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.”

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