Here it is, my own "PB's Origin" Story.
On a day like any other, Finn and Jake were at Marceline's house for a jam session. Trying out a new song, with new instruments Marceline got, including a saxophone and a lyre-guitar (real instrument I swear).
"I think we did good." Jake stated.
"Yeah," Finn said, "but I think I need to work on my high notes."
"That's what jam sessions are for," Marceline voiced, "getting better with the instruments."
Jake strummed the lyre-guitar again.
"I like this instrument," he said, "it reminds me of playing my viola, but without the bow."
Marceline nodded, but then there was a knock at her door.
"Oh, I thought I told people not to coming over during our jam sessions." she groaned.
Marceline floated over to the door and opened it, to her surprise, finding Princess Bubblegum standing on her doorstep in her old rock shirt.
PB smiled innocently.
"Hi." she said meekly.
"Oh...boy..." the Vampire Queen sighed, "it's that time of year again?"
"What time is that?" Finn asked.
"Mothers day." Marceline stated.
Bubblegum merely nodded.
"Oh yeah," Jake said, "that's why Lady's out with the pups today."
"I'll tell you the same thing I told you every other Mother's Day," Marceline told Bubblegum, "I will tell you...when one of us is on our deathbed."
"Come on, Marce," PB begged, "this killing me, just tell me what I wanna know!"
"Tell her what?" Finn asked.
"Marceline knows where I came from," Bubblegum whined, "and she won't tell me."
"Wow." Jake responded.
"Won't tell?" Finn voiced, "Why not, Marceline?"
"Because it's a terrible story," the Vampire Queen affirmed, "I die a little inside every time I think about it."
Her words caused looks of curiosity to grow on everyone else's faces.
"Plus," Marceline said, "I already caused enough trouble, if I told she either would not believe or just legitimately hate me."
With that, the pink, gooey princess, in a fit of hysteria, grabbed onto Marceline's shirt and dropped to her knees.
"Everything's fine now," Bubblegum cried desperately, "I don't hate you! I won't! Tell me, please TELL ME!"
Marceline grabbed PB's wrists and tore the princess's hands from her shirt.
"Fine!" she hissed, her demon eyes flaring up before she regained what little composure she had as she prepared to tell her grueling story. "I'll tell you," she said, "but I guarantee that by the time this story is over, you're gonna wish you never even asked."
Bubblegum stood up.
"Well," Marceline sighed, "everyone take a seat, because this is a long story."
~Flashback(Start of real story)~
Many years ago, nevermind how many, just know that it was a many, the world was a tad different. Ooo was still young but teeming with mystical life, all of which was a result of the Great Mushroom War. There was magic and monsters, witches and demons; the human species however had become critically endangered, leaving few unmutated specimens left.
One of these survivors was a young woman, a confident and resolute scientist named Belinda. Using what little human and or organic DNA she could scavenge, she was attempting to create a brand new dominant species to replace the human beings who would be gone soon.
Belinda used a desolate, but still-standng, warehouse as a home/science lab for her and the only other human she had around, her infant daughter, Carys. It was there she conducted all her questionable experiments; because there were so few living things at hand, Belinda would even resolve to using heavily radiation-ridden corpses, be they animal or human.
Over time, with none of her attempts at creating new life working, Belinda began to grow mad and reckless in her scientific pursuit. Now, not only was she using dead things and scavenged DNA, she was even using inorganic materials, like clay, metal, and even candy, as the warehouse was once used to store confectionery.
"Why is nothing working?" Belinda growled, finding that another one of her creations had failed.
She had attempted to create a being made of human, cat, and wolf parts, all of which she had hacked off of radioactive corpses. She thought for sure that the radiation would have had an effect on the corpses.
"I will not leave this world a failure! I won't!" the woman cried, knowing that death for her was imminent, giving how fast the humans were disappearing.
Suddenly, the door of her mutation machine fell of, causing a large amount of think, pink, bubblegum to spill out.
"Aaaaah!" Belinda screamed, "Not fair, God!"
Just then, Baby Carys started crying in her play pen, which was made out of an old crate, old window screens, and a discarded gym mat.
Belinda reluctantly walked over and picked up her baby, which she kept forgetting about.
"Sorry, Carys," she sighed, holding the baby in one arm, "mommy doesn't like to yell, but God likes to make things difficult for her. So what if I'm playing God? It's not like he's done anything for me lately, or anyone!" She added, "Our world's gone, my husband is gone, my real lab is gone! Everything is gone, and I won't stand for it!"
She placed the infant Carys on a cushion on the floor, then she walked over an open window.
"Do you hear me, O Father who art in Heaven!?" Belinda shouted at the sky.
As she ranted, Baby Carys rolled off of her cushion and started crawling toward the mutation machine.
"Your time is done," Belinda yelled skyward, "it's time for a new God, a Goddess that is, I'm bringing mortal life to this world now!"
The infant Carys crawled into the machine, amongst the gooey mess of bubblegum. The baby squealed in delight, quite enjoying the squishy sensation beneath her hands and knees.
"I am God now!" Belinda yelled, "And you can't stop me!"
Just then, as Baby Carys played in the mess machine, some of the gum seeped into some exposed circuits, causing the mutation machine to activate. When Belinda heard the machine powering up, she turned around. She was shocked to find the baby cushion left unoccupied, then she heard the baby giggling in the machine. Belinda then saw that Carys was in the machine.
"No! Not that!" the woman gasped.
