Okay, okay. I’ll come clean, it isn’t a bear and it doesn’t cheat death, it’s just very good at surviving.
The bear (or bears, really, as there are more than one thousand species) is a Tardigrade. A creature first observed in 1773 by Johann August who described them, in an admittedly adorable fashion, as ‘little water bears’.
This name stuck and now Tardigrades are known world wide as water bears. But what makes these creatures incredible? Their ability to survive longer in conditions that would kill…well, anything else, instantly.
Tardigrades range in size from 0.1mm to 1.5mm, tiny and almost indestructible.
How do we know this? Well, humans are, above all else, curious. And when driven by that curiousity we can do some pretty horrid things.
Such as expose different types of tardigrades to temperatures like 1K (-272 degrees Celsius) and 420K (150 degrees Celsius). 1K is a fraction away from absolute zero, the coldest temperature in the universe. And most of the hardy little tardigrades that were brutally exposed to it lived. That’s right, a little microscopic water bear survived the near-coldest temperature of the universe.
How?
The same way it survives in the vacuum of space, it’s cells become glass-like.
When a tardigrade becomes dehydrated, and by that I mean completely desiccated with it’s water percentage reaching as low as 1%, it’s metabolism slows right down (0.01%) and it seems to have died. This is called cryptobiosis and it’s surprisingly common among organisms (brine shrimp, yeast, round worms all share this ability).
Within each cell of a tardigrade when it becomes a ‘tun’, the term for a desiccated water bear, are groups of proteins called tardigrade-specific intrinsically disordered proteins (TDPs). These proteins work to vitrify the tardigrade, clumping around the mechanisms within each cell and preventing them from moving. Which essentially stops the cells from functioning.
Thus the tardigrades are put into cryptobiosis and they can survive in that state for upwards of a decade.
If you want to bring a tardigrade back to the world of the (functionally) living, simple add a drop of water.
The TDPs are extremely hydrophilic and when water comes into contact with them, they bind to it and release their hold on the cells.
The tardigrade starts to wiggle and writhe once more, able to resume its ponderous life.
This incredible ability may be transferable. TDPs have been shown to be transferable to simple organisms such as yeast. This could in the future unlock crops that can survive any temperature that it would be reasonable to find on earth; a potentially species saving intervention, given the steady increase of heat and number of extreme whether events that will only continue to plague the world into the future.
It should be noted that tardigrades can also survive levels of ionizing radiation at doses far above what a human can and in late 2020 a species of tardigrade was shown to have the natural ability to produce a ‘UV radiation shield’. When subjected to lethal doses of UV radiation, Paramacrobiotus produces an absorbing shield that emits a harmless blue light. This, like the transfer of hardy genes to crops, could have huge implications for the future of space travel for humanity.
But if you want to kill a tardigrade you can. Another study published in 2020 revealed that tardigrades are susceptible to high temperatures for long periods of time. Even half of the tun-state tardigrades exposed to 63.1 degrees Celsius for 24 hours died.
They aren’t indestructible, just skilled at surviving.
These little water bears live all around us. In ponds, gutters, and puddles. If you have a telescope, even a low power one, you can very easily sample some water from wherever you are, put it underneath the scope and directly observe this incredible creature.
Further Reading/References:
Tardigrades Use Intrinsically Disordered Proteins to Survive Desiccation: Molecular Cell
Weird Science: Cryptobiosis | manoa.hawaii.edu/ExploringOurFluidEarth
Physics and Fiction 
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