The Guardians of the Galaxy’s origins can be traced to Earth native Peter Quill, AKA Star-Lord, betraying Yondu Udontu and his Ravagers and pre-emptively stealing a valuable Orb they were after. In retaliation, Yondu put a bounty on Quill’s head, which mercenaries Rocket and Groot were eager to claim.
Also on Quill’s tail was Gamora, a warrior assassin and adoptive daughter to Thanos. She had been loaned out as muscle to the Kree zealot Ronan the Accuser in order to help him track down the Orb. Gamora decided to betray Ronan and Thanos, not wishing the Orb and its potentially lethal power to get in their hands, and made a deal to sell the Orb to a being known as the Collector.
On Xandar, Quill and Gamora battled over the Orb, as Rocket and Groot intervened to detain Quill. During their skirmish, they were all captured by the Nova Corps and arrested. The four were sent to a high security prison, the Kyln, where they were joined by fellow prisoner Drax, whose family had been killed thanks to Thanos and Ronan. While locked up, the wary partners formulated their escape and a plan to sell the Orb and split the profits. In the wake of their breakout, the five outlaws would all come realize the threat Ronan held, and put their own lives on the line to stop him, in the process becoming the Guardians of the Galaxy.