The first Mass Effect was clearly the start of something special. Bioware introduced us to the likes of the space station city of the Citadel and a number of cool and unique alien species to get to know. The game also would plant the seeds for storylines that would be explored in later games, one of which is the pro-human organization known as Cerberus.
While there is no Illusive Man or the iconic black, white and orange colorway associated with them, there a number of instances in Mass Effect 1 where Cerberus is referenced. All of which are hidden in side quests making it possible to do an entire playthrough without ever knowing Cerberus was even in the first game.
6 They Are Behind Admiral Kahoku's Missing Marines
In the Citadel Tower, you can see Admiral Kahoku near where the Council convenes to discover that he is trying to find out what happened to a squad of Alliance marines, triggering the side quest, UNC: Missing Marines. They can be found on the planet Elodus in the Sparta system of the Artemis Tau cluster.
Go directly north of where you land on the planet and you will find the marines dead and a thresher maw that needs fighting. After investigating the bodies and the distress signal that lured them there, return to Kahoku and tell him what happened. Then go to the galaxy map and he will reveal that Cerberus was responsible for what happened and that he now has to go into hiding.
5 Cerberus Later Kills Kahoku
Once the conversation ends, Shepard will be able to go to the planet Binthu in the Yangtze system of the Voyager cluster and clear out three Cerberus bases. Here, you will see that they have been experimenting on both rachni and Thorian creepers. Garrus and Tali both bring up these experiments when you recruit them in Mass Effect 2.
If that wasn't bad enough, Shepard will eventually find the dead body of Kahoku in one of the bases at the mercy of the creatures involved in the experiments, though it is later revealed that it was actually a lethal injection of some kind that killed him. Clearing the final base will trigger the UNC: Hades' Dogs quest, enabling you to avenge Kahoku and ending this lore-establishing questline.
4 ExoGeni Has Ties To Cerberus
There is actually a reason as to how Cerberus even got the Thorian creepers to begin with. As it turns out, Shepard can discover that ExoGeni is working with Cerberus.
The proof lies in a terminal on Feros in the ExoGeni facility, specifically where Shepard can activate the window shutters to take out the geth ship. It requires decrypting but doing so will reveal a log from an ExoGeni employee who expresses concerns about samples of an experiment they sent to Cerberus and that they don't trust them.
3 A Cerberus Team Turned A Settlement On Chasca Into Husks
Opening the aforementioned terminal will also trigger a side quest called UNC: Colony of the Dead. An appropriately named title considering what you will find on the planet Chasca where the quest leads you.
If you check out the civilian structures and the science station there, then you will find dragon's teeth surrounding them which means only one thing: husks. Going inside these structures will reveal that its inhabitants are all husks. While Cerberus is not named throughout the quest, it does showcase how far they are willing to go in the name of science.
2 Benezia Shipped Rachni To Cerberus For Experimentation
Benezia is an asari matriarch who is also the mother of Liara and is one of the strongest asari in the series. She was responsible for the reemergence of rachni on Noveria. It can be assumed that Cerberus received rachni from her since the rachni were extinct before a queen was hatched on Noveria.
In a series of side quests taking place in the Styx Theta cluster, it is revealed that rachni have gotten loose in some listening posts Alpha and Theta in the cluster. Clearing these places out will reveal that Cerberus was experimenting on rachni in a supply ship that clearly did not go well for Cerberus.
1 Cerberus Caused The Destruction Of Akuze
A few years before the first game, Akuze was a human colony that had been destroyed by thresher maws. It was said to be an accident but according to Corporal Toombs, an Alliance marine that survived the attack, it was no accident when confronting him on Ontarom about dead Alliance scientists in the UNC: Dead Scientists side quest.
Toombs will say that Cerberus was responsible for luring the thresher maws there as part of an experiment. They then kidnapped Toombs and performed experiments on him before he escaped. He killed the Alliance scientists claiming they were part of Cerberus and plans to do the same to Dr. Wayne, the sole surviving scientist. As it turns out, he was right as it is revealed in Mass Effect 3 at Cronos station that they did in fact cause it.