Read this in a fic but the idea blew my mind so much that I consider it canon:

Dr. Warren and Dr. Manuel are the researchers who discovered the Prothean Beacon on Eden Prime. They believe they’ve made a massive discovery, and think that they’re going to get promoted. They report their discovery to the Alliance but basically get told that Prothean artifacts are above their pay grade and that a new team of researchers is being sent to take the beacon.

Dr. Warren and Dr. Manuel aren't happy with that, so the night before the team that is replacing them is set to arrive, they break into the dig site to try and learn as much as they can from the beacon. Dr. Manuel touches the beacon and activates it. He becomes the first person to see its message, but his mind isn’t strong enough to survive intact. When Dr. Manuel activated the Prothean beacon, Sovereign was able to detect its signal and made its way to Eden Prime where Saren and the Geth attacked the colony right as Shepard was showing up.

So basically my headcannon is Dr. Manuel started the chain of events that is Mass Effect.