Some microbes can live for millions of years – perhaps even for a quarter of a billion years. How do they avoid succumbing to the inevitable wear-and-tear of old age?

Some corals live for thousands of years. American lobsters can live to at least 140. One tortoise lived to 250. And a mollusc called Ming was the ripe old age of 507 when researchers inadvertently killed him.

Forget these babies, though. The oldest living creatures on Earth can easily shatter their longevity records, which is not bad going for organisms that are too small to be seen with the naked eye.

In the coldest parts of Siberia, Antarctica and Canada lie soils that have remained permanently frozen for thousands to millions of years. Trapped hundreds of metres down between layers of this frozen earth, known as permafrost, are living bacteria as old as the ice itself.

Just how the bacteria survive is unknown, but some claim the microbes' secrets could unlock the key to immortality.

Russian scientist Sabit Abyzov was working at the Vostok station in Antarctica in 1979 when he discovered bacteria, fungi and other microorganisms 11,811ft (3,600m) beneath the Antarctic ice sheet, just above the subglacial Lake Vostok.

Brouchkov has even injected himself with the 3.5-million-year-old microbe

The ice had been frozen solid for hundreds of thousands of years, and yet bacteria were living quite happily inside it. There was no credible way that the bacteria could have made their way down there from the surface after the ice had formed, so Abyzov concluded that the bacteria must themselves be hundreds of thousands of years old – far older than any organisms found previously.

In 2007, the longevity record fell again. Eske Willerslev and a team from the University of Copenhagen made history when they discovered living bacteria half a million years old hidden deep below layers of permafrost in Antarctica, Siberia and Canada.

It was the first time that researchers had isolated DNA from such ancient but still active bacteria.

Then, just two years later, an even older microbe came to light – this time thought to be a remarkable 3.5 million years old.

It was unearthed by Russian scientist Anatoli Brouchkov. The bacteria came from ancient permafrost at a site known as Mammoth Mountain in Siberia.

Brouchkov has even injected himself with the 3.5-million-year-old microbe, known as Bacillus F, in the hopes that the "eternal life" bacteria will work their magic and extend his lifespan too.

He had already tested the inactivated form of the bacteria on mice, fruit flies and human blood cells with promising results. He claims that he has not caught the flu in the two years since his self-treatment with the ancient microbe.

What exactly makes scientists think that the bacteria inside permafrost are so old, and not simply descendants of the bacteria originally trapped thousands or millions of years ago?

At 250 million years old, these bacterial cells would have been alive when the first dinosaurs were just starting to walk the Earth

The answer is that the bacteria are so tightly trapped in the frozen soil that they do not seem to have room to divide. Even if they did divide, there is nowhere for new cells to go.

If reproduction is not possible, then the microbial cells that are found living in the permafrost today must be the same ones that were frozen in place when the climate cooled.

The same sort of reasoning is behind a controversial claim that some individual bacteria have lived for an astonishing 250 million years.

These bacteria come from the inside of salt crystals buried 1,970ft (600m) below ground at a site in New Mexico where a nuclear waste dump was being built. At 250 million years old, these bacterial cells would have been alive when the first dinosaurs were just starting to walk the Earth.

Russell Vreeland of West Chester University in Pennsylvania, who made the discovery, says the bacterium – known as Virgibacillus strain 2-9-3 – is remarkably similar to modern Virgibacillus found in the Dead Sea.

Once the ancient microbes were extracted from the crystals and placed in a nutrient-rich flask in the lab, they reawakened and began to grow.

None of these discoveries has been close to 250 million years old

Some researchers maintain that strain 2-9-3 must be much younger than 250 million years, probably arising from contamination in the lab. But Vreeland is convinced of their age, particularly because he says they were clearly trapped inside the ancient salt crystals.

"They were in the crystals and they were alive," says Vreeland. "The chances of them getting into a sealed crystal were about zero and the chances of this being a contamination event were around one in [a billion]."

What's more, he says that many other similar examples of salt-trapped bacteria have been discovered since. Most recently, bacteria between 33 and 48 million years old were found in salt crystals in an inland salt lake in central China.

However, none of these discoveries has been close to 250 million years old.

We know that some bacteria go into a dormant form called a spore when conditions turn particularly harsh

The ancient microbes found in permafrost or salt crystals are on the very edge of survival. Robbed of the ability to undergo cell division because of a lack of space, each one instead must have had to divert the little energy it could find into keeping its single cell alive.

"The bacteria could not have reproduced inside the salt crystal, as there would have been very few nutrients and it would have built up toxic wastes," says Vreeland.

But, to state the obvious, managing to stay alive for millions of years is an incredible feat. In particular, DNA and the proteins responsible for powering life's reactions inside living cells usually degrade over relatively short time frames, as a consequence of radiation damage. What is the secret to overcoming these problems?

Some scientists believe that the ancient bacteria can only be as old as they appear to be if they have mechanisms to repair their DNA and cell structures. But exactly what these active repair mechanisms are, and how they could work in such a hostile environment, is still unknown.

For instance, the bacteria in permafrost or in salt would lack good access to water, which is necessary to power the enzymes that normally perform cellular repairs.

