Whether the universe is filled with alien beings who wish upon stars, struggle to understand the subatomic realm and argue over who’s paying for dinner is not yet known. We’re looking for them.
But in the search for life, there’s another fundamental question that has gone unanswered for millennia. Step one in the development of any civilization is life itself. How, exactly, does life get going?
“The essential message of life has been copied and recopied for more than three billion years,” says Neil deGrasse Tyson, on this week’s episode of Cosmos: A Spacetime Odyssey. “But where did that message come from?”
Even on Earth, the origin of that on-switch is murky. We don’t know how a pile of organic molecules, their atoms arranged in intricate rings and bridges, gained the ability to survive and replicate, to wall themselves off from a young Earth’s iron-rich seas and oxygen-free air.
Some ideas suggest that life’s first gasp came from shallow ponds, warmed by a sun still in its childhood; others point to bubbling hot springs, clays, ice, or to warm, energy-rich vents erupting from the deep ocean floor.
For decades, scientists have tried to replicate the planet’s primordial recipe for life. They’ve mixed salty brews, spiced them with metals and smelly gases, and jolted the mixes with electricity, or sunlight, or heat, then reset the timer and started all over with a new handful of ingredients and instructions. These experiments have taught us a lot. Among other things, we’ve learned that amino acids are sort of easy to make from scratch, that complex metabolic pathways can emerge from a seemingly random mix of ingredients, and that single-stranded, ribonucleic enzymes can replicate themselves indefinitely.
But none of these experiments have produced the secret sauce that sparked the first single-celled organisms. Each time, when the oven timer chirped, there was no life.
Enter: Another theory that’s been simmering for years (millennia, even). What if, it asks, instead of being baked from scratch on Earth, life came from the stars?
“If life can withstand the hardships of space, and endure for millennia, then it could ride the natural interplanetary transit system from world to world,” Tyson says. “What this means is that life doesn’t have to start over again.”
Called panspermia, the theory suggests that organisms hitchhiking from one world to another can spread the organic seeds of life throughout the cosmos. Launched into space aboard blasted out bits of planetary debris, these space-faring life-forms could, upon arrival at an alien planet, survive and thrive – perhaps evolving into spiders, sharks (or spidersharks?), dandelions and elephants.
Obviously, no one knows whether panspermia actually happens. For years, the idea failed to gain strong scientific traction. But recent pieces of circumstantial evidence suggest that in some environments, such as the inner solar system, versions of panspermia aren’t so farfetched.
For starters, fragments of other planets have made their way to Earth. We have pieces of Mars and Mercury (maybe), and (probably) Venus on our planet. Pieces of Earth have undoubtedly made their way to our neighbors. This exchange of crusty planetary material, if it harbored the right kind of hardy organism, could conceivably transfer life from one world to the next, says astronomer Caleb Scharf of Columbia University.
“I’d say that a plausible, but entirely unproven, mechanism exists for the transfer of viable organisms,” Scharf says.
There are creatures on Earth that would probably consider an interplanetary trip a worthy challenge. Take tardigrades, for example, the tiny, tough invertebrates that have survived 10 days in space. Lichens have survived the same freezing vacuum for more than two weeks. Some microbes, like Deinococcus radiodurans, are especially tolerant of the levels of radiation they’d likely encounter during a trip to Mars. And organisms frozen for centuries beneath the ice in Antarctica have been revived in labs.
“We have no reason to believe that some microbes can’t survive interplanetary journeys inside of meteorites,” says astrobiologist David Grinspoon of the U.S. Library of Congress.
But, he says, spending two weeks in space and living to tell the tale is different from crash-landing after a decades-long interplanetary voyage and setting up shop in a new world. It isn’t enough to simply arrive – organisms have to thrive.
“We tend to separate the possibility of exchange of viable organisms between planetary bodies and the possibility that they can ‘seed’ a world,” Scharf says. “It’s just not clear that even the hardiest Earth microbes, dumped supersonically onto Mars (for example), are going to get a foothold. The Martian surface is nasty for terrestrial biology.”
Interplanetary panspermia as a dispersal mechanism seems fairly plausible, then, if unproven. Is it possible that young Earth, Venus, and Mars traded life-forms for a few hundred million years? (See Scharf’s treatment of a panspermic paradox here.)
Cosmos also introduced the idea of interstellar panspermia, which magnifies all the challenges associated with the inner solar system’s planets playing meteorite ping-pong. In other words, it’s a bit trickier to transport life from stellar system to stellar system. Distances are much greater, and the time it would take for a meteorite bearing life-forms to arrive on another world is substantially longer. To some scientists, it seems unlikely that such a thing is possible.
“Eventually, cosmic radiation would shred the genetic material beyond any ability to self-repair,” Grinspoon says.
On the other hand, he notes, stars are born in clusters. And at the age when young stars are busy assembling their planetary systems, the distances between them are much smaller. Our solar system would have exchanged material with these systems, Grinspoon says. Perhaps it’s during this period of stellar infancy that stars stud their sister systems with seedlings.
And there’s a third version of panspermia waiting in the wings, one proposed by Francis Crick and Leslie Orgel in 1973: Directed panspermia, or the idea that intelligent beings intentionally send life to other worlds.
“It now seems unlikely that extraterrestrial living organisms could have reached the earth either as spores driven by the radiation pressure from another star or as living organisms imbedded in a meteorite,” they wrote. “As an alternative to these nineteenth-century mechanisms, we have considered Directed Panspermia, the theory that organisms were deliberately transmitted to the earth by intelligent beings on another planet.”
But the pair concludes that there’s feeble evidence supporting the deliberate seeding of Earth with alien life, and similarly feeble evidence supporting the abiotic emergence of life on Earth. “Both theories should be followed up,” they wrote.
The idea that faraway, intelligent beings – or perhaps the bored teenagers of the species – might be intentionally hurling life at planets is truly science fiction. But it’s a universe of infinite possibilities, right?
“If you imagine that intelligent, technological life exists on other worlds – which I do imagine,” Grinspoon says, “Then what would lead you to conclude that nobody anywhere in the galaxy has ever tried such a stunt?”
Looking toward the stars for our origins seems, perhaps, like the kind of explanation one ought to turn to when all other attempts to flick life’s on-switch have failed. It’s more than plausible that the building blocks of life – amino acids, nucleobases, sugars – were delivered to Earth by asteroids or comets. We know that asteroids in the solar system are carrying complex organic molecules. And not only that, complex organic molecules have been spotted wafting through interstellar space.
Put more simply, the galaxy is littered with the building blocks of life. But it could also be littered with life itself. And maybe, from across the interstellar sea, some of those organisms came to Earth, crawled out of their space rocks and flourished on their new cosmic shores.