For scientists eying distant planets and solar
systems for signs of alien activity, University of Colorado at
Boulder Professor Carol Cleland suggests the first order of
business is to keep an open mind.
It may be a mistake to try to define life, given
such definitions are based on a single example -- life on
Earth, said Cleland, a philosophy professor and fellow at the
NASA-funded CU-Boulder Center for Astrobiology. The best
strategy is probably to develop a "general theory of living
systems," she said.
Many biologists agree the best definition of
living systems today is the "chemical Darwinian definition"
involving self-sustaining chemical systems that undergo
evolution at the molecular level, she said. But the theory is
limited in that life on Earth probably resulted from physical
and chemical "contingencies" present at the time of its origin
on the planet.
"What we really need to do is to search for
physical systems that challenge our current concept of life,
systems that both resemble familiar life and differ from it in
provocative ways," she said. Cleland participated in an
astrobiology symposium at the annual American Association for
the Advancement of Science meeting held in St. Louis Feb. 16
to Feb. 20.
In 1976, for example, NASA's Viking 1 spacecraft
conducted automated biology experiments on Mars by mixing soil
samples with radioactively labeled nutrients to determine if
metabolic "burps" from possible extraterrestrial microbes
could be detected, she said. Although positive readings
convinced at least some team scientists that life was present,
a subsequent investigation by a second Viking instrument
failed to find evidence of organic molecules on the planet's
"Initially, the scientists were ready to break
out the champagne," said Cleland. "But because subsequent
investigations yielded baffling results that didn't fit the
original metabolic definition of life they were working with,
NASA eventually concluded the original signal was not evidence
of life. This is an experiment that is still debated today,
and it's a classic example of an anomaly."
Although there are more than 100 combinations of
nucleic acids in nature, terrestrial life constructs all of
its proteins from only about 20 of them, suggesting a single
origin for life on Earth, said Cleland. "It's very difficult
to generalize about life based on just one example," she said.
An article by Cleland and CU-Boulder molecular,
cellular and developmental biology Professor Shelley Copley,
published online in the Jan. 16 International Journal of
Astrobiology, explores the idea that an "alternative microbial
life" may exist on Earth. Such a "shadow biosphere" could have
a different molecular architecture and biochemistry than known
life and would be undetectable with current techniques like
microscopy, cell cultivation and Polymerase Chain Reaction
amplification, the authors wrote.
Despite new suites of sophisticated instruments
developed in recent years, the ability of scientists to detect
life on Mars or in another solar system is probably very
limited, Cleland said. "If the DNA in an alien organism was
even slightly different than the DNA in life on Earth, with a
different suite of nucleotide bases to encode genetic
information, we probably wouldn't be able to recognize it. "
So what might be out there? "It's not too
far-fetched to imagine an alien microbe whose genetic material
directly and adaptively changes in response to different
environmental conditions," said Cleland. "Instead of looking
for life as we know it, scientists may be better served to
look for anomalies, which amounts to looking for life as we
don't know it."
In the past decade, scientists have discovered
more than 170 new planets around other stars, a number that
seems to grow by the month due to clever new planet-hunting
techniques, Cleland said. In the future, astrobiologists
surveying other planets will no doubt encounter non-living
systems that are "really weird," she said.
"In such cases, it probably is best to suspend
judgment," she said. "The great strength of science is its
tentativeness, and through history, it has been the careful
analysis of anomalies that have eventually changed scientific
Contact: Carol Cleland, (303) 492-7619, email@example.com
Jim Scott, (303) 492-3114
Editors: Content is embargoed until Feb. 18 at 2
p.m.CST when Carol Cleland will participate in a symposium,
"Astrobiology: Habitability of Worlds Around Other Stars," at
the annual AAAS meeting in St. Louis.
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