The Meaning of Life

The word, not the phenomenon – that’s a story for another blog. Carl Zimmer has an article at Txchnologist on the ongoing disagreements of how to scientifically define life. It seems intuitive that we know what life is – it’s… you know… living stuff. The reality is much less settled:

When NASA says it wants to find out if Mars was ever suitable for life, they use a very circumscribed version of the word. They are looking for signs of liquid water, which all living things on Earth need. They are looking for organic carbon, which life on Earth produces and, in some cases, can feed on to survive. In other words, they’re looking on Mars for the sorts of conditions that support life on Earth.

But there’s no good reason to assume that all life has to be like the life we’re familiar with. In 2007, a board of scientists appointed by the National Academies of Science decided they couldn’t rule out the possibility that life might be able to exist without water or carbon. If such weird life on Mars exists, Curiosity will probably miss it.

Defining life poses a challenge that’s downright philosophical. There’s no ambiguity in looking for water, because we have a clear definition of it. That definition is the same whether you’re on Earth, on Mars, or in intergalactic space. It is the same whether you’re dealing with water as ice, liquid, or vapor. But there is no definition of life that’s universally agreed upon. 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…

[Edward Trifanov, biologist at the University of Haifa] 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…

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…

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 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.

I have to agree with Szostak; the definition of life is, at least from science’s perspective, irrelevant. However, there is a standard set of criteria used by biology, as far as I learned in school and Wikipedia has to say:

• Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
• Organization: Being structurally composed of one or more cells, which are the basic units of life.
• Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
• Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
• Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism’s heredity as well as the composition of metabolized substances, and external factors present.
• Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and by chemotaxis.
• Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.

Seems like pretty basic, inclusive criteria. Homeostasis is a word you don’t hear often, but it’s important; it’s basically what keeps organisms being themselves. It’s an organism’s negative feedback mechanisms, always adjusting to changes and trying to keep its state in the ideal place – ideal temperature, ideal CO2 level, ideal blood sugar, anything and everything. Also note that viruses aren’t considered to meet this definition of life; they can’t reproduce themselves per se, they’re not composed of cells, and they don’t grow, as far as I’m aware. 

Carl Zimmer posted a response to his article from an evolutionary biologist named David Hillis that I found interesting and insightful:

Like all historical entities (including other biological taxa), it is only sensible to “define” Life ostensively (by pointing to it, noting when and where it began, and following its lineages from there) rather than intensionally (using a list of characteristics). This applies to the taxon we call Life (hence capitalized, as a formal name). You could define a class concept called life (not a formal taxon), but then that concept would clearly differ from person to person (whereas it is much less problematic to note examples of the taxon Life). So, I’d say that I can point to and circumscribe Life, and that it the appropriate way to “define” any biological taxon. A list of its unique characteristics is then a diagnosis, rather than a definition. So, I’d argue that any intensional definition of Life is illogical (does not recognize the nature of Life), no matter how many words are used.

Defining Life (the taxon) is like defining other particular historical entities. We don’t “define” Carl Zimmer or the United States of America by listing out their attributes. Instead, we point to their origin and history. The same should be true for Life. If we ever discover a Life2, we’ll have a new origin and history to point to…

So that is another way of looking at it that I had never heard before, and it seems like the reasonable way to think about life. I don’t know if we’ll find any revolutionary kind of life in my lifetime, but if we do it’ll be pretty interesting to see different fields struggle with the implications. I hopefully will not be too worried about that – I’ll just want to pick its brains.