The planet Earth, third from the sun in its Solar System in the Milky Way galaxy, is a planet populated by a wide variety of native organic lifeforms, most notably, humans. This planet is one of the very few in existence that (without being colonized or modified with some form of technology) can support organic life. It is plentiful in many forms of energy sources, and different means by which to produce them.
Many Autobots in many different continuities have noted Earth's beauty and have stated that "if Earth was their home, they'd definitely take pride in it." There are many different wonders on the planet including its weather patterns, and countless systems and cycles the planet relies on to remain inhabitable.
Versions of this planet are known to exist in many dimensions within the multiverse, and it frequently becomes involved in the conflicts of the Transformers. Perhaps too frequently. Whenever something (or someone) leaves, escapes, or is otherwise ejected from Cybertron, there's a disproportionate chance it will eventually wind up on Earth.
The Transformers cartoon
Around nine million years BC, the Earth was visited by the Transformers Starscream and Skyfire, but Skyfire was caught in a polar wind storm and separated from Starscream, buried in ice. Then, four million years BC, the Transformers' spacecraft, the Ark, crash-landed in what would become the north-western United States, entombing the robots within stasis.
Earth is home to many unusual creatures and "hidden civilizations." The underwater city of Sub-Atlantica, populated by water-breathing fish-like humanoids, gave rise to the legends of Atlantis and other sunken cities. Also, fire-breathing dragons exist naturally as late as the medieval era, though by the 20th century, they appeared to have died out. A possible descendant was the unique, reptilian creature known as the "Beast of Borneo" captured by big game hunter Lord Chumley.
The magma energy of the Earth's core is channeled by a series of green crystalline structures, which the Decepticons attempted to exploit in 1984, exposing a crystal shaft in the North Pole and tapping the core's energy (while, at the same time, happening upon and reactivating the body of Skyfire). Another unusual aspect of the planet is the natural existence of electrum, a metallic alloy that renders Transformers invulnerable when coated in it.
By the year 2005, human interaction with the Transformers had resulted in an advanced level of technology worldwide, and the foundation of the Earth Defense Command, which operates a series of space stations. Saturn's moon, Titan, was colonised, and an EDC base was established on Mars, while travel between star systems was hugely enhanced by the creation of warp-gates. Earthling Spike Witwicky became Earth's ambassador, first to Cybertron, and then to other alien worlds as, through the Transformers, humanity came into contact with other races.
Japanese animated continuity
In the expanded fiction of the additional Japanese-exclusive animated series, the existence of the sunken continents Atlantis and Lemuria was confirmed. Later, it was revealed that a small group of Autobot and Decepticon Pretenders crash-landed on the planet in the Stone Age. While the Autobots used their Pretender abilities to adopt the form of humans, the Decepticons assumed monstrous forms and were mistaken by early man as "demons." Finally defeating the Decepticons and sealing them away - within the pyramids of Giza, the ruins of Atlantis and beneath the Nazca Lines of Peru - the Autobots then integrated themselves into human society, serving as secret protectors who quietly and subtly guided the evolution of human society.
Marvel Comics continuity
The Ark crashed into Earth four million years ago. Already present, and the site of the first Transformer battle on Earth, was the Savage Land - a hidden jungle in Antarctica where dinosaurs still live. It is believed the Savage Land was created by another, earlier race of aliens.
In the year 1017AD, the Autobot Navigator and his companion arrived on Earth searching for the lost Ark. Their ship landed in England and the companion would look around every few centuries, and he would be named the Man of Iron by the awed residents of Stansham; his first appearance turned the tide of a great battle.
By the 1980s, Earth was involved in a Cold War driven by the United States of America and the Soviet Union. The planet was also threatened by the international terrorist group Cobra, who was opposed by America's G.I. Joe organisation and its international equivalent Action Force. There also existed a group called S.H.I.E.L.D., an American spy organisation that primarily handled an unknown dinosaur variant, and otherwise seemed to have little prominence.
By a quirk of genetics, the odd human proved capable of wielding superpowers. The earliest known was a vigilante named Spider-Man who vanished after 1984 for unknown reasons, though he did clash with the Decepticons and caused Megatron to rethink how dangerous humans were.
