Mass Effect - Major Events Timeline

PC Gamer had a look at the timeline for the Mass Effect series.

There's a lot to it. And there will be spoilers.

The history of the Mass Effect universe begins some time before 1,000,000,000 BCE—dates are given in the format of CE (Common Era) and BCE (Before Common Era)—when a species called the leviathans controls the galaxy. Observing a repeating cycle in which civilizations collapse after creating synthetic lifeforms that turn on their creators, the leviathans seek to break this cycle by, ironically, creating a synthetic intelligence. They call it the Catalyst. 

The Catalyst is intended to serve as a bridge between organic and artificial life, working to preserve organic species at any cost. The Catalyst, believing conflict to be inevitable, sees the best way of achieving this to be taking organic civilizations at their peak, shortly before they are responsible for their own downfalls, and absorbing their genetic material into new lifeforms that it can maintain forever. It preserves them, but only in the sense that it turns them into jam.

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Thanks Farflame!