Whether it is a supernova or an asteroid impact, should a cosmic calamity strike, it seems there will be at least one form of life left: a tubby, microscopic animal with the appearance of a crumpled hoover bag.
The creatures, known as tardigrades, are staggeringly hardy animals, a millimetre or less in size, with species living in wet conditions that range from mountain tops to chilly ocean waters to moss and lichen on land.
“They can survive incredible conditions – we are talking close to absolute zero, the vacuum of space, exposure to radiation that would kill us, and these things just walk away from it like nothing happened,” said David Sloan, an astrophysicist from the University of Oxford.
Now new research by Sloan and colleagues has shown that the creatures would survive any cosmic disaster that might conceivably be thrown at Earth – a discovery that could have implications elsewhere in the solar system, and beyond.
“There are quite a lot of stars like our sun out there, and about 20% of these stars have an Earth-like planet around them,” said Sloan. “What you then want to ask is if life started on one of these planets, what are the odds that it is still around?”
Writing in the journal Scientific Reports, the researchers describe how they probed the conundrum by exploring the likelihood of a variety of catastrophes serious enough to wipe out tardigrades on an Earth-like planet, including a nearby supernova, a burst of gamma-rays, and an impact by a large asteroid powerful enough to cause the oceans to boil away.
But the team found that the chances of such events were so remote as to be extremely unlikely – there was little chance of a supernova occurring close enough to an Earth-like planet to kill off the creatures, and it would take an impact from an asteroid or dwarf planet near the mass of Vesta for the oceans to boil. “There are about 17 [asteroids] this big in our solar system, but they are all on sufficient orbits that they will never intersect with us,” said Sloan.
The upshot, he said, was that it was very unlikely any cosmic event would be so catastrophic as to sterilise an Earth-like planet where life, of the sort we know, had got going. “Because [tardigrades] are so hardy it means that events that we are worried about as human beings, and rightly so, certainly wouldn’t concern you if you just considered all life,” said Sloan.
Matthew Cobb, professor of zoology at the University of Manchester, who was not involved in the study said the findings were reassuring for the future of life on Earth.
“It suggests that the complete eradication of life on Earth is extremely unlikely until we get to the point that the sun enlarges and all the oceans boil away,” he said. “Many organisms, in particular animals and bacteria, live in the deep ocean, which the authors show would be unaffected by any conceivable cosmic cataclysm.”
Mark Blaxter, professor of evolutionary genomics at the University of Edinburgh, agreed, adding that there are other organisms have a similar survival strategy to tardigrades. What’s more, he said, there are also organisms that live very deep underground in hot water within continental and under-sea-floor rocks. “Sterilising the planet would have to deal with these too,” he noted.
He also stressed that even tardigrades are not invincible. “[Land-based] tardigrades stay alive in extreme conditions by drying out completely. So if there was no water left … there would be no “live” tardigrades, just dried up ones,” said Blaxter. “And if there was some water left so that the tardigrades could re-animate, if there was no food left they eat algae and fungi - they too would be dead in a couple of weeks.”
Cobb, too, noted that even if the tardigrades were the only survivors, they would face a struggle. “For the tardigrades to inherit the Earth, whatever catastrophe swept over the planet would have to return to normal-ish conditions within a matter of decades at most, or it really could be curtains,” he said.