The full details of Sarah’s early history remains unrevealed prior to her joining the underground community of Morlocks. She was a young child living with the Morlocks when they were targeted by the agents of the evil Mister Sinister, the Marauders, who began massacring the community. Although the X-Men managed to thwart the Marauders, scores of mutants were killed. Sarah was one of the few who survived, as she was rescued by Gambit (the leader of the Marauders to the tunnels.) Over the years, the Morlock tunnels were occupied again. Sara somehow ended up back in the tunnels, and she was raised by their leader Callisto into a young woman.
Years later, the Morlocks' tunnels were flooded by the mutant Mikhail Rasputin, but not before he teleported a select group of the Morlocks away to another dimension. Sarah, now calling herself Marrow, became the leader of a faction of the Morlocks calling themselves Gene Nation. She led Gene Nation in a mass murder at a New York club before being stopped by the X-Men.
With her companions, she eventually confronted Storm, who was once a Morlock leader, in a one-on-one battle on the anniversary of the Marauders’ invasion. Sara threatened to blow up New York's subway system, along with a hundred commuters, as payback for the mutants killed. During their battle, Storm was forced to pull out Marrow's heart to stop the bomb that was attached to it. Believing that she was dead, Storm left her in the tunnels; however, Marrow survived, having two functioning hearts as a part of her mutant physiology. After a while, Marrow resurfaced at a party with Callisto at her side, briefly clashing with Storm and her ally Cable.
During a massive attack on the nation's mutants by the government's Operation: Zero Tolerance, Marrow and Callisto were ready to kill anti-mutant activist Senator Robert Kelley, but they were stopped by the hero Spider-Man. A few days after, Marrow decided to get back at humanity by slaughtering everyone at a police department. However, the X-Men's Iceman and the mutant Doctor Cecilia Reyes were nearby, ambushed by the robots on behalf of Operation: Zero Tolerance, the Prime Sentinels. Marrow saved the two and joined them in confronting the Operation's leader, Bastion, and stopping his plans. The threesome returned to the X-Men's mansion headquarters, and Marrow was invited to stay with the team.
Marrow remained filled with hatred in regard to the world, humanity and the X-Men, whom she referred to as the "Beautiful People" due to the fact that they are mutants who happened to be normal looking. Her anti-social behavior caused major problems in the X-Men. The X-Men's Wolverine took it on himself to rehabilitate Marrow, which often sparked more fights than it solved. Shortly after this incident, Marrow revealed that she was spending double duty, healing the wounded Callisto in the Morlock tunnels while living with the X-Men. She became close to teammate Archangel, with whom she had a Christ-fixation from witnessing his struggle during the Mutant Massacre. Eventually, Marrow began to mellow somewhat and was confronted by her teammates about hiding her insecurity behind a wall of hate and violence.
Later, during a time-travel adventure with the team to the alien Skrull homeworld, Marrow was mortally wounded and put in a healing matrix. When she came out, Marrow was transformed into a beautiful young woman with apparent control over her mutant power. Marrow continued to adventure with the X-Men, and soon displayed the ability to retract her bone spikes into her body, although it still took extreme concentration. At one point, she and Colossus were captured by Mikhail Rasputin. Marrow confronted Mikhail and blamed him for making her the psychopath she had become, although, in a surprising move, she spared his life. After several more adventures with the X-Men, Marrow had learned enough control over her powers and personality that she left the team to pursue an university education.
At some point, however, Marrow was discovered by the espionage agency S.H.I.E.L.D. and brainwashed into becoming one of their undercover agents. She maintained an identity Sarah Rushman, student at Empire State University and waitress. Marrow apparently killed a record producer and was confronted by the hero Spider-Man before escaping. Spider-Man again confronted Marrow, learning that she was actually tracking down and destroying life-like androids on behalf of S.H.I.E.L.D. Marrow was still consumed with self-loathing, and despite Spider-Man's attempt to help, she threw herself off of the Brooklyn Bridge.
Somehow, Marrow survived her fall and found her way to the mysterious Weapon X program, a clandestine organization associated with experimentation on superhuman beings and with mercenary activities. Presumably, her powers were becoming uncontrollable once again and she needed help. The Director of Weapon X used an experimental procedure in slowing and stopping Marrow's bone growths, granting her a normal human appearance for the first time. Marrow was then asked to perform an assassination on the program's request, but she refused. She instead went out on the town and met a guy named Junichi, and for the first time in her life felt comfortable being intimate. Upon returning to Junichi's place, however, he revealed himself as Mesmero, an agent of Weapon X sent to watch over her. Furious, Marrow beat Mesmero to a pulp, and killed her target, just to prove herself. She returned to Weapon X and joined their mercenary team.
Over time, Marrow began to suspect that the agents of the Weapon X program were not told everything about the program and its ultimate motives. Capitalizing on this suspicion, she was approached by the agent Brent Jackson, persuaded to join a group he was forming to stage a coup with the Director. Marrow agreed, and Jackson's allies joined the mutant team Underground, led by the soldier-of-fortune Cable who hoped to take down the Weapon X program. The Underground was able to infiltrate the Weapon X complex, ousting the Director and nearly crippling the program. However, before the final defeat, agent Jackson betrayed the Underground, turning Cable's psychic powers against his allies. Jackson captured the Underground members and altered their memories to erase everything they saw and learned about the program. Marrow, in the meantime, had escaped to personally confront Weapon X's former member Sabretooth, although Sabretooth escaped when a burst of water accidentally carried him away. Marrow returned to rescue the members of the Underground and return with them to their base.
However, this proved to be a front, as Marrow tricked Cable into believing that the Underground was successful, thereby gaining information about the various Underground cells around the country. She then proceeded to convert the Underground cells into a new version of Gene Nation, hoping to once again create a mutant terrorist army. Marrow and Gene Nation were systematically hunted down by Weapon X's operative Agent Zero, who killed every member of Gene Nation but left Marrow alive to humiliate her.
Following the advent of "M-Day," when the majority of Earth's mutants were depowered, a still-powered Marrow returned to the Morlocks to help in their hour of need.