Growing up in troubled times strongly affects lifespan in baboons, a new study shows. Although previous studies had demonstrated that early adversity can affect health and longevity in another social primate—humans—this is the first birth-to-death study in a wild animal that links multiple forms of early hardship to shortened adult lifespan. This means that a shorter lifespan may not be a consequence of simply the stresses of living in modern societies but rather a feature that has long been part of our evolutionary history.
Finding a definitive link between experiencing tough times as a youngster and dying earlier as an adult has proven difficult. Studies in humans have suggested that adversity in childhood is associated with the development of poor health habits, such as smoking and alcohol use, as well as poorer access to healthcare, all of which can affect lifespan. But it had been impossible to tease apart the direct biological effects of early stresses on child development from the chronic effects of lack of access to healthcare and poor health habits later on.
To simplify matters, researchers turned to wild baboons. The animals can’t smoke or drink alcohol, and healthcare isn’t an issue. Scientists with the Amboseli Baboon Research Project, which was initiated by Jeanne Altmann of Princeton University in 1971, used data collected from more than 1,500 savannah baboons in east Africa to investigate whether early life stresses, such as drought, low social rank of the mother, social isolation of the mother or the loss of the mother, would shorten the lifespan of the baby baboons that survived to adulthood.
The researchers looked at how many adverse events baby baboons experienced and then watched what happened to them once they became adults. Baboons live in large social groups. Males tend to leave the group upon reaching sexual maturity, while females stay home. For that reason, the team focused on females; males are a lot harder to keep track of.
Baby baboons that did not experience any adverse events lived about 10 years longer, once they reached adulthood, than those who experienced three or more of these episodes. Baboons that experienced more than three sources of adversity had a median lifespan of only nine years compared with 24 years among animals that had experienced no adversity. “It was a shockingly large effect,” says one of the project scientists, Susan Alberts, an evolutionary biologist at Duke University. The findings appear today in Nature Communications.
“Females who got a good start in life, who were born of high-ranking mothers when there was a lot of food around, lived a lot longer than females who did not get a good start in life,” says Joan Silk, an evolutionary anthropologist at Arizona State University who was not involved in the study. “Although it is intuitive that this might be the case, no one had ever shown this before. No one had the kind of the data that you need to show this,” she adds.
The vulnerability of primate species such as baboons and humans to early adversity has deep roots in evolutionary history, the researchers say. “Early life disruption of development has long-term effects on lifespan, and that is probably something that has affected our broader lineage for a long time,” says Jenny Tung, an evolutionary anthropologist at Duke University.
Experiencing rough patches early in life changes the developing brain and body in ways that affect health for the rest of an individual’s life. “So it's not just these differences in health habits that are the mechanism—there's also these fundamental mechanisms of biology of how the organisms are put together during development that lead to these differences in lifespan,” says Elizabeth Archie, an evolutionary biologist at the University of Notre Dame.
Although a shorter life may result from early adversity, it is possible that this vulnerability is not a bug, but a feature of development, says Alberts. She wonders, “What is the upside of being so sensitive to these things like social isolation and social status?” It is possible that hardships of this sort force organisms to make adjustments during development that have negative consequences in the long term, even if they help with immediate survival, she says.
When baby baboons are faced with a lot of really bad things happening, they might change their development to adapt to life in poor conditions, and those changes could indirectly affect lifespan, Silk suggests. “Maybe all the early life adversity events are telling you something about the world you have to live in, and you have to adapt to that in certain ways,” she suggests, “and maybe there's a tradeoff with longevity.”
No one knows the exact mechanisms that account for the effect of early adversity on lifespan, but the researchers suspect that stress hormones and changes in the epigenetic markers that control gene expression might be involved.
Though this is a study of baboons, the research implies that suggestions that banning tobacco and alcohol and giving healthcare to all people would give everyone an equal chance at a long life might not be enough, says Tung. “Animal research is very important in suggesting that [inequality] is not simply a health policy problem, she says, “but that you actually have to look at the physiology and biology of organisms as well.”