Can We Recognize Aliens?

The National Research Council has released a report on the search for alien life forms that warns -- rightly, in my view -- against "Terran" thinking. Terran, or Earth-based thinking, is the kind of necessarily blinkered expectation that takes only the Earth as its model. Life out there, Terran thinking assumes, looks roughly like life here, perhaps with some fanciful variations. When H.G. Wells wanted to imagine sentient Martians he essentially made them great big octopi -- a better move than handsome people in silver jumpsuits, but just as provincial.

I spoke with one of the NRC panel members, Steve Benner, recently in the course of researching an article on the RNA World hypothesis -- the suggestion that RNA predates DNA, and was once the only chemical coding system on the planet. Benner is a terrifically fun person, and has that condition someone once used to describe Murray Gell-Mann: "He has three brains, and each of them is faster than yours." He's also doing amazing things: the Benner lab pioneers a field called "synthetic biology‚" which is just what it sounds like. They have already synthesized an expanded DNA alphabet that generates its own proteins as well as self-replicating molecular systems that undergo chemical evolution . . . on their own. When I asked him how this was different from creating life in a test tube, he laughingly said, "If I called it that, I wouldn't get funding."

At the time I spoke with him Benner had already been commissioned to write up a series of NASA Astrobiology Institute guidelines on "what to look for when looking for aliens" -- our most far-reaching guesses at what galactic biosignatures might look like. In those as well as the current NRC report, he underscores the real possibility that we might travel all those millions of miles and miss the prize because it doesn't occur to our robotic probes that, say, the ammonia-rich ice crystals they're rolling over might be alive.

Could we really meet ET without knowing it? I'm going to hazard a guess here and suggest that it"s very unlikely we *would* know it, at least at a first pass. (Even life on Earth is far more diverse, and frankly weird, than most people imagine.) The Martian meteorite ALH 84001 gives an excellent example. It looked to some people like microfossils. Others said no, these were simple geological formations. The latter view has won out, barring dramatic new data. But the whole tenor of the discussion encapsulates the problem: are we looking at living things or not?

And that's a relatively simple case. Presumably, the farther we go from microbes, the more complex the problem. One of the many strengths of Stanley Kubrick's 2001: A Space Odyssey is the out-and-out weirdness of the stargate sequence: what on earth are we looking at? Is this machinery, natural phenomena, a hallucination, a dream? All attempts to read the sequence literally are bizarrely frustrated, throwing us up against that realization that it can't be understood in the terms we are used to applying. The film compellingly replicates, at least for me, the experience that I imagine awaits us when we actually encounter sentient technological civilization: a big part of the challenge is going to be just figuring out what is happening.

At a lower-brow level, when you bring up the question of defining life at parties someone invariably mentions the Horta, a silicon-based organism from the original Star Trek series that looks a lot like an extra underneath a rug. This rocky critter shuffled around with an eerie scraping sound, suggesting it was midway between a huge amoeba and a pile of bricks. Could life really be based on silicon? I ran this question past a panel of SETI researchers at a Harvard conference two years ago and the response was actually pretty skeptical: don't rule it out, but, as one panel member said, the biproduct of cellular respiration is carbon dioxide. What's another name for silicon dioxide? Sand.

The point is double: it's hard to imagine how sand could be part of an active, metabolizing system. It seems the very definition of inert. Second point: who knows? So far we have an N of 1 regarding life blueprints: carbon-based, water-solvent, DNA-coded, and more or less smooshy. But does any living thing have to meet these requirements? If we write silicon life out of the definition a priori, you can be guaranteed we won't ever find it.

Other possibilities: life that uses methane as a solvent (Titan, anyone?), life that uses RNA instead of DNA (Benner and others speculate that RNA-based life might even still be present on Earth, in super-remote locations such as under the ocean floor), life that uses some as-yet undiscovered coding mechanism; life in a gaseous form. No one to my knowledge has suggested plasma-based life, though it appears in David Brin's novel Sundiver and, actually, in Arthur C. Clarke's original Space Odyssey, a much more literal sci-fi treatment than its famous film version. Our stargate-hopping hero witnesses a permanent solar flare he suspects of sentience. I personally can't help but think of the alien zoo in Stanley Kubricks's earlier film Dark Star that includes, among other organisms, a species that seems to be nothing more than a hovering cloud of lights.

Is that absurd? If nature has shown herself to be anything, it's resourceful. When we do succeed in finding, and recognizing, life out there, the aliens of popular imagination will no doubt seem the most absurd suggestion of all.