Jenova
Approximately two thousand years before the main events of the game, Jenova arrived on the Planet, having traveled through space on a meteor. Its landing caused a giant wound to the Planet that would later came to be known as the "Northern Crater," a blight on its surface that the Planet would channel its Lifestream to in an effort to heal even up to the beginning of the game.After its arrival, Jenova emerged from the Crater and approached the Cetra (also known as "the Ancients"😉, using its mimic abilities to glean the memories and emotions of the Cetra, and then shifting its form in response in an effort to get close enough to them to infect them with its own cells, which then drove those infected insane and turned them into monsters. This was accomplished by taking on the forms of their deceased loved ones.
Most of the Cetra were mutated in the time that followed, though the survivors attempted to defend the Planet despite their dwindling numbers. They were abandoned in the defense of the Planet by their common human cousins (those descended from the Cetra who gave up their Planet-revering lifestyle and subsequently lost their close relationship with the Planet), though a small band of survivors mounted an assault on the Calamity from the Skies and managed to defeat it nonetheless. The creature effectively rendered neutralized, it was sealed away in the Northern Crater. As a result of Jenova's onslaught, only a few Cetra would survive into the modern era.
Approximately 30 years before the main events of the game began, Professor Gast excavated the creature and mistakenly identified it as a Cetra. Being Shin-Ra's top scientist, the Jenova Project was approved under his guidance and carried out in Nibelheim. This was a project that was intended to produce an individual with the abilities of the Cetra in the modern era, and that would result in the birth of Sephiroth, child to Gast's assistants (Hojo and Lucrecia), and inheritor of Jenova's power. Some time after the birth of Sephiroth, Gast was informed by Ifalna, one of the last Cetra, that Jenova was not an Ancient, and he fled from Shin-Ra in regret of what he had done. However, Hojo continued Gast's research and would later use it to augment select candidates for becoming members of SOLDIER, Shin-Ra's elite force.
Only 5 years before the game would begin, Sephiroth would learn of the JENOVA Project and read the flawed reports identifying Jenova as an Ancient. Further, he would misinterpret them as stating that he was produced from Jenova's genetic material. Believing himself to be the sole survivor of a race abandoned to death by the species that currently dominated the Planet, he lashed out in rage and razed the village of Nibelheim, also slaughtering most of its citizens in his blind wrath. He then attempted to retrieve the remains of Jenova from the nearby Mako Reactor, but due to the interference of Cloud Strife, only managed to escape with the creature's head, retreating with it into the Lifestream. In the time that followed, Sephiroth would gain mastery over Jenova's powers, essentially acting as an evolved form of the creature. Its own mind -- that of an instinctual virus seeking to replicate -- ceased to be and the mind of Sephiroth was all that remained.
After drifting through the Lifestream, Sephiroth arrived at the site of Jenova's impact. While in the Lifestream, he had absorbed the knowledge of the Cetra and now set in motion a simple, yet devastating plan--scarring the Planet, so that when it healed, he would be situated in the middle of the wound. He would then absorb the Lifestream's energy and be reborn as a "god," becoming the Planet itself. Jenova's body was moved to the Shin-Ra HQ in Midgar, where it remained for five years until the events of Final Fantasy VII, when Cloud tried to save Aeris. It was then that "Sephiroth" reappeared and took Jenova's body after murdering President Shinra and a multitude of the Shin-Ra personnel working in the building.
In the time immediately following Sephiroth's departure into the Lifestream, Hojo, believing Sephiroth dead, began experimentation on other humans, specifically the survivors of the Nibelheim Incident. Injected with Jenova cells and showered with mako, all but Zack suffered from mako poisoning. Due to their escape, neither Cloud nor Zack were numbered with tattoos as the later subjects were. The results of the experiment were known as 'clones', but this was only for their similarity to the Sephiroth experiment, not for being genetic copies.
Mother is waiting...