For most of human history, people have sought tricks to extend their lives, but new research suggests that one key may be not having a father.
Researchers in Japan created 13 mice using DNA material from two mothers, and compared their lifespans with mice born under normal circumstances. They found that the mice with no father lived roughly 842 days, with control mice living 186 fewer days, on average, findings that suggested a benefit to not having a father.
"We have known for some time that women tend to live longer than men in almost all countries worldwide, and that these sex-related differences in longevity also occur in many other mammalian species," said Tomohiro Kono, one of the study authors from the department of bioscience at Tokyo University of Agriculture, in a statement. "However, the reason for this difference was unclear and, in particular, it was not known whether longevity in mammals was controlled by the genome composition of only one or both parents."
In their paper, the researchers noted: "Our results further suggested sex differences in longevity originating at the genome level, implying that the sperm genome has a detrimental effect on longevity in mammals."
But while researchers have hit on a provocative issue about the possibilities of genetics -- namely whether having two mothers is better than having a mother and a father -- it's unclear how applicable those theories are.
"In laboratory rodents kept under controlled conditions the relationship of sex to longevity is variable, with males sometimes showing greater longevity than females and with life span being dependent on factors like breeding and diet," the late Dr. David W. E. Smith, a pathologist at Northwestern, wrote in a 1988 paper on male and female longevity.
That variability has led to a suggestion that the entire paper must be scrapped, interesting though some of its findings may be.
"If you look at the record for this particular type of mice, the males live longer than the females. So this was an inappropriate model to deal with this question," said David Harrison, a professor at the Jackson Laboratory in Bar Harbor, Maine. "The whole hypothesis cannot be tested using this strain, because the males live longer than the females."
Harrison cited past research from the Jackson Laboratory and a study from the Tufts University School of Veterinary Medicine showing, he said, a lifespan of approximately 973 days for females and 1,065 days for males in the strain used in the study.
In addition, Harrison noted, the lifespan of typical mice in the study was much shorter than those average life expectancies, something he said revealed possible flaws in the Japanese laboratory. (The lifespan of mice with two mothers was close to typical.)
"Their explanation here has got to be wrong," he said, although he added, "I have no idea what could be wrong."
As of press time, Kono had not responded to a request from ABC News for comment.
Parenting Leaves An Imprint
The notion of creating an embryo from two parents of the same sex has been a challenge to scientists, who would need to overcome a phenomenon in genetics known as imprinting.
After DNA is passed by parents on to their offspring, it undergoes a process known as methylation, where a molecule of carbon and hydrogen attaches to DNA, marking whether it came from the mother or the father, and dictating how it will impact the embryo as it grows.
This process has been a barrier to same-sex parenthood, since the methylation prevents an embryo with same-sex parents from developing normally.
"The reason you can't ordinarily do that…I would guess isn't as related to aging as to the way males and females have different interests," said Harrison.
Since males are not guaranteed to be the partner of the female, they would have more interest in having a child grow quicker to ensure immediate survival, for example, while a female -- who often needs to raise the children -- would have an interest in smaller children who are easier to raise. That idea is known as the parental conflict hypothesis.
"There's some tendency for the male imprinting pattern to increase growth rates," said Harrison.
In their paper, the researchers noted that smaller size -- a feature of female interests -- seemed to correspond with longer life.
But Harrison noted that "generally, the smaller mice live longer. However, smaller mice don't always live longer, it's a generalization."
But longer lives may not always be an evolutionary advantage.
"Post-reproductive lifespan is going to be irrelevant, primarily in the context of survival of one's offspring," said Keith Latham, a professor of biochemistry at the Fels Institute for Cancer Research and Molecular Biology at Temple University.
Aa Machiavellian as it may sound, longer life, he said, has little evolutionary benefit once an animal has had children, but can be harmful to the population of animals, since they still need things like food.
"Shorter lifespan, in a population, would tend to eliminate post-reproductive individuals and [expand] the environmental resources available to the population," said Latham.