogers, an ill-fitted man from New York, became the world's first superhero, "Captain America", but not without cost. Erskine was assassinated by HYDRA operatives and what remained of his serum was destroyed before the project could be pushed into widespread use. In spite of this, Rogers proved to be a brilliant military expert and soldier and led many soldiers into victory, eventually bringing down HYDRA and defeating Schmidt. However, a final mission against HYDRA left Rogers stranded in the Arctic, where he was left frozen for the next seven decades. In the end, the war was won, not with super soldiers, but with many countless lives and simple, bloody warfare.

Howard Stark, who had assisted with Erskine's program, went on to develop numerous advances in technology. He accomplished much over the next several years, assisting in the founding of the world-spanning security agency, the Strategic Homeland Intervention Enforcement Logistics Division (S.H.I.E.L.D.). Learning from the recovered Tesseract, he went on to develop prototype arc reactors, powerful sources of energy in their own right. He eventually died in a car accident, leaving his business empire to his young son, Tony, and his company, Stark Industries, grew into a world renowned supplier of weapons technology.

21st Century
"If we can't protect the Earth, you can be damn sure we'll avenge it!"

- Iron Man to Loki

The dawn of the superhero age might have looked like a brief spark to those who grew up reading the exploits of Captain America, but it was soon to flare into a far brighter flame. In the 1980s, the unexpected departure into outer space by United States pilot Peter Quill instigated his eventual role as the leader of the intergalactic justice squad, the Guardians of the Galaxy, who would later save the galaxy, the Earth included, from Ronan the Accuser. The secrets of Erskine's formula had baffled scientists for decades until Dr. Bruce Banner, a graduate from Culver University, explored a new direction and bombarded himself with gamma radiation to prove his theories. His experiment was successful in augmenting the human form, but the extent of the mutation was far beyond anything anyone could have anticipated. Banner was transformed into a raging green beast that was later dubbed the "Hulk", and found himself being hunted by military operatives that sought to exploit the transformation for the army's weapons division. The Hulk, however, would prove to have the heart of a hero by stopping maddened soldier Emil Blonsky, who used a similar variation of Banner's formula to transform himself into a grotesque, Hulk-like creature that rampaged through Harlem.

In response to threats posed by such things as the Hulk and the Tesseract, governments of the world formed the World Security Council, which in turn channeled resources into S.H.I.E.L.D., appointing Nick Fury as the agency's director. Idealistic himself, Fury saw more potential in superhumans than the council did, and spent the next several years putting together a list of dossiers regarding certain individuals with superhuman abilities. This list would later be called the Avengers Initiative. Tony Stark, who had spent the years since his father's death learning the role of businessman and further developing his father's company, became a pivotal participant in the superhero age. An inventive genius himself, he was personally responsible for many of the innovations that made Stark Industries a world leader in weapons technology. In 2010, while on a trip to Afghanistan, Stark's convoy was ambushed by the Ten Rings terrorist group and he was held captive and forced to develop weapons for the organization. He escaped from his captors by building a battlesuit for himself rather than the missile system they wanted, but the whole incident had a profound effect on him. He turned his company away from the path of violence, while constructing a series of high-tech battlesuits that he hoped to use to achieve world peace. Stark's vigilante actions had the effect of ushering in a new superhero age, and taking a leaf from the press, he began publicly referring to his heroic persona as "Iron Man".

S.H.I.E.L.D. soon took an interest in Iron Man's activities. Nick Fury approached Stark on several occasions and reviewed him for potential inclusion into the Avengers Initiative, but ultimately withdrew from seeking his personal involvement due to his erratic nature. Instead, S.H.I.E.L.D. turned their attentions to exploring Stark's weapons technologies. Their mandate was to protect the world from such powerful threats as the Hulk, and to this end they began to grow from a clandestine police agency into a powerful military force. The most significant development in human history took place in the summer of 2011, when the Asgardians, thought to be nothing more than a myth after centuries of absence, returned once again to the world when Thor, the heir to the throne of Asgard, found himself banished to Earth after unintentionally brewing another war with the Frost Giants. While his initial appearance went unnoticed to all but a few scientists, his hammer Mjølnir caused quite a stir after it impacted in Puente Antiguo, New Mexico, and also drew the attention of S.H.I.E.L.D. After numerous altercations, Thor's adopted brother, Loki, sent the Asgardian Destroyer to Puente Antiguo to kill Thor and ensure Loki's own coronation. However, Thor reclaimed Mjølnir and defeated the Destroyer, and announced that S.H.I.E.L.D. could thereafter count him as an ally. This event revealed to humanity that they were not alone in the universe, and the now paranoid World Security Council withdrew all of S.H.I.E.L.D.'s resources away from the Avengers Initiative, and more on developing weapons to deter possible alien threats. Later that same year, a S.H.I.E.L.D. expedition team found Steve Rogers' frozen body in Antarctica, and due to the effects of the super soldier serum in his body, he was still alive and revived. Having nowhere else to go, Rogers joined S.H.I.E.L.D.'s services and became a superhero once again.

By 2012, the Avengers Initiative had been dropped on the council's orders, but was soon brought into effect when Loki, having being banished from Asgard after the events in Puente Antiguo, made a surprise appearance and stole the Tesseract away from S.H.I.E.L.D. in a campaign to conquer Earth. In a desperate bid to counter this threat, S.H.I.E.L.D. reactivated the Avengers Initiative, calling in Tony Stark, Bruce Banner, and Steve Rogers. Thor and elite S.H.I.E.L.D. agents Natasha Romanoff and Clint Barton also provided support, and together, the six individuals formed the first official Avengers lineup. The team battled Loki and his allies, an alien race called the Chitauri, in New York City, and eventually defeated them both. The Avengers Initiative was reactivated in light of its success, and S.H.I.E.L.D. began seeking out other superhumans. Phil Coulson, a high-ranking agent, was assigned leadership over an elite team of operatives that specialized in seeking out such individuals.

Over the next few years, the Avengers would continue to act independently, fighting off numerous threats that S.H.I.E.L.D. could not. Tony Stark, suffering from severe posttraumatic stress disorder following his experience with the Chitauri, came into conflict with an old enemy of his, Aldrich Killian. Killian attempted to initiate a world takeover through use of a formula called Extremis. With help from his two close friends, Pepper Potts and James Rhodes, Stark defeated Killian, but pockets of Extremis were recovered by agents of the Centipede group, who then came into conflict with S.H.I.E.L.D., more specifically Coulson's team. In 2013, a cosmic phenomenon known as the Convergence occurred in London, and Malekith, the leader of a hostile race called the Dark Elves, tried to use it as a means of unleashing the destructive Aether upon the Nine Realms. Malekith nearly succeeded in annihilating all of the Nine Realms, including Earth, but was defeated by Thor. Pieces of Dark Elf technology left behind after the battle were recovered by S.H.I.E.L.D.

Trivia

 * Because of the numerous alternate universes of Marvel Comics, this Earth from the Marvel Cinematic Universe is unofficially known as Earth-199999.