Tardigrades Survived All Five Mass Extinctions, They’ll Outlive Us Too.

There is a thought experiment that biologists sometimes use to illustrate the concept of extremophiles. Imagine an animal that can survive the vacuum of space, temperatures of -272°C and +150°C, radiation doses 1,000 times lethal to humans, complete desiccation for decades, and pressures six times greater than the deepest ocean trench. Then imagine that this animal is half a millimeter long, lives in the moss on your garden wall, and was first described by a German pastor named Johann August Ephraim Goeze in 1773, who called it Kleiner Wasserbär — little water bear.

The tardigrade is not a thought experiment. It is real, it is almost certainly within walking distance of wherever you are reading this, and it is the most resilient animal ever documented on Earth.

The Tun

The tardigrade’s survival capabilities are not always active. They are triggered by environmental stress through a process called cryptobiosis, specifically a form called anhydrobiosis when the trigger is desiccation. When a tardigrade’s environment dries out, it expels almost all the water from its body, retracts its legs, and contracts into a barrel-shaped structure called a tun. In this state, its metabolism drops to 0.01% of normal. It is not dead. It is not alive in any conventional sense. It is waiting.

Researcher Takekazu Kunieda at the University of Tokyo has studied the molecular mechanisms of tardigrade desiccation survival in detail, identifying a class of proteins unique to tardigrades called tardigrade-specific intrinsically disordered proteins, or TDPs, that replace the water in cells during desiccation, forming a glass-like matrix that protects cellular structures from damage. When water returns, the matrix dissolves, the proteins resume their normal functions, and the tardigrade rehydrates and walks away.

The tun state can persist for decades. Researcher Diane Nelson at East Tennessee State University documented tardigrades reviving from museum moss samples that had been dry for over 100 years. The animals that emerged were not the descendants of the original tardigrades. They were the original tardigrades, resuming activity after a century of suspended animation.

Space Survival

In 2007, the European Space Agency sent tardigrades into low Earth orbit on the FOTON-M3 mission, exposing them to the vacuum of space, cosmic radiation, and ultraviolet radiation from the sun without any shielding. When the samples were returned to Earth, a significant proportion of the tardigrades survived and reproduced normally.

This was the first confirmed survival of any animal in the vacuum of space without life support. Researcher Ingemar Jönsson at Kristianstad University led the experiment and has continued studying tardigrade space survival, finding that the combination of vacuum tolerance and radiation resistance makes tardigrades uniquely capable of surviving conditions that would kill every other known animal within seconds.

The radiation resistance is particularly remarkable. Tardigrades can survive doses of ionizing radiation up to 570,000 rads. A dose of 500 rads is lethal to humans. The mechanism involves a protein called Dsup, damage suppressor, identified by Kunieda’s group, that physically binds to DNA and shields it from radiation damage. When the Dsup gene was introduced into human cultured cells, those cells showed significantly increased radiation resistance, a finding that has attracted interest from researchers studying radiation protection for cancer therapy and space travel.

Everywhere and Invisible

Approximately 1,300 tardigrade species have been described, found in every habitat on Earth including deep ocean sediments, hot springs, Antarctic ice, and the Himalayas at altitudes above 6,000 meters. They are most abundant in mosses and lichens, where a single gram of dry moss can contain thousands of individuals.

Despite their abundance, tardigrades were essentially unknown to most people until a combination of improved microscopy and social media made their appearance widely visible. The eight stubby legs, the circular mouth with stylets for piercing plant cells and algae, the barrel-shaped body, and the lumbering walk that gave them their common name, water bears, are now recognizable to anyone who has spent time looking at science content online.

Researcher Bob Goldstein at the University of North Carolina has worked to make tardigrade biology accessible to a broader audience while conducting serious research on their development and genetics. His lab’s time-lapse videos of tardigrades moving, feeding, and reproducing have been viewed millions of times and have introduced more people to the animal than any textbook.

Five mass extinctions. The asteroid that killed the dinosaurs. The Permian extinction that eliminated 96% of marine species. The end-Ordovician, the Late Devonian, the Triassic-Jurassic. The tardigrade survived all of them, in its tun, waiting for conditions to improve. Whatever comes next, the reasonable scientific expectation is that the tardigrade will survive that too. The moss on your garden wall is home to the toughest animal that has ever lived, and it has been there, in one form or another, for 500 million years.