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 ev
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Contact: Carol Cleland
carol.cleland@colorado.edu
303-492-7619
University of Colorado at Boulder
18-Feb-2006