The World’s Oldest Animals: Why are they so old?

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The Greenland shark recently hit the record for the world’s longest-living vertebrate, whose average lifespan was estimated by Carbon dating to be almost 400 years (392 ± 120 to be exact). This is a phenomenally long time. To put this in context, a shark that died this year might have been born when we still thought the Earth was flat and could have reached sexual maturity as the American revolution began. This sort of timescale for a life is pretty mind-boggling by human standards, but shockingly there are plenty of other creatures whose lives extend even longer. Many species of trees live for thousands of years, some microorganisms for 10,000 years, and some aquatic animals appear not to age at all – some can even age backwards. Although rare in nature, this kind of extreme longevity is fascinating – the following article will try to explain why it occurs, and whether we could ever find the fountain of youth ourselves.

Hint: For those who want to know more, explanations of underlined words can be found in the Jargon Buster at the bottom of the article.

Live fast, Die young

Let’s come back to the Greenland shark – the reason they age so slowly is essentially because they don’t do much. It is cold blooded, so gets the majority of energy for metabolism from being heated up by the surroundings. Living in the arctic ocean, there’s not much heat available so it has to release its energy slowly and use as little as possible. It has a very slow metabolism, swims at just over 1 km/hr and typically scavenges or eats sleeping animals rather than chasing down prey.

Why does living slower help it to live longer?

Having a slower metabolism means the chemical reactions that occur in its body take place more slowly. Metabolic chemical reactions often produce very reactive molecules called free-radicals as by-products, which can damage cells. This cumulative damage is thought to contribute towards ageing – so by having a slower metabolism, free-radicals are not produced as frequently, and their potentially ageing effects are felt less often.

So, there is actually some scientific merit to the phrase ‘live fast, die young’. Well – sort of. You might wrongly conclude from the above that the secret to living longer would be to lie down all day and eat less, but this would make us very unhealthy. This is because there is actually nothing we can do to alter our basal metabolic rate (our metabolic rate at rest), which is responsible for 60 – 75 % of calories burned – it is this that is responsible for most of the free-radical production.

Although it might live long, it essentially lives in a zombie-like half-slumber. In its 400 year life, it does nothing much but cruise around eating carcasses while its eyes are slowly rot until it’s blind. I’d rather spend 80 years having fun.

That’s not a tear, that’s a parasite that commonly infects the Greenland shark’s eye, rendering most of them near blind. They get so old they’re practically rotting alive. Image source: BBC

 

The curious case of Benjamin Button

The mysterious beetle, whose larvae can age backwards

Now we get onto the really weird science. The larvae of the beetle Trogoderma glabrum can actually grow backwards to an earlier stage of their development. When the larvae have enough food, they usually shed their skin, growing larger and larger until they eventually morph into insects. However, when starved of food, they can preserve their energy by shedding their skin and growing smaller to previous stages of development – the idea being that the small and shrinking larva requires less food to sustain itself than the large and growing one.

By this ‘reverse development’, the beetle can extend its lifespan from 8 weeks to around 2 years. The beetle still ages though – otherwise it would simply never die. The free-radical damage mentioned above for example, still damages its tissues and it eventually deteriorates to a point at which it can no longer live. Like nearly all organisms, its tissue cells can only divide a certain number of times – known as the Hayflick limit – which is the biggest limiter to life.

Live forever?

However, there are some animals whose lifespans don’t appear to be capped, so could theoretically live forever. An example of this is the Hydra – an animal related to Jellyfish. To explain why this is, we need to look more closely at the cells of such organisms. Cells themselves have a limited lifespan, but divide to give new cells allowing for growth and maintenance of tissue. As mentioned before, typical cells can only divide a certain number of times – known as the Hayflick limit. This limit exists because structures called telomeres which allow DNA in cells to replicate, gradually become shorter after each division – until they eventually become too small for another division. At this point, DNA can’t replicate without exposing itself, so cells can’t divide anymore. There are some cells that don’t follow this rule – stem cells, for example. Some stem cells contain an enzyme called telomerase, which helps to repair the telomeres allowing them to keep dividing forever. The Hydra is thought to have plenty of these stem cells, which can continually divide to their hearts content, creating new tissue via differentiation. Why don’t we all just inject ourselves with telomerase to stay forever young? Well, some cancer cells can create this enzyme too – some cancer cells can also infinitely replicate, hence why they’re so uncontrollable – so maybe this isn’t the best idea.

