"What is the purpose of life?" It is perhaps the grandest question ever pondered by man and the possible answers have divided the world for thousands of years.
Marissa Ayala is pretty certain of the answer, however, at least in her case.
She was born to save her older sister Anissa who, after being diagnosed with leukemia at age 16, desperately needed a bone marrow transplant.
After a year of unsuccessful cancer treatment and similarly disappointing public pleas to find a bone marrow match for Anissa, her parents made an unusual, controversial decision to have another child in hopes that that baby could provide the life-saving marrow.
Nearly two decades later, Anissa, 36, is perfectly healthy and owes her life directly to her 18-year-old sister, Marissa.
"It was really difficult for my parents at the time," Anissa said in an interview with "Good Morning America." "I think that having another child was something that I was really, really excited about. It gave me something to live for -- to be a big sister."
After the decision was made, her father, Abe Ayala, had a 14-year-old vasectomy reversed, and her mother, Mary Ayala, conceived Marissa. But they did not find out whether the child's marrow would match until Mary was six months pregnant.
"It was like Christmas," Mary Ayala said when she learned the bone marrow was compatible. "It was a wonderful, wonderful feeling. I couldn't believe that she matched perfectly."
While the Marissa's birth brought tears of joy to her family, it also brought sharp criticism from some who questioned the ethics of having one child to save another.
"When you have a little child who's going to be a donor of an organ like bone marrow, you have a sense that the child is being created for that purpose and really doesn't participate in that choice," Alexander Capron, professor of law and medicine at the University of Southern California told "Good Morning America."
Marissa, who is only two years older than her sister was when she found out she had cancer, is still working to define herself beyond the moniker of her sister's savior.
"I'm pretty much immune to everything, like the whole story -- my friends knowing, my classmates knowing, everyone knowing ... But I don't really like talking about it," she told "Good Morning America." "I want to be me. That's not my whole life and it's a part of me but I want to be defined as myself."
"Even though, yes, I gave her a second chance at life, we're just normal sisters. I never think about it at all," she added.
Anissa does think about it, though, and has taken steps to make sure the priceless gift of life is not easily forgotten.
On "Good Morning America" today she announced that she is founding a research grant called the "Marissa Eve Ayala Research Grant" with the Leukemia and Lymphoma Society.
Anissa is making the first donation, $10,000, towards a goal of $100,000.
"It's going to be in honor of [Marissa]," she said.
"It's really just brought so much joy to our lives. She's such a beautiful girl and she's such a beautiful person."