7.2 Astrobiologists
http://arstechnica.com/journals/science.ars/2008/06/11/astrobiologists-ponder-the-laws-of-life
Astrobiologists
ponder the laws of life
By John Timmer | Published:
June 11, 2008 - 12:59PM CT
How do we look for life on
other planets when we have only the single example of life on earth to inform
our search? That question is so challenging that the National Academies of
Science convened
an expert panel to try to answer the question. The World Science Festival
had a panel of its own on the topic, consisting of Steve Benner, Paul Davies, and Margaret Turnbull. Turnbull
has almost no web presence, but is involved in planning space-based
observatories designed to observe extrasolar planets.
Davies gave a great
explanation of why the problem is so hard. Most definitions of life include a
list of properties it has, and nearly every one of them can be found in
nonliving matter; it's really the combination that's distinctive. Benner argued
that our definitions are unduly influenced by the biological theories we have,
noting that even one of the best—'a self sustaining chemical system capable of
Darwinian evolution'—is "laden with our theories." Davies more or
less concurred, pointing out that, even though the extremophiles we've found on
earth tell us a bit about what's possible, they still act more or less like us,
with ribosomes, DNA, etc. Still, this might be helpful, as evolutionary
convergence suggests that some physical or chemical features of earth life
might have analogs elsewhere.
What might non-earth life
look like? Benner described creating a 12-base DNA system that worked in the
lab, although graduate students had to continue to feed the artificial bases to
the organisms. Going further afield, Davies described how lots of people have
been excited by the prospect of some form of life catalyzed on the surface of
clay, but suggested things of that nature should still be happening in the
wild, and they're apparently not. "Even if it's happening," he said,
"it's not doing anything especially interesting."
After Benner and Turnbull
agreed that water was probably a prerequisite, Davies averred, "I'm not a
big fan of water." Asked for a solvent with a similar set of properties,
he suggested formamide. Davis was a fan of chirality, suggesting that
biochemistry needs a bias in the handedness of molecules, so looking for that
could help us spot life, even if we aren't sure of its actual chemical basis.
It seems that a chiral bias could also help us identify the chemical processes
unfamiliar life was engaging in, although Davies didn't mention this
specifically.
The discussion also turned to
the key question of instrumentation. Davies argued that the microscope
completely changed our concept of what life on earth was like, since it
revealed a vast single-celled world. After Benner mused that the new microscope
is the telescope, Turnbull took over, since she designs them for a living. She
describes how NASA's planned Kepler orbiting observatory will be able to detect
an earth-like planet transiting in front of a star; if it works, then follow-on
observatories may be able to get spectra of its atmosphere; she mentioned one
potential design that included a specially designed sunshade orbiting 80,000 km
from a four meter telescope.
With the possibility to
observe the components and changes in a planet's atmosphere, we can watch for
things like water (or formamide), hints of tectonic activity, potential energy
gradients that could be harvested by life, etc. We wouldn't know whether there
was life there, but we could definitely identify places with favorable
conditions.
So, are there any laws of
life? Turnbull was skeptical. "We're just trying to put things in boxes,
and it's really a continuum," she said, and argued that there won't be
anything like a phase change between living and nonliving. Benner said the laws
we have—cell, gene, and evolutionary theories—aren't like the laws of physics.
"If we really understand them, then we can repeat the process in the
lab," he suggested, "we're not there yet." Davies' take is that
"biology doesn't have theorists in the same way that physics does,
possibly because it's all history and contingency." Until we find a second
example of life, it's hard to separate the basics from the contingencies.
Overall, the takehome message
seemed to be that the question itself was a really hard one, and we can expect
the answers to change as we get a better grip on biology and origin of life
issues in the coming years.