The Autobot and Decepticon war would rage until 1991 before a brief lull; both sides would relocate their bases to multiple locations, and while often battling in America they would also strike in the United Kingdom, Peru, Canada, Mexico, Japan and Argentina. Responsibility for dealing with them in the States fell to the Intelligence and Information Institute, another American spy group, and their military wing RAAT. Diplomatic relations were tried early on with the President of the United States, but failed.
In 1993, the Decepticons struck Earth with overwhelming force, devastating cities and slaughtering millions, merely as a way of attracting Autobot attention. Even this was dwarfed when the Cybertronian warlord Jhiaxus blew up San Francisco the following year. The long-term impact this had on Earth is unknown.
In an alternate timeline, the bulk of the Autobot/Decepticon war moved off-world in 1990 and a small Earthforce remained to contain two duelling Decepticon factions (who later united). The Autobot base was located in Canada, where an abnormally high number of polar bears roam the wilderness; the animals have even become bold enough to approach the Autobot base.
In several alternate timelines where Unicron attacked in 2006, the Autobots were allowed to build a city on Earth and it would become a major location for scattered Autobots to regroup after Decepticon conquest of Cybertron. Diplomatic relations were strong between human and Autobot, and futuristic technoloy such as weather-control machines were gifted to mankind.
In yet another timeline, the Americas were brutally conquered after 2005 by Galvatron.
Dreamwave Comics continuity
IDW comics continuity
Earth was invaded by the Decepticons sometime during 2002, presumably for the purposes of harvesting its local resources. However, the recent discovery of Ore-13 by Starscream's team prompted the treacherous lieutenant to break protocol in an effort to rebel against Megatron.
A detachment of Autobots, led by Prowl, is also present on Earth in an attempt to prevent the Decepticons from pillaging the planet. One of its members, Ratchet, also broke protocol by initiating direct contact with three humans, who proved invaluable in gaining intelligence on Starscream's operations.
The discovery of Ore-13 threatened to make Earth the new front line in the war between the Autobots and Decepticons. Megatron carried on the standard strategem of igniting conflict between humans to weaken their kind for conquest, but Autobot interference made him escalate things drastically. The Transformers are now known to humanity - and the Autobots have temporarily withdrawn from Earth (bar a few AWOL agents), leaving it vulnerable to the Decepticons. This will not end well.
The mysterious aliens known as the Vok chose prehistoric Earth, which they dubbed Nexus Earth, as the site of an experiment, and seeded the planet with vast amounts of raw Energon, installing a second, artificial moon into the planet's orbit and placing several devices at strategic points on the planet. Around the time of the emergence of proto-hominids, a group of Maximals and Predacons from Cybertron's future travelled back in time to prehistoric Earth, where they waged a war they dubbed the "Beast Wars," and encountered the Vok, interfering in their plans. Deciding that the Beast Warriors had contaminated their experiment, the Vok activated the second moon, transforming it into a giant laser weapon with which they planned to detonate the Energon seeded throughout the planet, wiping it clean of life. This plan was foiled by the Maximal leader, Optimus Primal, but the vast majority of the raw Energon on the planet was transformed into stable, cube form. Eventually, the Maximals returned to Cybertron with the Predacon leader, Megatron their prisoner. The only surviving Predacon, Waspinator, was left on the planet as the leader of a tribe of protohumans. Although they eventually rejected him and he too returned to Cybertron, it is unclear precisely how much influence Waspinator may have had on the evolution of humanity and their culture.
Interestingly in Japanese version of it, Earth and its sector in space was declared off-limits by order of the Maximal High Council, and likely all information about it tightly regulated. It's unknown precisely when and why this ban was initiated. However, it related to the events in BWII.
(Note: in the Japanese dub of Beast Wars, the Transformers referred to the planet they were stranded on as "Energoa", before they realised it was prehistoric Earth.)
