Published online 15 February 2008 | Nature | doi:10.1038/news.2008.571

Column: Muse

There's no place like home

...but that won’t stop us looking for the familiar in our search for extraterrestrials, says Philip Ball.

In searching the skies for other worlds, are we perhaps just like the English tourists waddling down the Costa del Sol, eyes lighting up when they see “The Red Lion” pub with the Union Jack in the windows and Watneys Red Barrel on tap? Gazing out into the 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 Solar System surrounding a distant star1 (see 'Solar System match made in the heavens'). There’s sound reasoning in looking for a place like Earth, because the one thing we can say for sure about that is that it is capable of supporting life. And it is entirely understandable that extraterrestrial life should be the pot of gold at the end of the extrasolar-planet rainbow.

Yet I doubt that the cold logic of this argument is all there is behind our fascination with Earth-likeness. Some of it is surely down to our desire to seek out the comfortingly familiar.

Science-fiction writers and movie-makers have sometimes had fun inventing worlds very different from our own, peopled (if you can call it 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 just somewhere in the Sahara, populated by extras from the Arabian Nights. And Jeanette Winterson’s new novel The Stone Gods features a moribund, degenerate Earth (here called Orbus) and a pristine Blue Planet that offers somewhere to escape to.

Most fictional aliens have been very obvious versions of ourselves, distorted or enhanced, because in the end our stories of other worlds, 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.

Life as we don't know it

The desire — it is more than an unspoken expectation — to find a place that looks like home is also 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 imagined2. But in the event, the participants did not feel comfortable looking beyond the Earth-bound dimensions of that question. They debated whether proteins could function in the dry or in other solvents on Earth, rather than whether other solvents could support the evolution of a non-protein equivalent of enzymes elsewhere.

Attempts to re-imagine biology in, say, liquid methane or ammonia, have been rare3. An even more fundamental question, which I have never seen addressed, 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.

Maybe these questions are too big to be truly scientific. Better perhaps, then, to break off smaller chunks.

One bit of the question

Marcelo Gleiser and his co-workers at Dartmouth College in New Hampshire have tackled one bite-sized piece of the bigger question in a recent preprint4. They asked, in effect, whether a ‘replica Earth’ would share our left-handed proteins and right-handed nucleic acids. (The handedness here refers to the mirror-image shapes, or enantiomers, of the biomolecular building blocks.)

In principle, all our biochemistry could be reversed by mirror reflection of these shapes, and life would still 'work'. So the question is why one set of enantiomers was preferred over the other.

One possibility is that it is 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 some 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 of course simply defers the question though).

Gleiser and his colleagues argue that these ideas might all be irrelevant. They say that environmental disturbances strong and long enough can re-set the handedness of enantiomers in a prebiotic environment.

One way or another

The US researchers consider a system in which enantiomers are autocatalytic: they promote the formation of more of themselves, so that differences in the proportions of handedness tend to get amplified. In this case, a uniform mixture of roughly equal quantities of enantiomers 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, such as the planet-sterilizing giant impacts experienced by the early Earth, can shake the salad dressing, breaking up the blobs. When the process begins again, the 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 on it.

Gleiser's paper isn't the beginning or the end of such arguments. Some say that the unstirred solvent considered by Gleiser’s team isn't very relevant to ponds on the turbulent young Earth, and others say that the notion of resetting by shaking is in some ways obvious. But the conclusion is noteworthy: aliens' biochemistry has an even chance of being the mirror image of our own.

Of course, the more fundamental astrobiological issue is whether there is the slightest reason to think that alien life will use amino acids and DNA at all.

So that's one more illustration of our tendency to frame questions parochially. In the quest for life elsewhere, whether searching for planets or considering the molecular parameters of potential living systems, we are in some ways more akin to historians than to scientists: our data are a unique narrative, and our thinking is likely to stay trapped within them. 

  • References

    1. Gaudi, B. S. et al. Science 319, 927-930 (2008). | Article | PubMed | ChemPort |
    2. Phil. Trans. R. Soc. Lond. Ser. B 359 (2004).
    3. Benner, S. A., Ricardo, A. & Carrigan, M. A. Curr. Opin. Chem. Biol. 8, 672 (2004). | Article | PubMed | ISI | ChemPort |
    4. Gleiser, M., Thorarinson, J. & Walker, S. I. Preprint at http://arxiv.org/abs/0802.1446 (2008).
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