Though tardigrades aren't the smallest creatures on this list, they're definitely the toughest. Also known as water bears, these 1 millimeter-long lifeforms can survive in conditions that would kill most other animals. They can survive without water for up to ten years, can survive radiation, and thrive in temperatures as frigid as -272 degrees Centigrade (-457 Fahrenheit) and as thermal as 150 degrees Centigrade (302 Fahrenheit). These critters are so hardy that in 2007, scientists shot thousands of them into space, to see what would happen. They not only survived, but the females laid eggs that hatched into healthy infants.

On top of all that, they can return from the dead. In 1948, a Italian zoologist added water to some 120-year-old, dried-out tardigrades, which immediately started moving. We should all do our best to never anger tardigrades.