All people get older. It is part of our biology and limits our lifespan to just over 120 years.
Not all animals experience aging during their lifetime. Some animals’ bodies do not gradually degenerate with age, like our bodies.
But for people who reach about age 30, their chance of dying doubles roughly every eight years. So even if you are lucky enough to live to be a centenarian, your chances of dying every year will be high.
This high mortality reflects many other health problems, such as loss of muscle mass and general frailty, cognitive decline, loss of vision and hearing, and many other degenerative changes that characterize the human aging process.
And the reason humans age so dramatically may be due to the fact that our ancestors evolved around the time of the dinosaurs.
Compared to other mammals, humans have long lives. We have the longest lifespans of any land mammal, and of all mammals only whales are likely to outlive us. I say ‘probably’ because you have to keep animals in captivity to do a detailed study of lifespan, which is virtually impossible for whales due to their size and long lifespan.
We know that whale and dolphin species exhibit menopause, and that all mammals exhibit some form of reproductive decline with age. In fact, all mammals studied show physiological aging and increased mortality with age, even though some species – such as mice and voles – age much faster than others – such as humans, whales and elephants.
But many species of reptiles, amphibians and fish show no signs of aging. Examples include turtles and tortoises, salamanders and rockfish.
A study of 77 species of reptiles and amphibians, published in Science in 2022, shows that age-related increases in mortality are not observed in many reptile and amphibian species. It’s as if these animals don’t age at all. Some of these animals, such as turtles, probably live longer than humans.
If we study these apparently non-aging species long enough, perhaps they will show signs of aging. But good luck studying animals like the Greenland shark, which is estimated to live almost 400 years.
For now, at least, we can say that among reptiles, amphibians and fish, some species not only live longer than the longest-lived mammals, but also age significantly more slowly. Furthermore, some of these non-aging species continue to grow throughout their lives, meaning older females lay more eggs, again in stark contrast to what happens in mammals.
These animals die mainly because they are eaten by predators and diseases. Most wildlife does not die of old age, and of course most people died from infectious diseases until the 20th century.
Some reptiles, amphibians and fish are also known for their ability to regenerate tissue.
Press mammals
Amphibians evolved from fish about 370 million years ago, and reptiles evolved from amphibians about 50 million years later. Mammals then evolved from reptiles about 250-300 million years ago.
We are all products of evolution, which we see in relics like our tailbone. Our evolutionary history can have a profound influence on modern times. For example, humans retain evolutionary traits from when our ancestors roamed the savannah that are no longer suited to the modern world, from sugar cravings to behaviors that lead to prejudice.
About 200 million years ago, massive volcanic eruptions wiped out 76% of marine and terrestrial species. Afterwards, dinosaurs became the dominant predators in the country. To survive and avoid being hunted to extinction by dinosaurs, mammals became small, nocturnal, and short-lived.
Our ancestors of this time were not at all like us. They looked more like voles and mice, small animals that set out in the dark to catch insects. Under pressure from the dinosaurs, ancestral mammals had to reproduce quickly, just as mice and rats do today. And like mice, rats and voles, our ancestors had short lifespans.
For a hundred million years, during the time of the dinosaurs, mammals were at or near the bottom of the food chain. Mammals were more often prey than predators. During this time, there was no reason for mammals to maintain processes and genes associated with longevity, such as DNA repair and tissue regeneration systems.
My longevity bottleneck hypothesis posits that repair and regeneration systems were lost, mutated, or inactivated through the evolution of early mammals. This imposed biological constraints that determine how mammals age to this day.
After the dinosaurs disappeared when an asteroid hit Earth 66 million years ago, mammals conquered the world. An astonishing diversity of species with a variety of lifespans emerged. Some species, such as humans, have evolved long lifespans, but they may have done so under constraints, left over from the age of the dinosaurs.
Why dinosaurs made a difference
We can take a chance by looking at species that haven’t undergone the same evolutionary pressures as early mammals.
The tuatara, a reptile endemic to New Zealand, may look like a lizard, but it diverged from snakes and lizards about 250 million years ago. Due to its slow evolution, it is sometimes called a ‘living fossil’.
Tuataras are believed to live more than 100 years and age much more slowly than humans, as a 2022 DNA analysis study found. Perhaps they have retained their anti-aging genes, unlike even the longest-lived mammals.
Our lifespan may be limited because of our evolutionary history.
Joao Pedro de Magalhaes, Chair of Molecular Biogerontology, University of Birmingham
This article is republished from The conversation under a Creative Commons license. Read the original article.
