In 2018, a spacecraft departed the International Space Station carrying unusual cargo: Colonies of bacteria that had spent years hanging out in space. These intrepid microbes were the final samples to be returned to Earth as part of the Tanpopo mission, a Japanese astrobiology experiment studying the effects of the space environment on simple organisms. If the microbes survived long-term exposure to the vacuum, it would be a significant boost for a controversial theory known as panspermia, which suggests that life hitches a ride between planets on asteroids, comets, and space dust.
On Wednesday, new research from the Tanpopo team was published in Frontiers in Microbiology that details how multiple species of Deinococcus bacteria survived three straight years of exposure to the hostile space environment. This type of bacteria is renowned for its unusual ability to resist genetic damage from high doses of ultraviolet radiation, which classes it among other so-called “extremophiles” like tardigrades. But researchers weren’t sure exactly how it pulled off this feat.
“Deinococcus is known to have several mechanisms to survive in harsh environments,” says Akihiko Yamagishi, a professor at Tokyo University and the lead scientist for the Tanpopo mission. “We tested which mechanisms are responsible and found, among others, that its DNA repair system is important for surviving in the space environment.”
As part of the Tanpopo experiment, Yamagishi and his colleagues exposed dried colonies of three different species of Deinococcus to the vacuum of space in an experiment module attached to the outside of the space station. When the researchers rehydrated the colonies back on Earth, they found that their outermost layers had died from exposure to high doses of UV radiation. But the dead layers of bacteria protected the DNA of the microbes underneath from getting too damaged to survive. While the number of intact bacterial genes gradually decreases from exposure to space no matter how thick the colony is, the team’s results show that a pellet of bacteria just half-a-millimeter deep could survive for up to eight years in space.
It’s good news for proponents of panspermia, a theory that dates back to the early 1970s and suggests that life—including life on Earth—is seeded throughout the galaxy by microbes catching a ride on space rocks. It’s far from a mainstream idea, but one of its earliest proponents, the mathematician Chandra Wickramasinghe, argues that it can explain several thorny problems with the emergence of life on Earth.
The typical explanation is that life emerged from a bunch of organic molecules slamming into each other in a roiling primordial ooze and gradually formed more complex molecules. Eventually these molecules combined to form single-celled organisms like bacteria, which then evolved into multicellular organisms, and so on. But evolution of life on Earth, as far as scientists can tell, proceeded in fits and starts. There were short spikes of speciation interspersed by longer periods of stasis. Once bacteria hit the scene about 4 billion years ago, they remained the dominant lifeform for 2 billion years. Then there was an explosion of slightly more complex single-celled organisms called eukaryotes, which dominated for another billion years before more complex organisms finally started to crop up.
Accounting for these long evolutionary lulls is tricky. One explanation is that these periods of equilibrium were punctuated by mass extinction events that created new opportunities for speciation. Believers in panspermia believe Earth’s unusual evolutionary timeline can be explained if early life was given a little nudge by wayward extraterrestrial microbes.
One panspermia theory, known as lithopanspermia, suggests that asteroids and meteorites slamming into Earth may have contained some basic organisms or genetic material that shifted the evolutionary trajectory of life on the planet. The number of bacteria arriving on a single asteroid aren’t likely to shift evolution on an entire planet. But if biologically-rich space rocks were common in this part of the galaxy, they argue, the heavy bombardment that Earth experienced 4 billion years ago might have been enough to do the trick. It’s a big assumption, but there’s some evidence to back up the idea. “When protected deep inside a rock, calculations have shown that bacteria can survive up to millions of years,” says Avi Loeb, a physicist at Harvard University.
Simulations have shown that the process can go the other way, too. If life had already kicked off when Earth was getting pulverized by asteroids billions of years ago, some of those terrestrial microbes may have been blasted from the surface and ended up on another planet in the solar system. It sounds far-fetched, but researchers have discovered rocks from Mars on Earth, and there’s good reason to suspect that we might one day find Earth rocks on Mars, says Loeb.
So while there’s a chance that we’re all Martians, there’s also the possibility that if NASA finds evidence of life on Mars, it actually came from Earth. Indeed, if scientists discover life on Mars, one of the biggest challenges will be determining its provenance. But there are certain tell-tale clues. For example, all DNA on Earth twists in the same direction; if it goes the opposite way on Mars, that pretty strong evidence that life originated independently on the Red Planet and didn’t catch a ride from Earth.
Even if you don’t buy the panspermia hypothesis, the Tanpopo team’s results have serious implications for planetary protection, says Loeb. NASA and other space agencies go to a lot of trouble to make sure that spacecraft don’t carry any biological material from Earth along for the ride when they’re launched to Mars. They don’t want to inadvertently spoil the pristine environment where they’re searching for faint signs of ancient alien life. But now Yamagishi and his team in Japan have shown that certain types of bacteria can survive in deep space long enough to make an interplanetary journey without any protection.
“Any millimeter-thick aggregates would appear as dirt to the eye and would be wiped off the surface of a clean room in preparation of a related mission to Mars,” says Loeb. “But these results remind us of the importance of sterilizing all spacecraft sent to other planets in search of life.”
In 2018, Yamagishi and his colleagues conducted a series of high-altitude experiments on Earth using planes and weather balloons and found traces of Deinococcus bacteria nearly 8 miles up in the atmosphere. That’s well above the cruising altitude of a passenger jet. But that doesn’t mean that Earth is leaking these hardy microbes into space; small colonies of microbes floating on the breeze don’t have the velocity to escape the pull of Earth’s gravity. But as Loeb detailed in a paper he co-authored earlier this year, it may be possible that asteroids and comets that graze Earth’s atmosphere, like a stone skipping on a pond, pick up some microbes in the atmosphere and carry them into interstellar space.
The theory of panspermia remains controversial and isn’t widely accepted in the scientific community. But, for now, experiments like Tanpopo continue to challenge our assumptions about the conditions that are necessary for life.
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