Image via flickr (though probably didn’t originate there)

Tardigrades (meaning “slow walkers”) are microbial creatures that are resilient against all manner of extreme conditions (heat, cold, pressure, radiation, dehydration). They can survive in temperatures as low as 1 degree calvin — 0 degrees calvin being the temperature at which molecular motion ceases. They can survive space. They can survive being dehydrated and rehydrated like instant coffee.

They’re extremely common (there are probably some in your back garden) and examples have been found dating back to the Cambrian period, when they were less evolved: fewer legs, simpler head shape and no posterior appendages.

Image via the incredible water bear.

They occupy their own phylum in the animal kingdom (tardigrada), they are so unique. Their closest relatives are fruit flies (arthropoda) and nematodes (nematoda).

More info at Wikipedia, where there are links to yet more info, photos, and videos.