She ran to the machine, but it was too late; just as the machine finished the job, it exploded, throwing Belinda against the wall. The radiation and fire from the exploding machine, left the young woman severely burned, but even so, she forced herself up and went to investigate what the machine had done to her baby.
Belinda approached the remains of the machine, and found large wad of bubblegum. At first, she thought that her baby had been vaporized by the explosions, but upon further inspecting, she found that something else had happened. She soon saw that the wad of pink, gooey gum had eyes, and a mouth.
Belinda looked on in sheer horror as the gum heap looked at her and giggled. It was then she figured out what really happened: Carys had mutated with the Bubblegum in the machine, and was no longer her baby, but instead a blob of sugar-ridden, chewing confectionery.
"God...damn...damn you!" Belinda wheezed as the effects of the machine started to sink in.
She could not tell if she was dying or what, all she knew was that she could not stay there. With what little strength she could gather, Belinda got out her journal and proceeded to record the day's events up unto that very moment. She then wrote a note and attached it to the journal, then she placed the journal by the pink wad that used to be her baby, before finally fleeing the scene.
Unfortunately, she did not get very far, as she collapsed ten yards from the old warehouse. As she lay seemingly dying, she turned over to face the sky.
"Well played God." Belinda gasped, before finally succumbing to the effects of the explosion.
Inside the warehouse, the pink wad of gum that used to be Carys began to change back into a humanoid baby shape, only now the baby was made of gum, inside and out. The baby started to become more aware of its surroundings, and when she realized that she was all alone, she started to cry.
Meanwhile, not to far away, a young, teenage vampiress, Marceline, was flying around, noting the day's damage.
"More dead trees, more dead animals," she muttered, "a lot of dead things."
Marceline flew on, until she spotted an old warehouse; she decided to investigate, to see if there was anything new in it. Although, as she got closer to the warehouse, she spotted something on the ground. With a closer look, she saw that it was dead human who had apparently been badly burned.
"Well, there it is," Marceline sighed, shrugging, "another one down."
Just then, she hear a noise coming from inside the warehouse, which at first sounded like a cat. But as she got further inside, she realized that it was not a cat, but it sounded more like a baby crying.
Marceline noticed the destroyed machine and remains of dead things lying around the place; it looked like the former home of a mad scientist. It was then Marceline approached the machine and found the source of the source of the noise. A pink, squishy, crying baby girl.
"A baby!?" she uttered, "A kind of baby I've never seen before."
The baby stopped crying when it noticed the flying visitor.
"Hi, baby girl," Marceline said, "where did you come from?"
Then, she noticed the journal and the note next to the baby; she picked up the note first that told "whoever finds this" to read the last page of the journal for the explanation.
Marceline read the recommended page, which detailed how the pink baby had come to be. Shockingly, the writer had ended the page with "Whoever finds this...put her out of her misery."
She dropped the journal in disgust, then she heard the baby fussing. Marceline picked the little baby up off the ground, which made the baby happy, as pink, little bundle of joy started cooing happily.
"You don't have to be put down, you're not in pain," Marceline told the baby, "you're...sweet and gooey."
The baby started grabbing at her hair.
"Momma?" the baby cooed.
Marceline flinched, she was not sure if she really wanted be a "momma", but she could not leave the baby there.
"No, no, baby," the vampire girl said, "not "momma", I'm Marceline, Marcy, N-Nanny Marcy...yeah, that's it, I'm Nanny Marcy."
"Mah-cy." the baby squealed.
"That's right," Marceline said, "and you'll be...you'll be..."
She referred back to the appalling journal for the baby's name.
"Carys?" Marceline scoffed, "What kind of name is that? No, your name will be...uh...Bubblegum...um...Bonnibel Bubblegum. Yeah, that'll work."
The baby giggled happily.
"Oh, you like that, little Bonnie?" Marceline responded, "Yeah, that'll work. Now I just have to find a place to put you, and someone to look after you while I'm not around."
The newly named Bubblegum buried her face into her new caretaker's shoulder affectionately.
"Okay," Marceline said, "let's get to work."
~End of Flashback~
"And that's what happened." Marceline sighed.
Princess Bubblegum, along with Finn and Jake looked on in awe.
"Whoa." Finn responded.
"Crazy." Jake added.
PB tried to make sense of everything she had just heard.
"M...My...My mother...was a mad scientist?" Bubblegum asked meekly.
"Technically." she stated.
"And because I turned into a blob of gum she didn't want me anymore?" Bubblegum asked forlornly.
"From what I read of that woman's writing she was nuts!" Marceline explained, "She was scavenging, dissecting, and mixing up dead bodies like she was Dr. Frankenstein or something."
"So that's it?" Bubblegum uttered emotionally, "I was an accident? I wasn't supposed to be like this?"
"I told you it wasnt a good story." Marceline stated.
Marceline left the room and soon returned a book.
"In case you don't believe me," Marceline said, handing the book over to Bubblegum, "here's her journal that details all of her experiments and the events of that day, leading up to the...incident."
Bubblegum took the book.
"I might read it later." she whimpered.
"Well what happened after that, Marcy?" Finn asked.
"That's a different story for a different day," Marceline said, "for now, let's keep on jamming."
"Okay." Finn said.
Bubblegum stared woefully at the journal in her hands.
"Come on, Bonnie," Marceline offered, "come jam with us."
"Okay." Bubblegum sighed.
Very, very unrealistic and probably poor, but it was interesting to write.
The original version of Marceline's story was much darker, this was as toned down as I could make it without it being utterly boring.