Friday, February 15, 2008

There’s no place like home
… but that won’t stop us looking for it in our search for extraterrestrials.

[This is the pre-edited version of my latest Muse column for Nature news. Am I foolish to imagine there might be people out there who appreciate the differences? Don't answer that.]


In searching the skies for other worlds, are we perhaps just like the English tourists waddling down the Costa del Sol, our eyes lighting up when we see “The Red Lion” pub with the Union Jack in the windows and Watneys Red Barrel on tap? Gazing out into the unutterably vast, unnervingly strange depths of the cosmos, are we not really just hankering for somewhere that looks like home?

It isn’t just a longing for the familiar that has stirred up excitement about the discovery of what looks like a scaled-down version of our own solar system surrounding a distant star [1]. But neither, I think, is that impulse absent.

There’s sound reasoning in looking for ‘Earth-like’ extrasolar planets, because the one thing we can say for sure about these places is that they are capable of supporting life. And it is entirely understandable that extraterrestrial life should be the pot of gold at the end of this particular rainbow.

Yet I doubt that the cold logic of this argument is all there is behind our fascination with Earth-likeness. Science-fiction writers and movie makers have sometimes had fun inventing worlds very different to our own, peopled (can we say that?) with denizens of corresponding weirdness. But that, on the whole, is the exception. That the Klingons and Romulans looked strangely like Californians with bad hangovers was not simply a matter of budget constraints. Edgar Rice Burroughs’ Mars was apparently situated somewhere in the Sahara, populated by extras from the Arabian Nights. In Jeanette Winterson’s new novel The Stone Gods, a moribund, degenerate Earth (here called Orbus) rejoices in the discovery of a pristine Blue Planet in the equivalent of the Cretaceous period, because it offers somewhere to escape to (and might that, in these times haunted by environmental change, nuclear proliferation and fears of planet-searing impacts, already be a part of our own reverie?). Most fictional aliens have been very obviously distorted or enhanced versions of ourselves, both physically and mentally, because in the end our stories, right back to those of Valhalla, Olympus and the seven Hindu heavens, have been more about exploring the human condition than genuinely imagining something outside it.

This solipsism is understandable but deep-rooted, and we shouldn’t imagine that astrobiology and extrasolar planetary prospecting are free from it. However, the claim by the discoverers of the new ‘mini-solar system’ that “solar system analogs may be common” around other stars certainly amounts to more than saying “hey, you can get a decent cup of coffee in this god-forsaken place”. It shows that our theories of the formation and evolution of planetary systems are not parochial, and offers some support for the suspicion that previous methods of planet detection bias our findings towards oddballs such as ‘hot Jupiters’. The fact that the relatively new technique used in this case – gravitational microlensing – has so quickly turned up a ‘solar system analog’ is an encouraging sign that indeed our own neighbourhood is not an anomaly.

The desire – it is more than an unspoken expectation – to find a place that looks like home is nevertheless a persistent bugbear of astrobiology. A conference organized in 2003 to address the question “Can life exist without water?” had as part of its agenda the issue of whether non-aqueous biochemistries could be imagined [2]. But in the event, the participants did not feel comfortable in straying beyond our atmosphere, and so the debate became that of whether proteins can function in the dry or in other solvents, rather than whether other solvents can support the evolution of a non-protein equivalent of enzymes. Attempts to re-imagine biology in, say, liquid methane or ammonia, have been rare [3]. An even more fundamental question, which I have never seen addressed anywhere, is whether evolution has to be Darwinian. It would be a daunting challenge to think of any better way to achieve ‘design’ and function blindly, but there is no proof that Darwin has a monopoly on such matters. Do we even need evolution? Are we absolutely sure that some kind of spontaneous self-organization can’t create life-like complexity, without the need for replication, say?

Maybe these questions are too big to be truly scientific in this form. Better, then, to break bits off them. Marcelo Gleiser and his coworkers at Dartmouth College in New Hampshire have done that in a recent preprint [4], asking whether a ‘replica Earth’ would share our left-handed proteins and right-handed nucleic acids. The handedness here refers to the mirror-image shapes of the biomolecular building blocks. The two mirror-image forms are called enantiomers, and are distinguishable by the fact that they rotate the plane of polarized light to the left or the right.

In principle, all our biochemistry could be reversed by mirror reflection of these shapes, and we’d never notice. So the question is why one set of enantiomers was preferred over the other. One possibility is that it was purely random – once the choice is made, it is fixed, because building blocks of the ‘wrong’ chirality don’t ‘fit’ when constructing organisms. Other explanations, however, suggest that life’s hand was biased at the outset, perhaps by the intrinsic left-handedness in the laws of fundamental physics, or because there was an excess of left-handed amino acids that fell to Earth on meteorites and seeded the first life (that is simply deferring the question, however).

Gleiser and his coworkers argue that these ideas may all be irrelevant. They say that environmental disturbances strong and long enough can reset the handedness, if this is propagated in the prebiotic environment by an autocatalytic process in which an enantiomer acts as a catalyst to create more of itself while blocking the chemical reaction that leads to the other enantiomer. Such a self-amplifying process was proposed in 1953 by physicist Charles Frank, and was demonstrated experimentally in the 1990s by Japanese chemist Kenso Soai.

The US researchers show that an initially random mixture of enantiomers in such a system quickly develops patchiness, with big blobs of each enantiomer accumulating like oil separating from vinegar in an unstirred salad dressing. Chance initial variations will lead to one or other enantiomer eventually dominating. But an environmental disruption, like the planet-sterilizing giant impacts suffered by the early Earth, can shake the salad dressing, breaking up the blobs. When the process begins again, the new dominant enantiomer that emerges may be different from the one before, even if there was a small excess of the other at the outset. As a result, they say, the origin of life’s handedness “is enmeshed with Earth’s environmental history” – and is therefore purely contingent.

Other researchers I have spoken to question whether the scheme Gleiser’s team has considered – autocatalytic spreading in an unstirred solvent – has much relevance to ‘warm little ponds’ on the turbulent young Earth, and whether the notion of resetting by shaking isn’t obvious in any case in a process like Frank’s in which chance variations get amplified. But of course, in an astrobiological context the more fundamental issue is whether there is the slightest reason to think that alien life will use amino acids and DNA, so that a comparison of handedness will be possible.

That doesn’t mean these questions aren’t worth pursuing (they remain relevant to life on Earth, at the very least). But it’s another illustration of our tendency to frame the questions parochially. In the quest for life elsewhere, whether searching for new planets or considering the molecular parameters of potential living systems, we are in some ways more akin to historians than to scientists: our data is a unique narrative, and our thinking is likely to stay trapped within it.


References


1. Gaudi, B. S. et al. Science 319, 927-930 (2008).
2. Phil. Trans. R. Soc. Lond. Ser. B special issue, 359 (no. 1448) (2004).
3. Benner, S. A. et al. Curr. Opin. Chem. Biol. 8, 672 (2004).
4. Gleiser, M. et al. http://arxiv.org/abs/0802.1446

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