Earth

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

- Iron Man

Earth is the third planet from the Sun and is known to the people of the Nine Realms as "Midgard". After Thor Odinson of Asgard was exiled there, he made it his duty to protect it, as it was the home of his lover Jane Foster. S.H.I.E.L.D. director Nick Fury formed The Avenger Initiative to "make a statement" to the Nine Realms that Earth was ready for war.

The Dark Ages
Asgardians came to Earth, visiting the peoples of Northern Europe and teaching them language and culture. Early Norse cultures learned much from these visits, although in time the Asgardians would withdraw and their memory would fall into legend.

20th Century
The 1940's were a tumultuous time. Depression years and racial disharmony in Europe sparked World War II that pressed 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 a Tesseract, and in studying this device HYDRA scientists were 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 a soldier from an ordinary man into something altogether stronger, faster, and more resilient. As a result of this experimental process, Steve Rogers became the world's first superhero, "Captain America", but not without cost. Erskine was assassinated and most of the remaining samples of the serum were destroyed before the project could be pushed into widespread use. 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. 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 Stark Industries grew into a world renowned supplier of weapons technology.

21st Century
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. The secrets of Erskine's formula had baffled scientists for decades until Dr. Bruce Banner 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".

In response threats posed by such things as the Hulk and the Tesseract, governments of the world formed the World Security Council, which in turn channelled resources into a world-spanning security agency they named, Strategic Homeland Intervention Enforcement Logistics Division (S.H.I.E.L.D.). The young Tony Stark had spent the years since his father's death learning the role of businessman and further developing his father's company. An inventive genius himself, he was personally responsible for many of the innovations that made Stark Industries a world leader in weapons technology. On a trip to Afghanistan his convoy was ambushed, and he was held captive and forced to develop weapons for terrorists. He escaped from his captors by building a battlesuit for himself rather than the missile 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 wore into battle himself. Tony'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. They approached Stark on several occasions and reviewed him for potential inclusion into a select group of individuals, but ultimately withdrew from seeking his personal involvement due to his erratic nature. Instead they turned their attentions to exploring 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 Asgardians, thought to be nothing more than a myth after centuries of absence, returned once again to the world when the heir to the throne of Asgard found himself banished until he could prove himself worthy to rule Asgard. While his initial appearance went unnoticed to all but a few scientists, his hammer Mjolnir caused quite a stir after it impacted in Puente Antiguo, New Mexico, and also drew the attention of S.H.I.E.L.D. Thor did eventually reclaim his birthright and announced that S.H.I.E.L.D. could thereafter count him as an ally.

Alliances would needed again soon enough. Thor's brother, Loki, made a surprise appearance and stole the Tesseract away from S.H.I.E.L.D. In a desperate bid to counter this threat, S.H.I.E.L.D. reactivated an old plan to assemble a group of extraordinary individuals. They called on Tony Stark and Bruce Banner for their scientific expertise. Captain America, reawakened after decades of being frozen in Arctic ice, was called upon to be a super soldier once more. Thor, and S.H.I.E.L.D. agents Natasha Romanoff and Clint Barton stepped in to round out the new team. Calling themselves the Avengers, they fought Loki and his army of Chitauri to a standstill.

Trivia

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