Cancer cells under microscope. Image source: The Daily Mail

Since the Hydra’s cells continue to replace themselves, the free-radical damage doesn’t actually age the Hydra in the same way because it can replace damaged tissues indefinitely. It’s hard to actually age hydrae because their cells change so much throughout their lives, but some estimate that the oldest have been around for 10,000 years. Although immortal, they’re not invincible – like everything, they’re susceptible to disease and accidents.

The ‘immortal’ Hydra. Image source: Biology Stack Exchange

Something else interesting is one of its genes, FoxO. When this gene is suppressed, the Hydra ages – so the gene could be key to keeping it young. What’s even more interesting is that the same gene is also found in humans. But clearly in the Hydra it’s expressed in such a way that allows it to continually replenish itself – how this is or why, nobody knows for sure.

So what’s in it for us?

All of the creatures mentioned are deeply fascinating, but the very reasons they leave so long are probably not within direct reach of us. These organisms all have radically different lifestyles and traits to what make us human. If we were to live so long, we would in most likelihood cease to be anything we could recognise; it probably wouldn’t be worth it anyway. That said, more research into genetics and stem cells could help us to understand why we get old and help us to combat some of its effects – cancer, Parkinson’s, dementia and countless other diseases that make being old suck.

 

Jargon Buster

  • Carbon-dating is a technique used by scientists to predict the age of very old objects made up of Carbon-containing molecules, for example: Ancient Egyptian mummies, bones and now the Greenland shark’s eye lens. It exploits the presence of a radioactive form of the element Carbon: Carbon-14 (14C). 14C is present in low amounts in proteins and other Carbon-containing molecules that make up living things when they are first formed. 14C decays over time, so the amount of 14C in these objects will decrease over time. Measuring the amount of 14C in the object can therefore be used to predict its age: the older the object is, the less 14C it will have.

  • All the chemical processes in a living thing that are required to keep it alive.
  • A free-radical is a molecule that is very unstable. In basic terms it is unstable because it has an unpaired electron – electrons are more stable in pairs, so it reacts with other molecules to desperately try and get an electron to complete its pair, or rid itself of an extra electron so that all electrons on the molecule are paired up. Because of this reactivity, free radicals can react with proteins, DNA and other important molecules in our cells that could have a damaging effect. The free radical theory of ageing suggests that the accumulation of this damage is what causes ageing. There are many variants and alternatives to this theory – it isn’t unanimously agreed upon, but it’s clear free radicals play a part. Free radicals aren’t all bad though – they are important in biology as signalling molecules, which help the body to regulate its different processes. 
  •  In order to divide, a cell must replicate all of its DNA. DNA in cells is wound up into structures called chromosomes. At the end of each chromosome is a structure called a telomere, which becomes a little shorter after each division. When it eventually becomes too short, then another division of the cell would mean the telomere could no longer protect the rest of the chromosome – the DNA within them would ‘unravel’. The telomere is essentially just more DNA, but it is non-coding meaning it doesn’t do anything, so it doesn’t matter when it is worn away (unless you want to live forever).  
  • An enzyme is something which helps a chemical reaction or process in the body to get going and maintain it. In the case of the enzyme telomerase, it help a reaction which restores the telomeres, allowing cell division to continue with no theoretical limit.
  • Differentiation is the process by which unspecialized cells (stem cells) become specialized. A specialized cell is one that is adapted for a particular purpose, e.g. a red blood cell is adapted for carrying oxygen in the blood, a nerve cell for transporting information through electrical signals.
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