Raúl Cano and his co-workers managed to revive 30-million-year-old bacterial spores from the stomach of an ancient bee

Vreeland is currently working with the Howard Hughes Medical Institute to sequence the genes of his strain 2-9-3 bacteria, which will tell us more about how they survive.

Some ancient bacteria may have an alternative long-term survival plan, however. They may go into a kind of stasis.

We know that some bacteria go into a dormant form called a spore when conditions turn particularly harsh. Spores are a bit like plant seeds: a "shell" grows around the vulnerable cell.

Unlike seeds though, spores are super-tough. They can survive blasts of radiation, and they can go for years without water or nutrients. Inside the shell the microbe lies in a completely inert state, but it can reawaken when conditions get better.

As far back as 1995, scientist Raúl Cano and his co-workers managed to revive 30-million-year-old bacterial spores from the stomach of an ancient bee. The bee and the bacteria inside it got trapped and preserved in a drop of tree sap that became amber.

But some scientists say even the ability to form a protective spore would not allow bacteria to survive for 250 million years. They say that over such longer periods of time the DNA in a microbe would inevitably degrade and disintegrate.

DNA faces a three-pronged attack from high-energy cosmic radiation, solar radiation in the form of gamma and ultraviolet rays, and from the radiation released by the spontaneous breakdown of atomic nuclei.

You only need a couple of hits from cosmic rays and that's it, it's dead

Paul Falkowski of Rutgers University did an experiment in which he collected five ice samples between 100,000 and 8 million years old from beneath the surface of a glacier in the Beacon and Mullins valleys of Antarctica. Along with his team he then tried to cultivate microbes inside the ice. He found that the older the ice was, the shorter the average length of DNA fragments inside, and the fewer the number of microbes that could be revived.

In other words frozen DNA is progressively degraded as time passes. Falkowski successfully cultivated microbes from a two-million-year-old block of ice, and calculated that after 1.1 million years half the original DNA had been degraded.

"I would guess if bacteria are exposed to radiation damage at the poles then the max cut off point would probably be about two to three million years," says Falkowski.

"You only need a couple of hits from cosmic rays and that's it, it's dead. So while the chances of a hit are low over a short period of time, over millions of years there's definitely going to be a hit. The chances of lightning at any specific place in time is low, but if you average it over millions of years virtually every place on Earth will have been hit."

Vreeland, however, remains convinced that bacteria can survive for much longer than Falkowski's experiments suggest, given the right conditions.

In salt a bacterium could take 1,000 times more hits to its DNA before lethal damage occurred

"The salt crystal is impervious to oxygen so no oxidation occurs, and it would be buried and dark so there would be no damage from UV radiation," he says.

Vreeland also notes that spore formation causes the DNA molecules to become very tightly packed, which makes them a smaller target for damaging radiation rays. The salt crystal itself also protects the bacteria from radiation by pushing out heavy metals that would radioactively decay over such long time periods.

This means that the only potential source of radiation from decay comes from potassium-40, a radioactive form of potassium with a very long half-life of 1.25 billion years. This long half-life means that the probability of potassium-40 emitting radiation is very low.

Finally, the salt in the crystals creates a water-free environment in which the chemical bonds inside the DNA become stronger. In other words, the DNA is made harder to destroy.

The nearest neighbouring galaxy to our Milky Way is only 2-3 million light years away

"We did a study and the bottom line was that in salt a bacterium could take 1,000 times more hits to its DNA before lethal damage occurred," says Vreeland. "So add it all up and you have a very stable place to hide."

Is there any broader significance to the ability of individual bacteria to survive, potentially, for 250 million years? In a word, yes.

If bacteria really can survive in stasis for millions of years, we cannot rule out the possibility that cells or DNA first arose on another planet in another solar system – even in a different galaxy – and then travelled to Earth on a comet or asteroid. After all, the nearest neighbouring galaxy to our Milky Way is the Andromeda galaxy, which is only 2-3 million light years away: an approachable distance for a bacterium that can survive for 250 million years.

Vreeland's work also adds weight to the idea that life could exist on Mars, as salt deposits have been discovered in Martian meteorites.

Pathogens with the potential to harm humans could be locked in Siberia's permafrost, ready to be unleashed when the ice melts

Long-lived organisms also have the potential to pose a problem for human societies today. Pathogenic bacteria or viruses might be among those trapped in ice.

One such virus was found frozen 98ft (30m) underground beneath a deep layer of the Siberian permafrost in 2014, where it had remained for at least 30,000 years. The ancient "giant" virus Pithovirus sibericum is so large, at 1.5 micrometres, that it can be seen under a normal microscope.

Once the scientists took it back to the lab, the virus sprang back to life and became infectious again. Humans are not in danger from this particular virus, as it only attacks single-celled amoebas. But the researchers believe that pathogens with the potential to harm humans could be locked in Siberia's permafrost, ready to be unleashed when the ice melts.

We cannot rule out the possibility that cells or DNA first arose on another planet in another solar system, and then travelled to Earth

Ancient forms of smallpox, for instance, could be trapped in ice.

Not all human viruses could withstand life in ice, though. Viruses like flu and HIV that are surrounded by a "fatty" lipid envelope are more fragile than viruses with an external protein shell.

Even so, research like this does come with a warning. The most long-lived lifeforms on Earth may be among the smallest, but they still have the potential to make a big impression on the modern world.

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