Earth is a little blue planet at the unfashionable end of the western spiral arm of the galaxy.
It's not a great place to live, being beset by terrorist organisations, rogue paramilitary groups, corrupted AIs, supervillains, extradimensional despots and two separate species of alien shapeshifters. Much of this is due to the presence of Ore-13, a potent fuel source with mystical applications seeded on the planet by an alien during the ice age.
The main life-forms inhabiting the planet are humans. Sucks to be them.
Earth formed 4.5 billion years prior to the present day.
Earth was seeded with Ore-13 by the Decepticon Shockwave with Ore-13 600,000 years ago, thanks to the planet being the resting place of the Enigma of Combination. On the pretence of claiming the missing artifact, "Onyx Prime" sent the Maximal starship Axalon to find the relic. They tracked it to Earth 20,000 years ago only for Shockwave to shoot them down.
The Dire Wraiths arrived on Earth early in the 1800s. Though initially planning to conquer the planet and move on, the Wraiths found the energies of Earth optimal for terraforming into a new homeworld. To that end, they established a subterranean portal beneath the Earth's crust to the realm of their deity, the Presence, so that she could terraform the Earth.
In the latter half of the century, the Maximal Autobot and Decepticons were awoken by American railway workers. A few years later, the Elder Gods invaded Earth but the Eukarian Autobots and their human allies managed to beat them back.
Cybertronians started arriving en masse on Earth in 2001, applying infiltration protocol as they engaged in covert warfare over the Ore-13 deposits. In 2005 the Cybertronians abandoned infiltration as the Great War came out of the shadows.
In 2008, the entire planet was subject to a massive invasion by the alien tyrant known as Megatron. Billions died with New York City being virtually destroyed before the Autobots defeated him and his forces. The Autobots remained stranded on Earth for a year before they left to fight Galvatron on Cybertron and cut ties with Earth for two years afterwards.
In 2016, following repeated Decepticon attacks on Earth, Optimus Prime took matters into his own hands by annexing Earth into Cybertron's Council of Worlds. Humanity, however, took this as a threat, and responded by reactivating the disbanded G.I. Joe and creating the Mobile Armored Strike Kommand. At the same time, Baron Karza discovered the existence of Earth and its Ore-13 deposits. He attempted to teleport the mineral into Microspace to save his realm but this had the side-effect of making the Ore-13 unstable, threatening to rip the Earth apart in mere weeks. Once the heroes of Earth stopped Karza, Optimus invited them all to work together.
Soon after, Karza returned to Earth with an army intending to conquer it as a new home for the people of Microspace. His plan was halted by the combined efforts of the Micronauts, Autobots and Time Travelers. In the aftermath, the Time Travelers proposed a treaty of friendship between Earth and Microspace which Optimus Prime agreed to.
In 2017, Earth appointed Marissa Faireborn as its official representative on the Council of Worlds. The induction ceremony however was interrupted when Joe Colton launched a genocidal campaign on Cybertron out of the belief that Earth could be only be protected by being cut off from the larger galaxy. Colton's plan was derailed however by Garrison Kreiger who revealed himself to be an ancient alien wizard known as Merklynn and used the Talisman to transform a portion of Cybertron's underground into "New Prysmos". The act drained nearly half of Cybertron's Energon which, ironically, made Cybertron need their alliance with Earth.
Nonetheless, there existed a portion of humans who believed Colton's actions had been justified in the name of Earth's sovereignty and demanded the Cybertronians release him.
When the Presence awoke, the Wraiths across Earth began open conquest of the planet until their goddess was forced back into her realm and the Wraiths were put down by human forces.
After Unicron destroyed Cybertron, the Cybertronian race evacuated to Earth where Shockwave planned to rebuild the Decepticon Empire only to learn that the Talisman's removal from Cybertron had allowed Unicron to survive the planet's destruction. The Maximals, now having defected to the Chaos Bringer, invaded Earth to remove the errant Talisman only for it to be teleported out of their grasp. Left scattered, the Maximals were attacked by the heroes and villains of Earth while Optimus Prime's attack felled Unicron, saving the Earth.
With the monster destroyed, Earth became the new home for the Cybertronian and Elonian species.