Beast Wars II
A future version of Earth was later visited by Maximals and Predacons under the command of Lio Convoy and Galvatron, respectively. At first known only as "Gaea", the world was known to have two sentient inhabitants, a girl named Artemis and her companion Moon. Although the presence of what seemed to be a decimated Tokyo Tower suggested that Gaea was Earth, it was not fully confirmed until release of the Robot Masters toyline's version of Lio Convoy, the bio which revealed that Earth had been devastated by mankind's tampering with the power source known as Angolmois energy.
Robots in Disguise
Thousands of years ago, the Autobot high council hid Fortress Maximus on Earth. The many parts of its activation mechanism, such as O-Parts, the Orb of Sigma, and Cerebros, came to be located within relics left behind by ancient human civilizations. In the 1940s, a crew of Autobots was dispatched to the planet to check up on things, unsuccessfully. In the present day, the biggest problem facing humanity is a lack of access to energy. Doctor Onishi is the world's foremost expert on locating and cataloguing new energy sources, and during the course of his work he located Fortress Maximus and some of the o-parts. Cue the second biggest problem for humanity, as their top energy expert is kidnapped by a giant robot hand.
After the first defeat of Unicron, Earth was chosen as one of the planets to receive a Cyber Planet Key, which would have reformatted the planet into a Transformer colony. However, the colony ship Atlantis, also carrying the Omega Lock, crashed onto the planet before it could begin its mission.
(Note: A lost undersea city was found earlier during the Armada series, one powered by a Mini-Con from the Air Defense Mini-Con Team. A hologram described that the civilization had been quite advanced, but was destroyed by its own power. The human children deemed this city to be Atlantis, but it is possible that it was a completely different civilization.)
Some time later, when the Mini-Cons fled Cybertron to escape being used as tools, their ship came out of warp above Earth. Impacting onto the planet's moon, the Mini-Con ship broke in half, sending a large chunk of the craft into Earth's atmosphere. Mini-Con storage panels were spread across the globe and the ship itself crashed onto the surface, where they laid dormant for nearly a million years. Earth's fate being tied to that of Cybertron in the Unicron Trilogy is a form of predestination. The human Rad visited Cybertron millions of years ago via a completely unexplained timeslip and encountered the Mini-Cons upon birth, releasing them from Unicron's control and pretty much sealing this fated relationship for good.
Dreamwave comics continuity
Live-action film series
In Ancient Times, long before the All Spark crashed on Earth, the Seven Primes built a Solar Harvester on the planet to harvest Energon, but wanted to stop upon the discovery of sentient life. One of the Primes decided to defy the others and attempted to activate the Harvester anyway, leading to a battle for the Matrix of Leadership that the other Primes couldn't win, so they stole the Matrix and sacrificed themselves to hide it from the Fallen.
The powerful All Spark crashed onto Earth around 10,000 B.C.E., followed a few thousand years later by the Decepticon field leader Megatron who would become buried at the Arctic Circle. The discovery of the later would either lead to the formation or retasking of the Sector Seven, a United States secret service organization that may or may not have previously dealt with other alien encounters. Hoover Dam would be built as a secret holding base for Sector Seven, near to populous Mission City.
Through Megatron, much of modern human technology would be created via reverse-engineering and much of modern American history driven by Sector Seven. Oppenheimer created the atom bomb through reverse-engineered technology; the "space race" (including more secret parts of it) was won by America solely due to Megatron's bits; and Three Mile Island was caused by poking at the All Spark.
In 1961, the Ark crashes on Earth's moon and is detected by NASA and the Russians. The space race that follows is done with the goal of getting to the moon and seeing the crashed ship. America gets a man on the moon first and collects samples from the ship while the Russians send probes and somehow manage to collect one of the ship's fuel cells which they experiment with at Chernobyl.
By the 21st Century, the United States had thus become so powerful the United States Military possessed many military bases in foreign countries such as Qatar and NASA had total supremacy over space-related matters. Sector Seven had its own paramilitary wing and prisons, all while being kept in secret and the Secretary of Defense was unaware of its existence.
In 2003, covert Transformers (both Bumblebee and Decepticons) arrived on Earth to search for the All Spark, successfully evading and sometimes killing any Sector Seven forces. Another and seemingly unrelated faction would attempt the same thing, even getting their operative Dewbot inside the Hoover Dam base. Operating at the fringes were human vigilantes such as the mysterious Agent X and the Lunchables Brigade paramilitary, the latter having some form of contact with the Autobots.
