National Geographic

An Alien Origin for Life on Earth

This is the second of three blog posts associated with this week’s episode of Cosmos: A Spacetime Odyssey, which addresses life in the universe. Read the first one here.

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.”

Illustration of the 1833 Leonid Meteor shower. Could meteorites have carried life to Earth? (Edmund Weiss/Wikimedia)

Illustration of the 1833 Leonid Meteor shower. Could meteorites have carried life to Earth? (Edmund Weiss/Wikimedia)

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.

NASA researchers have found the building blocks of DNA in a meteorite. (NASA's Goddard Space Flight Center/Chris Smith)

NASA researchers have found the building blocks of DNA in a meteorite. (NASA’s Goddard Space Flight Center/Chris Smith)

 

There are 10 Comments. Add Yours.

  1. Anarcissie
    May 20, 2014

    Postulating that life comes from elsewhere only begs the question, does it not? We still have to wonder how it got started somewhere.

  2. michael pearson
    May 20, 2014

    First we need to define Life. Today, as we know it, it requires an internal matrix of liquid water for dissolving and diffusing nutrients and metabolites. That’s going to shorten the list of places where it could originate. Forget stardust, icy comets and frozen planets. OCCAM’S RAZOR, people! Panspermia in any form isn’t a theory, not even an hypothesis – it approaches Voodoo Science.
    Peace and Love, Mike xxx

  3. David Grinspoon
    May 20, 2014

    Mike – It’s simply an intriguing possibility about the origin of life here or elsewhere that cannot, at present, be ruled out. That makes it worthy of discussion.

  4. JJ
    May 20, 2014

    An interesting way to test the idea would be to look for historical DNA markers in terrestrial life that could indicate unnatural “seeding” by non-terrestrial organisms. In other words, the DNA markers of terrestrial life “jump started” by an already existing alien organism would probably look different from terrestrial life that was “home grown” from scratch on Earth.

  5. phil
    May 20, 2014

    easy to ruled out.

  6. Peter
    May 21, 2014

    Sure, Panspermia exists. The Apollo missions more than likely seeded the Moon (unlikely to survive!). And of course there is the possibility of “hitch hikers” on the various probes and landers sent to Mars, Venus etc. Most importantly however, we now possess the ability to deliberately send life to Mars-hence panspermia.

  7. David Bump
    May 21, 2014

    The first two or three comments sort of sum up the situation: The lack of any known natural pathway for life to arise on Earth other than from previous life invites consideration of the possibility it started with life from somewhere else, even sent (with purpose and possibly design) by intelligent life, yet that simply suggests that there might be conditions that never existed on Earth which could produce life — but what is life? And life that formed under unearthly conditions wouldn’t likely survive here, or as JJ suggests, there would be something “wrong” with all life here. Then again, perhaps evolution would erase all traces of such a foreign origin — but then there’d be no way to tell!

    The root of the problem is that everybody is ignoring the elephant in the room: the formation of anything close to life, outside of reproduction, is contrary to the way nature works! It only happens in reproduction because of the complete system which already exists and has all the materials, structures, mechanisms, and coding required. There is a vast gulf between that and all processes outside of nature, and all scenarios for the ultimate origin of life have to work on the far side of that gap or admit that … uh oh, we can’t go there. So the origin of life, panspermia or no, may not be “voodoo science” but it is clearly supported only by philosophical necessity and/or plain wishful thinking. It reminds me of attempts to build “free energy” machines or to achieve perpetual motion. It’s covered up by optimistic headlines and statements such as “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” which, upon investigation (i.e. reading all the way to the end of the articles), turn out to leave out major problems — 1) having too many amino acids gums up (literally) the works, and you need to have the ones you want all of one handedness (chirality), something that doesn’t come close to happening outside of living things. 2) “Metabolic pathways” outside of life involve inorganic aspects which would be contrary to life, require seeding with artificially-supplied complex molecules, and are only degenerative (SO much like “perpetual motion” machines) 3). RNA self-reproduction schemes start with artificial conditions and are all basically dead ends, being so good at reproducing exact copies of an RNA sequence that it likely would never do anything else. And each scenario requires that “somehow” it gets together with the others as an integrated system, within some environment with just the right balance between shelter from and exposure to the environment, hopefully (wistfully?) with or developing an enclosing membrane which, again, allows just the right aspects of the environment in (and letting out waste products) while keeping out the unwanted ones.

    Meanwhile, neither in nature outside of living things nor in our attempts to re-create the origin of life under conditions we’ve never seen in nature have we ever seen a organized, dynamically complex system comparable to a wind-up toy come into existence.

    But, hey, science is all about eliminating the miraculous, right? We figured out lightning bolts weren’t being thrown by Zeus, that means everything that ever happened must have happened by natural processes, right? So… have at those windmills, er, giants, guys… have fun!

  8. Patrick O
    May 21, 2014

    Information. The “Universal Laws”. Mathmatical probabilities. All existed at the start of our universe (time). Hence the mathematical sequence that orders RNA/DNA have and will indefinately exist. This is “God’s message”! Wherever and whenever the conditions are right, life will begin.

  9. Mike from Ottawa
    June 25, 2014

    I find panspermia kind of dull as it just punts on the really interesting stuff: how life got started.

    Oh, and Mr Bump, what with the World Cup underway, the score is Insights Gained by Searching for Natural Explanations for Natural Phenomena: Billions and Billions – Insights Gained by Assuming Miracles: Nil.

    • David Bump
      June 26, 2014

      Cute, Mike, but I’m all for searching the natural world for insights into natural processes going on around us, it’s just when “searching for natural explanations” means saying things once (sometime, somewhere, long ago and far away) happened naturally that are exactly opposite to the way we’ve observed nature work “billions and billions” of times, I don’t call that gaining insight, it’s just being stubborn; dogmatic even.

      Looking at living things as coming into existence without any intelligence involved has lead to silly errors, from _Bathybius haeckelii_ to the Tasaday misinterpretation and hype, and more consequential errors such as frequent surgical removal of perfectly healthy and useful organs, and more. We have now seen that living things are full of micro-machines such as rotary engines and walking cargo transporters, that put our attempts at nanotechnology to shame. Why should we cling to the 18th-century philosophical notion that everything can be explained in terms of simple inanimate forces when there’s something about which it’s been blinding us to truths?

Add Your Comments

All fields required.

Related Posts