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CX2SA > SCIENC 21.03.06 04:29l 90 Lines 5122 Bytes #999 (0) @ WW
BID : 45779_CX2SA
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Subj: Search for alien life...
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Sent: 060321/0156Z @:CX2SA.LAV.URY.SA #:45779 [Minas] FBB7.00e $:45779_CX2SA
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To : SCIENC@WW
Search for alien life challenges current concepts
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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 surface.
"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
paradigms."
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