As a child, Sarah was one of the underground-dwelling Morlocks. When Callisto, the Morlocks' leader, kidnapped the X-Man Angel to be her husband, Sarah came across the captive Angel. Sarah believed Angel was beautiful. He would become an idol of worship for her.
When the Marauders massacred the Morlock community, a young Sarah was saved by Gambit, who was not yet known by the X-Men. Years later, most of the surviving Morlocks (including Sarah) were apparently killed by Mikhail Rasputin, although they were actually brought to another dimension. There, Mikhail had established a citadel for himself on the top of a massive hill. The Social Darwinist philosophy of "survival of the fittest" was the only thing which governed the society. If someone could reach the summit of the hill they were considered "fit" and worthy of being part of Gene Nation.
The former Morlock leader Callisto cared for Marrow during this period and displayed a maternal attitude towards the young mutant. When her powers fully manifested she was forced to "up the hill", becoming more savage and killing her opponents to survive. When she managed to reach the top, Mikhail considered her fit to join his elite fighting team Gene Nation, which eventually became a terrorist group whose mission was to punish humans for their hatred towards mutants.
Time passed quickly in that harsh dimension. When Marrow and several other members of Gene Nation (of which she had become leader) returned to Earth, they discovered only a few years had passed during their absence. Marrow and the other members of Gene Nation began a homicidal terrorist campaign against surface-dwelling humans.
After inconclusive battles with the X-Men and Generation X, Gene Nation was defeated by the X-Men. Marrow wired a time bomb to her own heart to force the X-Man Storm, who had herself led the Morlocks in the past, to either yield or kill her; Storm tore out Marrow's heart. However, due to Marrow's possession of two hearts to compensate for her random bone growth and superhuman regenerative abilities, she survived.
Marrow eventually reunited with Callisto, and while they continued their terrorist activities, including an attack on a Lila Cheney concert that brought them into battle with Cable, Callisto tried to hold back Marrow's more violent ways.
When Marrow and Callisto attacked Henry Peter Gyrich for his involvement in Operation: Zero Tolerance, Callisto was injured by a Prime Sentinel. She directed Marrow to seek the aid of the X-Men. After teaming up with Iceman and Cecilia Reyes, Marrow went to the Xavier Institute. She joined the X-Men for a time, coming under the mentorship of Wolverine and flirting with her teammate Cannonball.
During an adventure in another dimension, Gambit, unable to control his powers, accidentally injured Marrow. She was later hospitalized on the Skrull home-world, and thanks to the alien technology Marrow gained more control over her powers, giving her a "prettier" appearance. This led to major softening of her personality, to the point where she reached out to Jubilee following Wolverine's then recent disappearance. Following the reorganization of the X-Men's roster she disappeared from the X-Men.
She was next seen brainwashed and used as a sleeper agent by S.H.I.E.L.D. under the alias of "Sarah Rushman" (an alias that had also been used by the Black Widow). Her mission was to terminate rogue Life-Model Decoys. She had a split personality (Mutant Marrow and human teenager Sarah) and had to be periodically injected to keep her implanted memories. With Spider-Man's help, she freed herself from S.H.I.E.L.D.'s control by faking a suicide.
Marrow was later recruited by the most recent incarnation of Weapon X, who again normalized her appearance and her powers. She betrayed the Weapon X organization upon discovering their anti-mutant nature. She reformed the Gene Nation terrorist cell and, as their leader, led several terrorist attacks on Weapon X until the rest of the group was slaughtered by Agent Zero. Agent Zero left her alone, not wanting her to become a martyr, but he warned her that he would come after her again after he had hunted down the remaining members of Gene Nation.
Marrow was next seen as a spokesman of a band of Morlocks after M-Day. She gave an interview to Sally Floyd for her ex-mutant diaries. Marrow remained underground to protect and give hope to the few mutants that remained and those that feared going to the surface to live normal lives. Marrow's interview was heavy with subtext towards the upcoming Superhero Registration Act. Marrow told Sally Floyd that the louder Congressman Sykes called for mutant registration, the easier it would be for him to hide the truth. Americans were already labeled and registered a hundred different ways, such as social security numbers and credit cards. Such indignities afforded the privilege of police and ambulance services. Spider-Man explained that he trusted Sally Floyd because of this interview with Marrow.
Marrow came to associate with the terrorist X-Cell, confronting Siryn and M before they could assault Blob and Fatale. She still sported several bone growths on her forehead, but de-powered, she was forced to use throwing knives as weapons. She initially wanted Quicksilver to use the Terrigen Mist to restore her powers but Callisto convinced her not to, since she didn't trust Quicksilver's ways.
At some point, Marrow became pregnant. During her pregnancy, she approached Volga for an experimental procedure to restore her powers, despite the great risks this posed for her child. The procedure was successful, and Marrow's powers were restored at the cost of her child, and she was left for dead in Alexandria during the Alexandria Incident.
She was found by Cable, who provided her with an inhibitor collar built by Dr. Nemesis in order to suppress her memories and control her powers. She joined Cable's new X-Force team, dedicated to providing mutantkind with a black ops team to look after their interests. Her personality was notably different as well; she acted much more manic and even childlike.
Marrow was approached by Emma Frost to join her new incarnation of the Hellfire Club. She took Marrow shopping in order for her to dress more formally for her new position. After they left the store Marrow saw Emma Frost's companions surrounded by soldiers from the Office of National Emergency led by General Callahan. Callahan paid Frost a visit due to her not living up to her end of their agreement, so he killed the Vanisher with a bullet comprised of the cure serum created by the Beast to send a message.
Marrow hung outside of a diner with Elixir while Emma talked to a revived Cyclops, unaware that Emma was present and cloaking herself from him. After their business was concluded, the next day Marrow was thrown inside Emma's penthouse by a O.N.E. soldier who got her out of the way so Callahan could talk to Frost. Later that night, Marrow was in the Hellfire Club's parking garage when Mystique, disguised as Captain America, brought a captured Mister Sinister there. While there was some small talk among the group, Marrow asked if Sinister was going to be handed over to Callahan, but before Emma could finish talking the group noticed Wolverine and Kwannon appear to break up the party.
Marrow was eventually welcomed to the new mutant island of Krakoa, created by Xavier, Magneto, and Moira X. She entered through the teleportation gateway alongside other villainous and fractious mutants, who had been invited to join the nation in order to heal mutantdom and start over as a whole species together. She later had to room with Feral but initially did not get along with her and attacked her leaving her arm in a sling. However, after finding out she was a Morlock, she found some common ground with her. She protected her from some bullies and invited her to hang out.
She joined her fellow Morlocks in defending Lowtown from the Reavers and was present when the citizens announced that Lowtown was now Mutietown. She also joined Magik's team The Dark Riders, who helped Man-Thing take back control from The Harrower and prevent her using his powers to wipe out Earth's human population. She was also in the running to be voted as a member of the newest X-Men team but lost to Polaris.
