Jack Hart was born in New Haven, Connecticut. He is the son of Philip Hart, a scientist who created an energy source called "zero fluid", and Marie, an alien humanoid woman of the Contraxian race. After being exposed to a vat of zero fluid, Jack underwent a mutagenic change and gained the ability to project concussive energy from his body. After gaining these powers, Jack began his mission of vengeance against the corporation that killed his father. This led to Jack fighting other superheroes such as the Sons of the Tiger and the Hulk.
Later, Jack moved past his mission of blind vengeance and became a true hero, working with various other superheroes such as the Defenders, the Thing, Quasar, Spider-Man, Moondragon and the Silver Surfer. Jack Hart was reunited a college girlfriend named Marcy (who was also a fellow graduate student of Peter Parker, aka Spider-Man). Marcy, a scientist, and SHIELD offered to help find a cure for Jack's dangerous super powers. Marcy turned out to be a Contraxian agent sent to Earth to find Jack, and it was at this time Jack learned of his half-Contraxian heritage. Marcy took Jack to Contraxia where he saved the homeworld of his mother from its dying sun. The Contraxians could not find a solution for Jack's unstable powers, and he left Contraxia to wander space. While wandering space his containment suit was breached in battle with The Presence which led him to seek aid from the Silver Surfer, who found an alien armorer who designed a new containment suit for Jack. Shortly after this adventure, Jack fell in love with the alien warrior woman known as Ganymede, though she did not return his feelings. Eventually he joined the Avengers, becoming the fifty-second superhero to join.