Mars Announcement Raises Question: What Is Life?
2013-03-18 0:00


NASA officials announced March 12 that ancient Mars could have supported primitive life. But this begs the question: What exactly constitutes life? defines life as "an organismic state characterized by capacity for metabolism, growth, reaction to stimuli and reproduction." But thereís no single satisfactory definition of life.

"I think it is a mistake to try to define life, because we have only one example of life, familiar life on Earth, and we have reason to believe that this example may be unrepresentative of life in general," Carol Cleland, a philosopher of science at the University of Colorado, Boulder, told LiveScience in an email.

Defining life

Aristotle made the first attempt at a definition, describing life as something that grows, maintains itself and reproduces. But this definition would exclude mules, which are sterile, while including things like fire.

Calling life something that has a metabolism, the ability to take in energy to grow or move and excrete waste, is no good either; cars do this, for example.

In 1944, the physicist Erwin SchrŲdinger gave life a definition based on the second law of thermodynamics, which states that the entropy, or disorder, of a closed system increases over time. SchrŲdinger defined life as something that decreases or maintains its entropy. Yet this definition fails because it includes crystals, which resist entropy by forming highly structured lattices.

Trying to define life by its qualities is the wrong approach, Cleland said. As an example, she cites scientistsí early attempts to define water in terms of properties like being wet, transparent and a good solvent. "We didnít ídefineí water as H2O, but rather discovered, in the context of molecular theory, that it is a chemical substance composed mostly of H2O molecules," Cleland said.

Life on Earth is typically divided into two main groups: the cellular life forms, which include archaea, bacteria and eukarya (all the plants and animals), and non-cellular life forms, like viruses. Whether viruses, which can replicate only inside the cells of a host organism, count as "life" is debated.


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