WASHINGTON—The discovery of what may be the smallest living creature on Earth raises anew the ancient question: What is life?
Tiny microbes detected in acid waste from a California gold mine are smaller than any other known life form, a team of scientists reported in last week's Science magazine.
If further study confirms that the little bugs are truly alive, it may be necessary to redefine "the minimum conditions for life," the authors said.
Despite what appears to be a wide gulf between living and nonliving things, scientists say there's no definitive way to distinguish between them. It's an unsolved problem in science, philosophy and religion.
"In spite of generations of work by hundreds of thousands of biologists, in spite of countless studies of living organisms at every scale, from molecules to continents, we still have no widely accepted definition" of life, geobiologist Robert Hazen, of the Carnegie Institution of Washington, wrote in his recent book, "Genesis."
"It's impossible to draw a precise distinction between living and nonliving things," George Dyson, a science historian at the University of Washington in Seattle, writes in a forthcoming book, "Dangerous Ideas."
This isn't merely a semantic question. It has real-life consequences, such as deciding when life begins in a fertilized egg or whether life ends when doctors declare a person to be brain-dead.
Instead of a bright line between life and nonlife, the world contains a range of objects, from simple, carbon-based molecules to complex strings of DNA, from genes and proteins to entire cells, from organs such as a heart or a liver to a human being, tree or mouse.
Where to draw the line on that continuum is a puzzle.
"The transitions are gradual, and the point at which one chooses to apply the term `life' becomes a matter of personal taste, not science," Robert Shapiro, a professor of chemistry at New York University, said in an e-mail.
A common definition of life among scientists has four basic elements:
_Every living thing must be enclosed in a cell, separating what's inside from what's outside.
_An organism must consume energy and produce waste, a process called metabolism.
_A living creature must be able to reproduce, passing on its genes to the next generation.
_An organism must evolve as it adapts to changes in its environment and mutations in its genes.
But every definition, including this one, faces problems and counterarguments, according to Carol Cleland, a philosopher of science at the University of Colorado in Boulder. "They include phenomena that most are reluctant to consider to be alive or exclude entities that clearly are," Cleland said.
For instance, if the ability to reproduce is an essential requirement for life, is a sterile hybrid such as a mule not alive?
Similarly, two rabbits can reproduce like crazy, but how about one rabbit?
Another example: If life means the ability to consume energy, is a candle flame alive?
One definition of life "might be based on the ability to consume and convert energy in order to move, grow or reproduce," Cleland writes in a forthcoming book, "Planets and Life." "But fire, and perhaps even automobiles, might be said to satisfy some or all of these criteria."
Some scientists suggest that there should be three categories: living, nonliving and in between. A virus, which can't live by itself but must hijack the genetic material of a cell, would belong in the third group.
"I advocate a much richer taxonomy than just `alive' or `not alive,'" Hazen wrote in an e-mail. "That's a false dichotomy."
Some theorists even contend that computers and robots eventually will become so intelligent that they should be considered alive.
Despite the difficulties, scientists haven't given up trying to define life, especially since some researchers are trying to re-create life in the laboratory and others are hoping to find it on Mars or other worlds.
Said Cleland: "Someday we may have a well confirmed, adequately general theory of life that will allow us to formulate ... a scientifically satisfying answer to the question `What is life?'"
(For more information, go to www.whatislife.com)
(c) 2006, McClatchy-Tribune Information Services.
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