Tardigrades, also known as the water bear, are microscopic animals , which can survive in many extreme conditions , including space . ( Taylor Turner/The Washington Post )
Tardigrades have a reputation as the toughest animals on the planet. Some of these microscopic invertebrates shrug off temperatures of minus 272 Celsius , one degree warmer than absolute zero. Other species can endure powerful radiation and the vacuum of space. In 2007, the European Space Agency sent 3 ,000 animals into low Earth orbit , where the tardigrades survived for 12 days on the outside of the capsule.
To a group of theoretical physicists , tardigrades were the
perfect specimens to test life ‘ s tenacity. “ Life is pretty fragile if all your estimates are based on humans or dinosaurs,” said David Sloan , a theoretical cosmologist at Oxford University in Britain.
The tardigrade lineage is ancient. “ Tardigrade microfossils are reported from the Early Cambrian to the Early Cretaceous , 520 million to 100 million years ago ,” said Ralph O. Schill , an expert on tardigrades at the University of Stuttgart in Germany who was not involved with this research. “ They have seen the dinosaurs come and go .”
Sloan , with his Oxford colleague Rafael Alves Batista and Harvard University astrophysicist Abraham Loeb , decided to try to rid the planet of tardigrades. In theory , anyway, in a report published Friday in the journal Scientific Reports . Through the powers of mathematical modeling they tossed three of the most devastating cosmic events at Earth : killer asteroids, supernovae and gamma – ray bursts .
“ These are the biggest ways you can transfer energy to the planet, ” Sloan said. The tardigrades kept on theoretically trucking , outlasting 10 billion years’ worth of cataclysms . Until the point that the sun failed or engulfed the planet .
In picking their apocalyptic poison , the scientists first tried to sterilize the planet with radiation. In the lab, some tardigrade species can survive radiation doses of 5 , 000 to 6, 000 grays. (“ You would be very , very lucky to walk away ” from a dose of 5 grays, Sloan said.) But long before the scientists blasted Earth with enough radiation to kill all the tardigrades, they calculated that the radiation ‘ s energy would boil the oceans away . The sticking point for tardigrades, then, was the evaporation of the planet’ s water .
For an asteroid to deposit that much energy into the ocean, it would need a mass of at least 1 .7 quintillion kilograms. Of all the asteroids in the solar system, only 19 fit the bill. (By way of comparison , the asteroid that finished the dinosaurs was six miles across ; an asteroid called Vesta that is one of the potential ocean killers has a diameter of 326 miles .) The chances of such a massive collision are so small , the scientists said, that the sun would die first.
Likewise, the closest stars that could explode into supernovae are too far away to boil the oceans. Gamma – ray bursts were a bit more complicated — “ we don ‘ t really understand where they come from, ” Sloan said — but not impossible to calculate. And though the bursts would strip off parts of the atmosphere, killing animals like humans , tiny and durable creatures under the ocean, huddled around hydrothermal vents , would be “ sufficiently well -shielded ,” Sloan said .
But lumping all tardigrade species into one unkillable chimera was a fatal flaw in this argument , according to tardigrade expert William R. Miller. “ I can ‘ t say anything about the physics, ” he said, “ but they can ‘ t say anything about the animals .”
Tardigrades, also known as the water bear, are tiny aquatic animals , which can survive in many extreme conditions , including space . A dry water bear shows no signs of life , but a drop of water can bring it back to normal activity. ( Daiki D . Horikawa)Not all tardigrades dwell in water ; some species live in moss and lichens on trees . (Their variety of habitats is reflected in nicknames like “ water bear” and “ moss piglet. ”)
Miller, a biologist at Baker University in Kansas, said that the authors of the new work treat tardigrades as a single animal, ignoring that they are in fact a phylum of 1 ,250 different species . He compared this approach to arguing that “ a sixgill shark at the bottom of the ocean is the same as a snow leopard in Siberia .”
Sloan emphasized that he was approaching the tardigrade apocalypse as a physicist , not a biologist. He said such doomsday calculations commonly take a human perspective, but such an approach misses the true resilience of life . The cosmic implications of this study , he said, “ means that if life did get started on another planet in our galaxy , it probably should still be there. ”
Land- dwelling tardigrades endure extremes thanks to an ability called cryptobiosis , in which the animals lose all but 3 percent of the water in their bodies. It is in this state that tardigrades can survive the hottest heats, the coolest colds , crushing pressures or the complete lack of it . They desiccate , and then they persist . Joseph Seckbach, a biologist at the Hebrew University of Jerusalem , said that a tardigrade “ can be in dormancy for 30, 40 years, and wake up and say , ‘Hello!’ ”
But there is no indication that water – dwelling tardigrades are capable of the same process, Miller said. “ The illusion that marine animals survive with a cryptobiotic plan is just dead wrong .” Nor are they indestructible. “ We work with active animals and they’ re quite easily murdered,” he said. “ We kill thousands of them every day .”
Shill noted that tardigrades had evolved to survive in particular microhabitats. “ I believe that the resistance to radiation is a product of chance ,” he said . “ If an astrophysical event sterilized all life on Earth , it does look also bad for the future of these amazing animals.”
That ‘ s not to say cosmic tardigrades are out of the question. In 2014, Miller and physicist Ran Sivron calculated that tardigrades could survive the 4 .37-light year trip to Alpha Centauri (and then longer , if they presumably landed on a friendly exoplanet ). Even then, though, “ the ability to go into this cryptobiosis survival mechanism probably isn ‘ t going to work , ” Miller said, “ if they still don’ t have food, water , habitat or atmosphere. ”