What if there were creatures on Earth that had abilities similar to Deadpool? There does truly exist such a species - they are called the tardigrades.
Tardigrades, at first glance, are intimidating. They have podgy faces with folds of flesh, a bit like a Doctor Who monster. They have eight legs, with ferocious claws resembling those of great bears. Their mouth is also a serious weapon, with dagger-like teeth that can spear prey. But there's no need to worry. Tardigrades are one of nature's smallest animals. They are never more than 1.5 mm long, and can only be seen with a microscope. They are commonly known as "water bears".
They are truly ancient. Fossils of tardigrades have been dated to the Cambrian period over 500 million years ago, when the first complex animals were evolving. And ever since they were discovered, it has been clear that they are special.
It is virtually impossible kill a tardigrade. You can freeze it, boil it, crush it, zap it with radiation, and deprive it of food and water - for years! - and it will wiggle back to life. Here, we look at some of the defenses in the arsenal of the tardigrade.
If you go into outer space without protection, you'll die.
The lack of pressure would force the air in your lungs to rush out. Gases dissolved in your body fluids would expand, pushing the skin apart and forcing it to inflate like a balloon. Your eardrums and capillaries would rupture, and your blood would start to bubble and boil. Even if you survived all that, ionising radiation would rip apart the DNA in your cells. Mercifully, you would be unconscious in about 15 seconds.
But one group of animals can survive this: the tiny creatures called tardigrades about 1mm long. In 2007, thousands of tardigrades were attached to a satellite and blasted into space. After the satellite had returned to Earth, scientists examined them and found that many of them had survived. Some of the females had even laid eggs in space, and the newly-hatched young were healthy.
For most animals, life without water is completely impossible.
When a typical cell dries out, its membranes rupture and leak, and its proteins unfold and aggregate together, making them useless. DNA will also start to fragment the longer the cell is dry. Tardigrades avoid all this.
When a tardigrade dries out, it retracts its head and its eight legs. It then enters a deep state of suspended animation that closely resembles death. Shedding almost all the water in its body, the tardigrade curls up into a dry husk. Its metabolism slows to 0.01% of the normal rate. It can stay in this state for decades, only reanimating when it comes into contact with water.
As they start to dry out, tardigrades make protectants that allow them to survive becoming completely dry. But what exactly these protectants are is still a mystery.
When tardigrades start to dry out, they seem to make a lot of antioxidants. These are like vitamins C and E, that soak up dangerously reactive chemicals. This may mop up harmful chemicals in the tardigrades' cells.
The antioxidants may explain one of tardigrades' neatest abilities. If a tardigrade stays in its dry state of suspended animation for a long time, its DNA gets damaged. But after it reawakens, it is able to quickly fix it.
Tardigrades seem not to care what temperature it is. They can survive extremely hot temperatures.
In 1842, a French scientist named Doyère showed that a tardigrade in the state of suspended animation could survive being heated to temperatures of 125°C for several minutes. In the 1920s, a Benedictine friar named Gilbert Franz Rahm brought tardigrades back to life after heating them to 151°C for 15 minutes.
While we have some idea of how tardigrades cope with the cold, we have no idea how they cope with heat. At scorching temperatures like 150 °C, proteins and cell membranes should unravel, and the chemical reactions that sustain life cease to occur.
The most heat-tolerant organisms known are bacteria that live around the edges of hydrothermal vents in the deep sea. They can still grow at 122 °C. If Rahm is to be believed, tardigrades can survive even higher temperatures.
Many animals that have evolved to live in hot places, like hot springs and scorching deserts, produce chemicals called heat shock proteins. These act as chaperones for proteins inside cells, helping them keep their shape. They also repair heat-damaged proteins.
That's all well and good, but there is no conclusive evidence that tardigrades produce these chemicals. Factor in the other things they can survive, and the picture becomes even more baffling.
Even at extremely cold temperatures.
Gilbert Franz Rahm also tested them in the cold. He immersed them in liquid air at -200 °C for 21 months, in liquid nitrogen at -253 °C for 26 hours, and in liquid helium at -272 °C for 8 hours. Afterwards the tardigrades sprang back to life as soon as they came into contact with water.
The biggest hazard tardigrades face in the cold is ice. If ice crystals form inside their cells, they can tear apart crucial molecules like DNA. Some animals, including some fish, make antifreeze proteins that lower the freezing point of their cells, ensuring that ice doesn't form. But these proteins haven't been found in tardigrades.
Instead, it seems that tardigrades can actually tolerate ice forming within their cells. Either they can protect themselves from the damage caused by ice crystals, or they can repair it.
Radiation has no effect.
Nope. In 1964, scientists exposed tardigrades to lethal doses of X-rays and found that they could survive. Later experiments showed they can also cope with excessive amounts of alpha, gamma and ultraviolet radiation – even if they're not in the state of suspended animation.
Radiation was one of the biggest threats facing the tardigrades sent into space in 2007. Those exposed to higher levels of radiation fared worse than those protected, but the mortality rate was not 100%.
High pressure? No problem.
They can also cope with extreme pressure that would squash most animals flat, according to a study published in 1998 by Kunihiro Seki and Masato Toyoshima of Kanagawa University in Hiratsuka, Japan. They found that tardigrades in the state of suspended animation could survive a pressure of 600 megapascals (MPa).
This is beyond anything they might encounter in nature. The deepest part of the sea is the Challenger Deep in the Mariana Trench in the Pacific Ocean, which goes down 10,994 m. There, the water pressure is around 100 MPa. Somehow the tardigrades survived six times that.
At these crushing pressures, proteins and DNA are ripped apart. Cell membranes, which are composed of fat, become solid like butter in a fridge. Most microorganisms stop metabolising at 30 MPa, and bacteria can't survive much beyond 300 MPa.
The sheer variety of stressors that tardigrades can survive is almost dizzying.
How do they survive?
Unlike bacteria that live in boiling hot springs or other extreme sites, most tardigrades live in relatively unremarkable places. They tend to live in or near water, and there's nothing a tardigrade likes more than a good chunk of moss and lichen. Their lives aren't even that exciting: while most creatures their size dart about frantically, tardigrades are sluggish.
We still don't have a good solution as to why they evolved these amazing defenses, but perhaps we may find out in the future. After all, these chubby guys will be around long after humanity has gone extinct.
