A Methuselah tree in California’s White Mountains is a real old-timer. According to recent research, this ancient bristlecone pine turned 4,851 years old in 2020. This makes it one of the oldest individual organisms on earth.
The Methuselah is not the only organism making recent headlines about longevity. Researchers announced last week that they had found beds of kelp off the Shetland Islands, and in Irish and French Atlantic waters, that had survived for 16,000 years. And an international group of scientists announced that they had revived microbes that had lain dormant in mud on the sea floor for 100 million years.
The endurance of life
The physiological limits of life on earth can be pushed to spectacular limits — although each of these feats shows different aspects of the ageing process.
The Methuselah tree’s longevity shows the remarkable endurance of a single living thing. The kelp story reflects the extreme age of a population, but not of its individual plants, while the revival of ancient seabed microbes shows how suspended animation can be extraordinarily long-lasting. Nevertheless, there is a common theme to these research results, as an expert on ageing, Professor Tom Kirkwood of Newcastle University, points out: “These stories all speak to the endurance of life, and at a time when we have become fixated with ideas about our own mortality.”
The diversity of lifespans on earth is certainly extraordinary. Mayflies live for little more than 24 hours in their adult form, while small, freshwater organisms called hydra — which can regenerate damaged tissue — are considered to be effectively immortal. The fruit fly, from egg through larva and pupa to adult, lives for only 14 days, while the world’s oldest known land animal, a 188-year-old giant tortoise named Jonathon, is still happily wandering around his home on the grounds of the governor’s residence on the island of Saint Helena.
Differences in longevity
This great variation in lifespans raises a fundamental question, however: why do some animals and plants live to a very old age while other, similar lifeforms spend extremely short times on earth? One of the reasons is simple: animals that are prey tend to have shorter lives than predators, even though they have similar bodyweights. Everything is trying to get them, in other words.
This point is illustrated by the examples of the mouse and the bat. Both are small mammals, but they have very different life expectancies. A mouse is lucky to survive a year in the wild and will die within three years even if it is kept as a pet. By contrast, most bats live until they are around 30 years old, with some making it to 50 or more. So, why such a difference?
Part of the answer is simple, says David Clancy of Lancaster University. “One has wings, and the other does not. That means the mouse is prey for many different land predators, from cats to humans. By contrast, the bat has no real enemies because it can always fly from danger.”
However, it is not only the avoidance of predators that gives the bat its longevity, adds Clancy. Even when there are no predators, a mouse’s lifespan will probably still be only a tenth of that of a bat. There is a basic difference in the two creatures’ internal programming — involving processes such as the repair of damaged DNA — that is the reason for the difference in longevity.
Cell maintenance
“A mouse’s internal processes don’t invest very much in cell maintenance,” says Kirkwood. “There is no point, because the mouse is not going to live for very long. Why bother to keep cells in good repair if you are doomed to die in a year?”
Cell maintenance is a costly business that uses up internal energy resources, and an animal will provide just enough to keep itself in good shape for as long as it is likely to survive in the wild.
“Instead, the mouse ‘bets’ its survival strategy — unconsciously, of course — on having large numbers of babies as quickly as possible,” adds Kirkwood. But what would happen if the mouse evolved wings, he asks. It could then fly about and avoid predators, just like a bat. “However, its internal processing will still be providing poor maintenance to cells. It will die fairly quickly and so waste the benefit of evolving and growing those wings.”
Humans and chimps
Evolutionary change would then begin, according to Kirkwood’s hypothesis, which is known as the disposable soma theory. Mice with wings would start to live longer firstly by avoiding predators, while those flying mice with slightly better cell repair systems than others would live even longer and so begin a gradual improvement in cell maintenance, which, over generations, would produce a flying mouse — a bat, in other words — that lives for much longer. And that is what we see in nature, says Kirkwood.
A similar effect is seen in humans and chimpanzees. The two species evolved from a common ancestor about seven million years ago. “Over that time, we have evolved bigger and bigger brains that have made it easier for us to develop strategies for surviving for longer periods, and so our cell maintenance services have had to improve,” says Kirkwood. As a result, he adds, our lifespans are now twice those of chimpanzees.
“The crucial point to realize is that organisms didn’t evolve to die,” adds Clancy. “They evolved to survive. The first organisms that appeared on earth may only have had very short existences, but as new niches were found, expectations of living longer have slowly risen. Today, as a result, life endures for varying lengths of time across the globe in just about every possible place.”
© Guardian News & Media 2020