It's often believed that cockroaches — nature's ultimate survivalist — will live until the end of time. But new research suggests they've met their match in the tardigrade.
This tiny creature will likely be the last one standing after all other complex life on Earth is gone, says a study published in Scientific Reports.
In fact, it says tardigrades will be around for at least another billion years — likely a lot longer than humans.
Also known as "water bears," tardigrades are eight-legged microscopic animals believed to have been on Earth for about 500 million years. They live under water and only grow to about 0.55 millimetres.
Tardigrades may be small, but they are mighty: they're considered the most resilient form of life on our planet. They can live up to 30 years without water; survive in the extremes, such as in the vacuum of space or in the farthest depths of our oceans; and they have a lifespan of about 60 years.
Devastation from above
With the discovery of exoplanets orbiting sun-like stars, the research team of astrophysicists and a cosmologist wanted to see what it would take for life to exist in such conditions.
"We wanted to say what was the type of creature that could survive the most extreme conditions on Earth and use that as a benchmark to see what could survive elsewhere," said co-author and cosmologist David Sloan, from the University of Oxford.
So using Earth, they looked at some of the toughest creatures on our planet. Enter the tardigrade.
From there, they wanted to see what it would take to kill off the tough critter. They determined that the only way to wipe out the tardigrade was to have the oceans boil completely.
Luckily, nothing humans have in our arsenal (including all the nuclear weapons in the world) would be capable of doing this, they concluded. So they turned to the ultimate power: the universe.
The researchers looked at various scenarios, including an asteroid impact, a supernova and a gamma-ray burst (GRB). GRBs are short-but-powerful cosmic explosions that emit gamma-ray light, lasting from a few milliseconds to minutes.
They are so powerful that they are about a million trillion times brighter than the sun, as well as brighter than a supernova, which can outshine all the light in its parent galaxy.
If a GRB occurred close enough to Earth, it would strip it of its atmosphere. However, it still wouldn't be enough to boil the oceans, the researchers concluded. The tardigrade would remain the most complex life form on the planet, feeding on single-celled organisms that may live off hydrothermal vents in the ocean.
So what about a supernova — the violent, explosive death of a star?
Well, it turns out, there isn't a star that would die in that manner anywhere close enough to Earth to affect us.
(Some stars die in much less spectacular fashion. If you're curious, the closest star that could go supernova is IK Pegasi, roughly 150 light years from Earth.)
The only thing left, the researchers figured, was an asteroid impact. But organizations such as NASA's Center for Near-Earth Object Studies have so far found about 95 per cent of all large objects in the solar system — and none are large enough to make our oceans boil off into space.
"You'd need … something very, very big. And there are only about 17 or 18 objects like that in our solar system," Sloan said.
So no matter what is thrown in Earth's way, insofar as we know, these microscopic water bears will still be around when, in one billion years, the sun begins its slow, five-billion-year death, the researchers concluded.
Location, location, location
What makes the tardigrade such a good candidate to be the "last man standing?" It's not just about its adaptability, but also about where it lives.
These creatures can live at the very bottom of our oceans. So even if something were to strip off our atmosphere or deliver extreme radiation, tardigrades would continue to thrive far below where deadly radiation could reach.
Ultimately, that is why the tardigrade gets the last laugh over the cockroach.
"Not only are [cockroaches] weaker to the impact of radiation than the tardigrade … because they live on the surface of the planet, mostly, if you were to strip the Earth's atmosphere, cockroaches would die off," Sloan said. "But tardigrades, which are underwater, would continue to live on."
As a cosmologist, Sloan said he's amazed at just how hardy tardigrades are.
"I'd always thought of life as being very, very fragile," he said. "It came as a real shock to me that, in fact, all these doomsday scenarios are very unlikely to be sufficient to kill off all life.
"The general result to me was a real surprise. I was not expecting that life would be just so resilient."