ETI—the Search for Extra-Terrestrial Intelligence—may be searching the skies in altogether the wrong medium. If the theories of scientists like Chandra Wickramasinghe and Bruno D'Argenio are proven correct, then Earth is on the receiving end of an extremely slow, extremely lossy, yet extremely high-data-content digital broadcast. Its carrier? Microbial spores.
The theory of "panspermia"—the notion that life on Earth originated in space—was mentioned here in "The Dark Between the Stars" (October 2000), and discussed in passing as part of "Microscopic Ghosts of Mars" (August 2001). However, it's a rich and fascinating subject, and given some recent findings in biology and planetary astronomy, it deserves a column all to itself, where the implications can be explored in detail.
Basically, the idea is that outer space—interplanetary and probably interstellar—is heavily contaminated with bacterial spores, which are capable of remaining viable across absurdly long stretches of time and space, and which bloom easily when they come in contact with a suitable planet, such as the Earth of 3.8 billion years ago. The idea was first proposed in 1900 by Swedish chemist Svante Aarhenius, and although his theory gained attention in 1976 with support from the eminent astronomer and SF novelist Sir Fred Hoyle, and soon after from Nobel laureate Francis Crick (co-discoverer of the double-helix structure of DNA), it was considered kooky and wrong-at-a-glance by most scientists at that time.
Critics of the idea had a lot of ammunition. To travel to Earth, any life form would have to survive the vacuum, radiation and temperature extremes of outer space, perhaps for millennia or even eons. But however preposterous this may have seemed in the '70s, we now know that the spores of bacteria and archaea (the most primitive single-celled life forms) can not only survive the conditions of outer space, but can also remain viable for millions of years. Eerily, spores preserved in amber since the time of the dinosaurs have been successfully revived, with no apparent ill effects from their long slumber, and no observable differences from the bacteria of today. Too, there is increasing geological evidence that life on Earth appeared not after an extended period of agonizing, bit-by-bit biochemical experimentation, but all at once, almost as soon as the planet's surface had cooled enough to support liquid water.
The concepts of panspermia finally entered the mainstream debate with NASA's 1996 announcement of possible microfossils in the Alan Hills meteorite ALH84001 (again, see "Microscopic Ghosts of Mars"), but since then a bunch of other strange findings have been unveiled that have kicked the debate into higher gear.
Bacteria's behavior continues to baffle
First, in 2001, Wickramasinghe, of Cardiff University in Wales, used a high-altitude balloon to gather microbes from the upper stratosphere, where there should be no life. That is, bacteria can sometimes be blasted up into the stratosphere by volcanic explosions and such, but according to theory they should quickly settle back down, like sand grains in a glass of water. In the lower atmosphere—the troposphere—wind carries dust and other material (including bacteria) off the ground and can lift it high into the clouds. But with their very different temperature and density profiles, the troposphere and stratosphere are like oil and water—they don't mix. So once the bacteria fall back into the troposphere, they can't get back up again until another volcano comes along. And the thing about that is, there weren't any big explosions in the months before Wickramasinghe's experiment. So why did the balloon's sample jars come back with bacteria in them?
What's more, Wickramasinghe claims that the patterns of their occurrence—denser at higher altitudes—indicate that they are falling from above, rather than rising from beneath. He hopes to do an isotopic analysis, which is difficult because the samples are so small. But the isotope ratios for each element—slight variations in the mass and composition of an atom's nucleus—form a "planetary fingerprint" which will determine conclusively whether the bacteria originated on Earth or, as Wickramasinghe suspects, on a comet traveling through interplanetary space. In related news, he claims to have identified the characteristic photon emissions of bioluminescent bacteria in the cosmic dust of our galaxy's star-forming regions. Both claims have met with considerable skepticism, but then, so did Wickramasinghe's original 1976 paper co-authored with Fred Hoyle, whose arguments have largely been borne out.
An even more controversial claim came from Naples University's D'Argenio, who claimed in mid-2001 to have isolated and revived bacteria he found in a 4.5-billion-year-old meteorite. "Their genetic code is unlike any known on Earth," he told reporters. Then, in September 2002, astrobiologist Dirk Schulze-Makuch from the University of Texas at El Paso announced that some of the chemicals in the hot, dry, acidic atmosphere of Venus looked a little suspicious. There was not enough carbon monoxide (something Earthly bacteria eat), and much too much hydrogen sulphide, sulphur dioxide, and carbonyl sulphide—short-lived gases which don't normally stick around unless replenished by bacteria. It's not quite a smoking gun, but it's ... hard to explain.
