Without water, a human can only survive for about 100 hours.
But there's a creature so resilient that it can go without it for decades.
This one millimeter animal can survive both the hottest
and coldest environments on Earth,
and can even withstand high levels of radiation.
This is the tardigrade,
and it's one of the toughest creatures on Earth,
even if it does look more like a chubby, eight-legged gummy bear.
Most organisms need water to survive.
Water allows metabolism to occur,
which is the process that drives all the biochemical reactions
that take place in cells.
But creatures like the tardigrade, also known as the water bear,
get around this restriction with a process called anhydrobiosis,
from the Greek meaning life without water.
And however extraordinary, tardigrades aren't alone.
Bacteria,
single-celled organisms called archaea,
plants,
and even other animals can all survive drying up.
For many tardigrades,
this requires that they go through something called a tun state.
They curl up into a ball,
pulling their head and eight legs inside their body
and wait until water returns.
It's thought that as water becomes scarce
and tardigrades enter their tun state,
they start synthesize special molecules,
which fill the tardigrade's cells to replace lost water
by forming a matrix.
Components of the cells that are sensitive to dryness,
like DNA,
proteins,
and membranes,
get trapped in this matrix.
It's thought that this keeps these molecules locked in position
to stop them from unfolding,
breaking apart,
or fusing together.
Once the organism is rehydrated, the matrix dissolves,
leaving behind undamaged, functional cells.
Beyond dryness, tardigrades can also tolerate other extreme stresses:
being frozen,
heated up past the boiling point of water,
high levels of radiation,
and even the vacuum of outer space.
This has led to some erroneous speculation
that tardigrades are extraterrestrial beings.
While that's fun to think about,
scientific evidence places their origin firmly on Earth
where they've evolved over time.
In fact, this earthly evolution has given rise
to over 1100 known species of tardigrades
and there are probably many others yet to be discovered.
And because tardigrades are so hardy, they exist just about everywhere.
They live on every continent, including Antarctica.
And they're in diverse biomes including deserts,
ice sheets,
the sea,
fresh water,
rainforests,
and the highest mountain peaks.
But you can find tardigrades in the most ordinary places, too,
like moss or lichen found in yards,
parks,
and forests.
All you need to find them is a little patience and a microscope.
Scientists are now to trying to find out whether tardigrades use the tun state,
their anti-drying technique,
to survive other stresses.
If we can understand how they, and other creatures,
stabilize their sensitive biological molecules,
perhaps we could apply this knowledge to help us stabilize vaccines,
or to develop stress-tolerant crops that can cope with Earth's changing climate.
And by studying how tardigrades survive
prolonged exposure to the vacuum of outer space,
scientists can generate clues about the environmental limits of life
and how to safeguard astronauts.
In the process, tardigrades could even help us answer a critical question:
could life survive on planets much less hospitable than our own?