We humans are unusually long-lived in comparison to our near primate cousins, and also compared to other mammals of similar body mass. We also exhibit menopause, an end to reproductive capability well before the end of life, which occurs in only a small number of other mammalian species. With a few noteworthy exceptions, such as naked mole-rats, some bats, and we humans, mammalian lifespan correlates quite well with some combination of body mass and resting metabolic rate. So why are the outliers long-lived?
Evolutionary theorists consider longer human life spans to be a consequence of our intelligence and culture. Once it became possible for grandparents to meaningfully contribute to the fitness of their grandchildren, a selection pressure for longer lives came into being. Unfortunately that selection pressure was not necessarily for anything other than an extended decline, given that the role of grandparents in the success of grandchildren is more intellectual and cultural than physical. At that point, we diverge from other primate species over time by gaining a much longer life span.
Today's research materials are an interesting look at some of the differences between humans and chimpanzees, the latter living a little more than half as long as we do. The effects of a sedentary lifestyle are clearly quite similar at the end of the day, despite the faster process of aging that occurs in our primate relatives. The pace of aging can be measured by an epigenetic clock that assesses characteristic changes in DNA methylation that take place with age, and researchers have recently extended that line of work into a number of species, chimpanzees the latest addition.
The world's population is ageing rapidly, presenting an urgency to address the health problems of the aged. Critical insights on these problems can be gained by examining how the ageing process has been shaped over evolutionary time, and how it is influenced by different environments and lifestyles. In this issue, we feature research conducted on humans in small-scale societies and on our closest primate relatives to ask how bodies, minds, and behaviour age outside of the usual research settings. These contributions shed light on the complex relationship between ageing and disease and offer clues to the social and ecological predictors of successful ageing.
Researchers examined cardiovascular profiles in chimpanzees living in African sanctuaries. These chimpanzees occupy large rainforest enclosures, consume a diet of fruits and vegetables, and generally experience conditions more similar to a wild chimpanzee lifestyle. They measured blood lipids, body weight and body fat in 75 sanctuary chimpanzees during annual veterinary health check-ups, and then compared them to published data from laboratory-living chimpanzees. Free-ranging chimpanzees in sanctuaries exhibited lower body weight and lower levels of lipids, both risk factors for human cardiovascular disease. Some of these disparities increased with age, indicating that the free-ranging chimpanzees stayed healthy as they got older.
Prior work suggested that chimpanzees have very high levels of blood lipids that are cardiovascular risk factors - higher than humans in post-industrial societies in some cases. The work also showed that chimpanzees living a naturalistic life have much lower levels even as they age, providing a new reference for understanding human health. In biomedical research labs, chimpanzees have more limited space and often consume a processed diet (food such as primate chow), unlike wild chimpanzees.
Many humans live to see their 70s and 80s, some even reach 100 years old. But life is much shorter for our closest animal relatives. Chimpanzees, for example, rarely make it past age 50, despite sharing almost 99% of our genetic code. While advances in medicine and nutrition in the last 200 years have added years to human lifespans, a new study suggests there could be a more ancient explanation why humans are the long-lived primate.
Studies have shown that certain sites along our DNA gain or lose chemical tags called methyl groups in a way that marks time, like a metronome. The changes are so consistent that they can be used as an "aging clock" to tell a person's age to within less than four years. The new study marks the first time such age-related changes have been analyzed in chimpanzees. Researchers analyzed some 850,000 of these sites in blood from 83 chimpanzees aged 1 to 59. Sure enough, they found that aging leaves its mark on the chimpanzee genome, just as it does in humans. More than 65,000 of the DNA sites the scientists scrutinized changed in a clock-like way across the lifespan, with some gaining methylation and others losing it.
The pattern was so reliable that the researchers were able to use DNA methylation levels to tell a chimpanzee's age to within 2.5 years, which is much more accurate than current methods for estimating a wild animal's age by the amount of wear on their molars. When the researchers compared the rates of change they found in chimps with published data for humans, the epigenetic aging clock ticked faster for chimpanzees.