Hmm.
And then there's Anatoli Pavlov from the Ioffe Physico-Technical Institute in St. Petersburg, Russia. He took a hard look at Earth's radiation-resistant bacteria (Deinococcus radiodurans, which are often found thriving in the deadly confines of nuclear reactors), and concluded that no environment on Earth could have produced them. When he tried to force evolution's hand by exposing normal bacteria to increasing radiation doses and then breeding the survivors, generation after generation, he found himself using doses higher than any natural bacterium— even one which had lain dormant for billions of years—could ever accumulate on Earth. "Perhaps they evolved on Mars" or elsewhere, Pavlov suggests, where the radiation hazards are greater.
Maybe he's a nut. Maybe these guys are all nuts, seeing what they want to see because they need, on some deep psychological level, to believe there is life out there somewhere. Indeed, that life on Earth is directly related, in an almost Star Trek-y kind of way, to life everywhere else in the universe. But the evidence for panspermia grows stronger year by year, while the arguments against it grow weaker. We used to believe biology was fragile, but as NASA's Everett K. Gibson says, "Once life takes root, we see now that it's very tenacious, very difficult to extinguish." And the chemical building blocks, once thought to be rare, are turning up in every corner of the universe.
The aliens may already be here
In the Journal of the British Interplanetary Society, Robert Zubrin, president of the Mars Society, has gone these studies one better with a statistical analysis of the bombardment of planets by killer meteorites, which are capable of lifting microbes into orbit and beyond. According to Zubrin, the interplanetary transport of life is already a certainty. "500 kg of unsterilized Mars rocks are estimated to fall on Earth every year," he says, noting that the transfer works both ways. He then follows up with a study of the dispersal patterns of interplanetary debris and the various mechanisms by which material can travel—slowly!—between the stars.
If even one life-bearing planet is smacked in the proper way, there'll be a cloud of bacteria, dried and shriveled into spores, slowly spreading across the void. Cosmic radiation slowly kills them off—half of the swarm slowly dies, and then half of those survivors are killed, and half again. After ten million years, only a tiny fraction of the original critters remain viable. But that could still mean a large, large number of individual organisms, covering an even larger volume of space. "The transmission of microbial life from one solar system to another is highly probable," Zubrin concludes. And even if Earth is not a receiver, over the 3.5-billion-year history of life it has surely served as a transmitter, capable of reaching an estimated 400 nearby star systems with an average of perhaps 0.04 grams of bacteria each. Thus, according to Zubrin, Earthlike microbes should be considered "widespread throughout the galaxy."
Are there competing formats? Could Earthly life be the Amiga or Betamax of the galaxy, or are we all standardized to a common genetic platform? With mass standardization comes mass vulnerability, as any Microsoft Outlook user will attest. If bugs from space have all the right biological machinery to feed on Earthly life, then they can make us sick. Maybe they already do; maybe that's where certain diseases come from. But orphan formats carry risks of their own. If there are alternate forms of life out there, perhaps they spread faster or use energy and chemical resources more efficiently. Maybe our kind of life is locked in a constant, invisible battle sprawling epically across the galaxy like microscopic Jedi and Sith. I can't decide if that's a cool thought or a horrible one, but it certainly gives us something to reflect on when we look up at the nighttime sky.
Anyway, assuming this is all true and bacterial spores really do rain down on our planet from broadcast points elsewhere in the galaxy, the next logical question is whether the signal is of natural or intelligent origin. To SETI or not to SETI? Are there secret messages hidden in the DNA? Astronomers Peter Ward and Donald Brownlee have argued that while microbial life may be common, complex multicellular life (and especially intelligence) must be terribly rare. Some might argue further that the arrival of squidgy biologicals, rather than energy creatures or hyperefficient diamondoid nanomachines, would rule out Little Green Johnny Appleseed as a probable source. Too, Occam's razor (the guiding principle that the simplest explanation is usually the best) would point us to the many natural mechanisms (e.g., meteor impact, volcanic eruption) that could lift microbes out into space without intelligent help.
But if the question is whether life exists elsewhere in the universe, then the medium itself may be tapping out our answer like Morse code: yes ... yes ... yes yes ... yes ...
Wil McCarthy is a rocket guidance engineer, robot designer, science-fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog, Asimov's, Wired and other major publications, and his novel-length works include the New York Times notable Bloom and The Collapsium.
