They say death and taxes are the only sure things in life, but is the former really so absolute and inescapable?

After all, with considerable gains in human life expectancy, and scientists discovering new ways to reverse aging and potentially extend human lifespans, one could be forgiven for thinking that the legendary elixir of immortality might be real after all.

Sadly, that's not on the cards, according to a sweeping new analysis of mortality patterns across both human and non-human species.

"No matter how many vitamins we take, how healthy our environment is, or how much we exercise, we will eventually age and die," says statistician Fernando Colchero from the University of Southern Denmark.

In a new international study by Colchero and 39 other scientists, the team found that the rate of aging is relatively fixed within species, even if species can figure out ways to maximize overall survival, which leads to individuals living longer lives on average.

Take humans, for example. While the maximum human lifespan has steadily increased by around three months every year since the mid-1800s, this doesn't necessarily mean that we've actually discovered a way to slow biological aging or push past what could be a hardwired limit on human longevity.

Rather, an alternative view – called the 'invariant rate of aging' hypothesis – theorizes that the rate of aging within a species is fixed, suggesting that gains in life expectancy don't show evidence of slower aging processes, but instead reflect lower levels of death in young individuals over time.

"In historical populations, life expectancy was low because many people died young," says demographer José Manuel Aburto from the University of Oxford.

"But as medical, social, and environmental improvements continued, life expectancy increased. More and more people get to live much longer now. However, the trajectory towards death in old age has not changed."

In the study, the team looked at the relationship between life expectancy (the average age at which individuals die) and lifespan equality (which measures the concentration of deaths at different ages). High lifespan equality means most people survive into old age, whereas low lifespan equality would mean people die at a range of distributed ages.

These mortality patterns were compared across a diverse set of human populations (including peoples from the 17th to the 20th century, and records from two hunter-gatherer groups) together with data from 30 non-human primate populations, including monkeys, baboons, gorillas, and chimpanzees.

Ultimately, the analysis suggests that the same general pattern of mortality exists across groups, suggesting differences in life expectancy and lifespan equality are primarily driven by age variations in which the young members of a population die, not the same kinds of variations in the old.

If lifespan equality can be increased – with less individuals dying young – then overall a population tends to live longer, the researchers say, but there is little evidence so far to suggest that slowed or delayed aging in older adult individuals has much effect on life expectancy.

"Populations get older mostly because more individuals get through those early stages of life," says evolutionary anthropologist Susan Alberts from Duke University.

"Early life used to be so risky for humans, whereas now we prevent most early deaths."

Of course, absence of evidence does not equal evidence of absence. The researchers acknowledge that declines in the baseline level of adult mortality since the middle of the 20th century "have very likely played an increasingly important role in industrialized societies". But so far, we're not seeing a huge signal of that in the data.

Otherwise, the research suggests that improvements in the environment – for instance, access to modern medicine and good nutrition, in the human context – are unlikely to translate into slower aging processes that could lead to a dramatic increase in lifespan.

But it does remain a possibility, the researchers say, even if their own findings point elsewhere.

"It remains to be seen if future advances in medicine can overcome the biological constraints that we have identified here, and achieve what evolution has not," the study authors write in their paper.

The findings are reported in Nature Communications.