The hardiest animal on Earth!! (v1.0)

A key reference is New Scientist. 

Tardigrades are an amazing evolution story!!

Tardigrades known colloquially as water bears are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Goeze in 1773, who called them Kleiner Wasserbär ("little water bear"). In 1777, the Italian biologist Spallanzani named them Tardigrada, which means "slow steppers". Tardigrades are usually about 0.5 mm (0.020 in) long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or suction disks. Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a low-power microscope, making them accessible to students and amateur scientists.

They have been found in diverse regions of Earth's biosphere – mountaintops, the deep sea, tropical rainforests, and the Antarctic. Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditions – such as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvation - that would quickly kill most other known forms of life. Tardigrades have survived exposure to outer space. A batch of them were included on an Israeli spacecraft that crash-landed on the moon. There are about 1,300 known species in the phylum Tardigrada. The earliest known true members of the group are known from Cretaceous period (145 to 66 million years ago) amber, found in North America, but are essentially modern forms. Their origin is therefore likely much earlier, as they diverged from their closest relatives in the Cambrian period over 500 million years ago.

Tardigrades are renowned for their ability to withstand extreme environments – now we know how they do it. Tiny molecular sensors in their cells can detect when they produce too many damaging molecules called free radicals, triggering a dormant state. “Tardigrades don't thrive under extreme conditions, but they can overwinter them,” says Derrick Kolling at Marshall University in West Virginia. “We wanted to understand how they're able to go into that.”

To investigate, Kolling and his colleague's exposed tardigrades to high levels of hydrogen peroxide, sugar or salt, or −80°C (−112°F) temperatures to induce tuns. As a result of these stresses, the tardigrades produce harmful, highly reactive molecules called oxygen free radicals. The free radicals then go on to react with other molecules, says team member Leslie Hicks at the University of North Carolina at Chapel Hill. Crucially, the team found that the free radicals oxidize an amino acid called cysteine, one of the building blocks of proteins in the body. These reactions cause proteins to change their structure and function, signaling the onset of the dormant state. In experiments where cysteine oxidation was prevented, the tardigrades weren’t able to enter a tun state. “Cysteine acts like a kind of regulatory sensor,” says Hicks. “It allows tardigrades to feel their environment and react to stress.” When conditions improved, the team found that cysteine was no longer oxidised, which told the tardigrades to wake up from their tuns.

“Whether this is a universally conserved protection mechanism and whether this is conserved across tardigrade species are really important questions,” says Hicks. The answers may help us better understand the ageing process and even how to achieve long-term space travel, she says.

What does a Tardigrade look like? Here is a picture: tardigrade pictures - Search Images (bing.com)