Human beings and other primates all inevitably age at fixed rates, according to a new study in Nature Communications. "Human death is inevitable," one of the researchers concludes gloomily in the accompanying press release. "No matter how many vitamins we take, how healthy our environment is or how much we exercise, we will eventually age and die."
The study aims to test the "invariant rate of aging" hypothesis, which posits that the rate of aging is relatively fixed within species. Bodies break down as their tissue and genetic repair mechanisms fail at species-typical rates, leading inevitably to death. The researchers explore this hypothesis by comparing patterns of births and deaths in nine human populations and 30 non-human primate populations, including gorillas, chimpanzees, and baboons living in the wild and in zoos. Their results, they report, imply the existence of "biological constraints on how much the human rate of ageing can be slowed."
To reach this conclusion, Fernando Colchero of the University of Southern Denmark and his team looked at the relationship between life expectancy—that is, the average age at which individuals die in a population—and lifespan equality, which measures how concentrated deaths are around older ages.
If deaths are evenly distributed across age groups, the researchers explain, "the result is high lifespan variation and low lifespan equality. If however, deaths are concentrated at the tail-end of the lifespan distribution (as in most developed nations), the result is low lifespan variance and high lifespan equality."
Human life expectancy has been increasing at the rate of about three months per year since the 19th century. The researchers report that most of that increase has been "driven largely by changes in pre-adult mortality." In the accompanying press release, Colchero notes that "not only humans, but also other primate species exposed to different environments, succeed in living longer by reducing infant and juvenile mortality. However, this relationship only holds if we reduce early mortality, and not by reducing the rate of ageing."
Historically, about 1 in 4 children died before their first birthdays and nearly half died before reaching adulthood. Globally, only 1 out 35 children today don't make it to their first birthday. The reduction of early adult deaths from accidents, natural disasters, and infectious diseases has also contributed to longer life expectancies. Consequently, global average life expectancy has more than doubled from just 31 years in 1900 to around 73 years now. Since more people are now dying at older ages, global lifespan equality has been increasing.
In the United States, average life expectancy at birth was 47 years in 1900; back then, only 12 percent of people could expect to live past age 65. Over the past 12 decades, life expectancy at birth in the U.S. has increased by 30 years; life expectancy at age 60 has risen by only 7 years. In 2014, U.S. life expectancy reached a high of 78.9 years before stalling out due to the rising deaths from despair among middle-aged whites and then from the COVID-19 pandemic. Nearly 88 percent of Americans can expect to reach 65 years of age.
Why do all animals, including human beings, age? One popular theory for how species-typical rates of aging emerge is that individuals are selected by nature so that they can keep their health long enough to reproduce and get the next generation up to reproductive snuff. If a body invests a lot of energy in repairing itself, it will reduce the amount of energy it can devote to reproduction. Thus, natural selection favors reproduction over individual longevity.
"Understanding the nature and extent of biological constraints on the rate of ageing and other aspects of age-specific mortality patterns is critical for identifying possible targets of intervention to extend human lifespans," the researchers note. Colchero optimistically adds: "Not all is lost. Medical science has advanced at an unprecedented pace, so maybe science might succeed in achieving what evolution could not: to reduce the rate of ageing."
The good news is that a lot of promising research on anti-aging and age-reversal interventions is advancing rapidly. In December, researchers at the University of San Francisco reported that a small molecule drug achieved rapid restoration of youthful cognitive abilities in aged mice, accompanied by a rejuvenation of brain and immune cells. Another December study found that dosing aged mice with a molecule called prostaglandin E2 can activate muscle stem cells to repair damaged muscle fibers, making the mice 20 percent stronger after one month of treatment. As we age, senescent cells accumulate and secrete molecules that cause various age-related diseases. Researchers are working on senolytic compounds that would help restore youthful vigor by clearing out these senescent cells.
The transhumanist biogerontologist Aubrey de Grey, co-founder of the SENS Research Foundation, argues that anti-aging research is on the trajectory to achieve that he calls "longevity escape velocity." That's when the annual rate of increase in life expectancy exceeds 12 months for every year that passes. De Grey recently tweeted that he thinks that there is a 50 percent chance that humanity will reach longevity escape velocity by 2036. If so, our species may finally be able to cheat aging and death.