13th
Can A Scientist Define “Life”?
“Defining life poses a challenge that’s downright philosophical. (…) When Portland State University biologist Radu Popa was working on a book about defining life, he decided to count up all the definitions that scientists have published in books and scientific journals. Some scientists define life as something capable of metabolism. Others make the capacity to evolve the key distinction. Popa gave up counting after about 300 definitions.
Things haven’t gotten much better in the years since Popa published Between Necessity and Probability: Searching for the Definition and Origin of Life in 2004. Scientists have unveiled even more definitions, yet none of them have been widely embraced. But now Edward Trifonov, a biologist at the University of Haifa in Israel (…) analyzed the linguistic structure of 150 definitions of life, grouping similar words into categories. He found that he could sum up what they all have in common in three words. Life, Trifonov declares, is simply self-reproduction with variations.
Trifonov argues that this minimal definition is useful because it encompasses both life as we know it and life as we may discover it to be. And as scientists tinker with self-replicating molecules, they may be able to put his definition to the test. It may be possible for them to create a system of molecules that meets the requirements. If it fails to come “alive,” it will show that the definition was missing something crucial about life. (…)
A number of the scientists who responded to Trifonov felt that his definition was missing one key feature or another, such as metabolism, a cell, or information. Eugene Koonin, a biologist at the National Center for Biotechnology Information, thinks that Trifonov’s definition is missing error correction. He argues that “self-reproduction with variation” is redundant, since the laws of thermodynamics ensure that error-free replication is impossible. “The problem is the exact opposite,” Koonin observes: if life replicates with too many errors, it stops replicating. He offers up an alternative: life requires “replications with an error rate below the sustainability threshold.”
Jack Szostak, a Nobel-prize winning Harvard biologist, simply rejects the search for any definition of life. “Attempts to define life are irrelevant to scientific efforts to understand the origin of life,” he writes (article PDF).
Szostak himself has spent two decades tinkering with biological molecules to create simple artificial life. Instead of using DNA to store genetic information and proteins to carry out chemical reactions, Szostak hopes to create cells that only contain single-stranded RNA molecules. Like many researchers, Szostak suspects that RNA-based life preceded DNA-based life. It may have even been the first kind of life on Earth, even if it cannot be found on the planet today.
Life, Szostak suspects, arose through a long series of steps, as small molecules began interacting with each other, replicating, getting enveloped into cells, and so on. Once there were full-blown cells that could grow, divide, and evolve, no one would deny that life had come to exist on Earth. But it’s pointless to try to find the precise point along the path where life suddenly sprang into being and met an arbitrary definition. “None of this matters, however, in terms of the fundamental scientific questions concerning the transitions leading from chemistry to biology,” says Szostak.
It’s conceivable that Mars has Earth-like life, either because one planet infected the other, or because chemistry became biology along the same path on both of them. In either case, Curiosity [rover] may be able to do some good science when it arrives at Mars this summer. But if it’s something fundamentally different, even the most sophisticated machines may not be able to help us until we come to a decision about what we’re looking for in the first place.”
— Carl Zimmer, popular science writer and blogger, Can A Scientist Define “Life”?, Txchnologist, Jan 10, 2012. (Illustration: Russell Kightley)