In 2007, Optimus Prime and the Autobots arrived on Earth to secure the All Spark, while the Decepticons learned of the existence of Archibald Witwicky's glasses. Open warfare would result, ending in a final battle with both the military and the Autobots fighting against the Decepticons at Mission City. Sector Seven were decimated, but the Decepticons were still defeated thanks in part to two teens. During the battle, several Decepticons, including Megatron, were killed and the Allspark was destroyed.
In 2009, thanks to a shard of the Allspark, its knowledge resurfaced in Sam Witwicky's mind and eventually the Decepticons, under the Fallen, began seeking pieces of it to resurrect Megatron. Ravage and his Microcons succesfully steal the shard from Diego Garcia and Megatron is resurrected. After a failed attempt to get the information on the location of the Matrix from Sam, Megatron kills Optimus Prime and has Decepticons land all over Earth and the Fallen makes a broadcast threatening the people of Earth to get them to find Sam for the Decepticons. Thanks to Jetfire, the Matrix is found once again and is used to resurrect Optimus Prime, but the Fallen steals it and activates the Solar Harvester. Thanks to Jetfire's sacrifice, Optimus becomes Jetpower Optimus Prime and destroys the Solar Harvester, saving the Earth and kills the Fallen.
In 2011, the fuel cell at Cherboyl is recovered by the Autobots and NEST before an Autobot mission to the moon is launched that recovers Sentinel Prime and five of his space bridge pillars. Eventually, Sentinel exposes his true colors and he and Megatron use the space bridge to bring an invasion force of hundreds of Decepticons from the moon. Later, the two use the space bridge to bring Cybertron to Earth in order to use Earth's natural resources and humans as a slave labor force to rebuild the planet. A battle erupts in which Optimus kills both Megatron and Sentinel and Bumblebee destroys the Control Pillar. As a result of Cybertron not being fully formed when the space bridge is destroyed, the planet collapses on itself and is destroyed, causing the Autobots to fully accept Earth as their new home.
There are several divergent timelines where the presence of other Transformers or different strategies undertaken altered the entire pattern of events.
Some have the same basic outcome, but with greater and more wide-ranging devastation to America and greater (but futile) resistance by Sector Seven. At least one resulted in not only wider destruction but in complete Decepticon victory.
In the most severe divergence, the rogue Decepticon general Mainframe reached Earth before the Autobots and established an army and base at the Arctic. He began taking out every Autobot agent arriving on Earth, whilst searching for the All Spark, and eventually began sweeping the Earth with large numbers of drone units. He was eventually defeated by the Earthbound Chevy Autobots unit; the impact this had on Starscream's attempts to gain the All Spark, not to mention Earth itself, are unknown.
In a further divergent timeline, the Decepticons succeeded at Mission City and conquered the United States. While NATO regrouped to try and oppose them, its forces proved no match for Decepticon retaliation.
Via a processing station at Savannah, Georgia, Megatron used the All Spark to begin transforming Earth into a new Cybertron. The Autobots eventually killed him and destroyed the All Spark, but the fundamental damage was already done: Earth was now being torn apart by techno-organic activity, and as a side effect a strange illness was affecting scores of people.
On top of this, attempts to rebuild the liberated United States were constantly thwarted by a Decepticon terror campaign; both Autobot and NATO forces (the American military having been decimated) operated on US soil, attempting to defend it.
It is revealed at that the core of the planet Earth is not (as science would have us believe) magma, but Dark Energon. This, combined with the strange weather phenomena around the Earth lead Ratchet to suggest that perhaps the core of Earth was actually the sleeping form of the Chaos Bringer — Unicron. This is confirmed when they tuned a scanner in on the core of the Earth to monitor seismic activity and the results indicate that the quakes were coming in a steady and predictable rate. When examined closer, the waves matched that of a Cybertronian heartbeat.
The true irony lies in the fact that Cybertron, Earth's opposite in a chain of 47 planets, has Primus, Unicron's brother and arch-nemesis, as its core.