free debate

January 17, 2012

Scientific Unknowns: What is Life Redux

Image Credit: JFantasy via Wikimedia Commons
Almost three years ago now, I wrote a post about the difficulty of defining life. Now this has come back into the news and I thought I would add my two cents.

This all started when Edward Trifonov, a biologist at the University of Haifa Isreal, proposed a three word definition of life. To quote from the article by Carl Zimmer on Txchnologist...
Trifanov 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.
In his article, Zimmer does acknowledge that there is a considerable amount of criticism but he mostly focuses on what this definition might be missing, metabolism, information and so on. I was interested, though, when I read a critical piece by Sean Carroll asking if reproduction should even be a part of our definition. He says...
...the idea of reproduction looms large in many people’s definitions of life. But I don’t think it really belongs. If you built an organism from scratch, that was as complicated and organic and lifelike as any living thing currently walking this Earth, except that it had no reproductive capacity, it would be silly to exclude it from “life” just because it was non-reproducing. Even worse, I realized that I myself wouldn’t even qualify as alive under Trifonov’s definition, since I don’t have kids and don’t plan on having any.
3 Legged dog on the right
Photo Credit: Jon Hurd via Wikimedia Commons
It is a really interesting question. We could define dogs in part as having four legs, but does that mean if a dog loses one leg it is no longer a dog? If we created a cell in the lab that is identical to a natural cell in every way except it can't reproduce is it alive? To give another example, I think it is easy to imagine a robot that can copy itself but that we would still not want to call alive.  My personal thought, is that reproduction is a necessary component for any type of life that will survive for long stretches of time. That doesn't mean reproduction is what makes something alive.

So what is life? I certainly don't have an answer. Right now we have a sample size of one*. Until we find life somewhere else in the universe, I wouldn't expect any real agreement on this deceptively simple question.

*After all, every living thing we have found on the Earth shares a common ancestor.