The Assassin's Creed timeline is a messy one, and determining when exactly its mysterious Isu people, also known as Precursors or the First Civilization, were actually alive is part of that mess. Assassin's Creed's nefarious Abstergo organization has muddied up the in-universe Isu timeline, going so far as to plant fake skeletons in Ethiopia in order to provide a false answer to the evolutionary gap in humans that came as a result of the Isu altering ancient primates. Unfortunately, Abstergo's efforts were also effective on a meta-level, as fans of the series are still trying to piece together the who, what, and when of Assassin's Creed's Isu civilization. With enough digging, though, the answers begin to shine through the dirt.
In Assassin's Creed lore, the Isu were an advanced humanoid species that established the first global civilization on Earth - hence the Precursor Race moniker. They created the Pieces of Eden, powerful artifacts of technology that have the ability to control or alter humans, whom they also created. Humans were originally intended to be used as a workforce for the Isu and were controlled by biogenetic systems in their brains connected to the Pieces of Eden.
However, in Assassin's Creed 2, it is revealed that two human-Isu hybrids, Adam and Eve, were able to resist the Apple of Eden's control, stealing it and using it to begin a human uprising that resulted in a decade-long war between AC's humans and Isu. During this conflict, three powerful Isu were attempting to find a way to avoid or survive a coming solar flare that was going to hit the Earth. These three Isu - Jupiter, Juno, and Minerva - were ultimately unsuccessful, and Earth was devastated. All but about 10,000 humans and even less Isu were killed, and the survivors were tasked with rebuilding and repopulating civilization.
When Assassin's Creed's Isu First Civilization Was Alive
The Earth of Assassin's Creed is the same Earth real-world humans currently inhabit, which opens up two major questions: When did the Isu civilization live, and when did it die? Records of the Isu date back to roughly 77,000 years ago, when Consus, an Isu scientist, developed the Shroud of Eden during an Isu war. That places the peak of the Isu's life around the same time real-world peoples began forming civilizations, according to a 2016 study published in the journal Nature, via CNN. But, while the Isu were active and powerful during that time, the genesis of their civilization is still ancient and unclear.
Regardless of when they originated, Assassin's Creed's First Civilization met its ultimate destruction around 75,000 years ago. This date coincides with an in-game and real-life historical event, the Toba supervolcano eruption, which scientists have theorized caused a volcanic winter and a cooling period of 1,000 years, according to Insider.
In-game, it's unclear if the Toba eruption was caused by the fictional solar flare or if it is mistaken for it. Either way, it led to another in-game and real-life event, a "genetic bottleneck," when humanity was whittled down to only a few thousand breeding pairs, globally. In real life, it's disputed whether the Toba eruption was the cause of this phenomenon. But in Assassin's Creed, the two are directly related, and the surviving humans and Isu produced hybrids, creating the bloodline of Assassin's Creed's titular Assassins.
That is the known chronology thus far, but recent games have not been shy in expanding on it. In Assassin's Creed Valhalla, Odyssey, and Origins, it is revealed that some of the gods worshipped throughout history were actually powerful Isu whose stories have survived the passage of time - and some even survived the catastrophe altogether. With an upcoming TV show, various books, and new games, players are bound to learn even more about the "Ones Who Came Before" and how they came to be.