Microscopic Tardigrade ‘Water Bears’ Will Be the Last Survivors on Earth
This creature can survive the risk of extinction posed from all known major astrophysical catastrophes, and suggests what one potential form of extraterrestrial life might look like.
Our mammalian ancestors survived the K-T extinction, but today’s mammals — including humans — would likely not be so fortunate should an event of greater scale happen again. The same holds true for birds, reptiles, and many of the world’s species.
At least one animal, however, is predicted to survive nearly every natural disaster, except for the death of the sun. According to a new study published in Scientific Reports, that extraordinarily resilient creature is the tardigrade, an eight-legged micro-animal that is also known as the water bear, space bear, and moss piglet. Whatever one wants to call it, researchers predict that this tiny species will be around for at least 10 billion years.
Tardigrades can survive for up to 30 years without food or water. They endure temperature extremes ranging from well below freezing to 302 degrees Fahrenheit, and have been found in the ocean depths. Although just .02 inches long, their lifespan is about 60 years.
To assess what apocalyptic event could lead to the annihilation of all life on Earth, the researchers used detailed mathematical modeling to predict what would happen to the tardigrade — the world’s most indestructible species — should there be a major asteroid impact, supernova, or gamma-ray burst. The overarching goal of the research was to find out how long life can survive on a planet once it has formed there.
“Most previous studies focused on the survival of humans, which are very sensitive to changes in the atmosphere or climate of the earth and can be eliminated by the impact of an asteroid, nuclear winter, or bad politics,” said senior author Abraham Loeb, who is a professor and chair of the Harvard Astronomy Department and is the director of the Institute for Theory and Computation. “To our surprise, tardigrades are likely to survive all astrophysical catastrophes.”
Loeb conducted the investigation with lead author David Sloan of the University of Oxford’s department of physics-astrophysics and his Oxford colleague and co-author Rafael Alves Batista.
Regarding asteroids, the scientists calculated that there are only a dozen known rocky sun-orbiting bodies with enough mass to boil the oceans — a calamity that could wipe out even the sturdy tardigrades. These include Vesta and Pluto.
“In terms of species death, a big asteroid would be the largest threat,” Sloan said.
But Loeb added that “such events are extremely rare and will not happen before the sun will die.”
In terms of supernovae, the researchers found that in order for an exploding star to boil the oceans, it would need to be 0.14 light-years away. The threat posed from these exploding stars to Earth is negligible, they determined, since the closest star to the sun is four light years away. The probability of a massive star exploding close enough to Earth to kill all forms of life on it, within the sun’s lifetime, is therefore very low.
If this phenomenon were to happen, Sloan said, “a nearby supernova would be a disastrous event for most life on a planet, but species like the tardigrade could well live on.”
Gamma-ray bursts are even brighter than supernovae, but the scientists believe that they are too far away from Earth to be considered a viable threat. To boil our planet’s oceans, they would need to be no more than 40 light-years away, so the likelihood of a burst occurring at such proximity to Earth is minor.
“Mainly we’ve found that, once life starts, it seems very unlikely that it will be killed off by any of the usual suspects for annihilating humans,” Sloan said. “If we find planets fit for life and search them, if life ever was there, it probably still should be in some form.”
The researchers speculate that tardigrades, or animals similar to them, could be living on planets other than Earth, so long as there is a source of water.
Last year, genetic analysis on one of the most stress-tolerant tardigrade species — Ramazzottius varieornatus — identified a unique protein that makes it so tough. An accompanying video (above) showed how a severely dehydrated tardigrade will display no visible signs of life before it is resuscitated with a drop of water.
“Water is needed to revive them, but they have been shown to stay alive after being dehydrated,” Loeb said. “Their systems shut down without water, but once water is introduced they are brought back to life and can reproduce again. This was demonstrated in an experiment done a decade ago where they were taken out to space for a week and exposed to vacuum and dehydration; when brought back, a significant fraction of them produced healthy embryos.”
Astronomers are soon hoping to perform spectroscopy, a process that seeks signatures for life, on exoplanets. The new experimental study suggests what they might find.
In the meantime, other scientists at Harvard are studying tardigrade genes to better understand what makes these animals so sturdy. They suspect that tardigrades can repair their DNA quickly, a survival tactic that likely evolved after years of withstanding punishing conditions in their often-extreme environments.
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