What is Life?

It has been suggested that the first posts of this book club be devoted to the Universal Rules of Life. So... What is life?

Jessica asks,

Carl, twice in the book you refer to viruses as "creatures." Perhaps you used the word metaphorically. In any case I'd love to know whether you think viruses qualify as being alive, and I'd love to hear your reasoning either way.

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Historically, viruses have been considered non-living. Some of the first discoverers of viruses, Frederick Twort for example, thought they were enzymes secreted by bacteria. Other biologists, such as Felix d'Herelle, contended that viruses were alive. The distinction lies largely on how "life" is defined.
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Viruses certainly do straddle the borderline between the living and the non-living. In some ways, they resemble chemicals. In essence, they are nucleic acids encased in protein. They are inert much of the time, only becoming active on finding and entering a host. They are unable to reproduce on their own. They cannot consume food, breathe, chase prey, respond to the environment in ways typical of the organisms most familiar to us.

But many organisms we classify as living occasionally show the inability to conduct activities we associate with life. For example, is reproducing on its own truly a characteristic of life? Obligate symbioses are common in the biological world. These organisms are unable to survive without the assistance of another organism. For example, some flowers cannot reproduce without assistance from bees. I would argue we are all obligate symbiotes deep down.

The best definition of life I have encountered comes from Salvatore Luria, he of the Slot Machine Experiment ably described by Zimmer in Microcosm.

"An organism is the unit element of a continuous lineage with an individual evolutionary history."

SE Luria, JE Darnell, D Baltimore and A Campbell (1978). General Virology, 3rd Edn. John Wiley & Sons, New York, p4 of 578.

With this definition, viruses are unequivocally alive. I've blogged briefly about this definition previously here.

Photo: A thin section of T4 phages hitting a microcolony of E. coli K-12 by John Wertz.

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Thank you.

Now, I'd like to hear our take on needs-a-new-name nanobacteria.

It has been shown that these little guys aren't bacteria at all, but still perform some of life's functions (most notably reproduction).

While the mainstream media seems to be running with the not bacteria = not alive meme, I'm really interested in where future research may lead, especially abiogenic possibilities (I say that fully realizing that they may be byproducts of more modern lifeforms, not life's chemical precursors, but they are highly interesting and require further study in any case)

By Jason Failes (not verified) on 04 Jun 2008 #permalink

I tell my freshman biology students that deciding whether the term 'life' should be applied in borderline cases is not worth the trouble. The criterion for what's of interest to biology is whether an entity is subject to evolution by natural selection.

Until recently the category 'nanobacteria' contained only the inanimate particles seen in some mineral hot springs and the Mars meteorite ALH84001, and the category 'nannobacteria' contained only some imaginary particles claimed to contaminate our precious bodily fluids. But this new evidence for very small bacteria looks pretty convincing, though I haven't yet carefully read the paper.

I'm currently partial to the definition of life given in the biophysics class I took last semester : a persistent state of being out of equilibrium with your environment; maintained by energy input. Even dormant viruses are out of equilibrium with their surroundings.

Microbial linages are so mixed up (horizontal gene transfer), I'm not really sure it is fair to call them "continuous".

I agree with comment#3, if it's subject to evolution, it's biologically interesting. Unfortunately, I've had teachers (and TA'd for instructors) who focus way too much on making a big deal of having us memorize definitions of "life" (which are invariably unique) and testing us on them. I think it gives the wrong idea of what's important and confuses people.

dormant viruses are out of equilibrium with their surroundings

?? How so? How, in any way different from, say, a quartz crystal?

By Sven DiMilo (not verified) on 05 Jun 2008 #permalink

Hmmm... well, if you change the definition of a word, you will undoubtedly change the things to which the world can refer.

This type of thing always seems to go round and round in circles - to the point where I wonder if it's really so important. Is a virus more meaningful if it is "alive" as opposed to something not. It's still a virus.

On the other hand, if we want to be clear then some folks should just pull the "Pluto Ex-Planet" maneuver and just make a semantic decision. I dunno, maybe life can be divided into "self-replicons" and "co-opting replicons" or something.

Oh, but remember, if we let viruses get life, soon prions will be banging on the door. And then the world will go to crap for sure.

I would argue we are all obligate symbiotes deep down

That made me smile.

I also like the definition from #5, though I don't have the scientific knowledge to be sure whether it doesn't apply to non-living things (there's some energy flowing into my computer, after all, and it remains hard and distinct from the air and the desk around it...

a persistent state of being out of equilibrium with your environment; maintained by energy input

Re: #6&8

I should have clarified better; I do realize that many counter-examples to the "definition of life by thermodynamics" exist and you both brought up good ones : naturally formed crystals and electronics. Generally the thermodynamic definition also includes restrictions such as "life must maintain its own non-equilibrium environment" and "must be able to direct it's own replication", etc. This brings us to increasingly complicated definitions that, as commenter #3 says, really aren't worth the trouble- unless you are debating for fun.

As technology improves, the lines get fuzzier and fuzzier. For example, what happens when we get nano-machines that self-maintain and replicate? For that matter, what happens when we get self-replicating, self-maintaining nano-machines running with evolutionary programming that keeps re-optimizing the population for a particular task? (I actually have a friend working in this area of research). What about synthetic organisms?

The definition of life is more of a fun philosophical question than a scientific one, I think. Carl Zimmer covers this really well in the next post. Personally I like the thermodynamic definition because "Equilibrium is Death!" makes for a cool battle cry. =-)

Until recently the category 'nanobacteria' contained only the inanimate particles seen in some mineral hot springs and the Mars meteorite ALH84001, and the category 'nannobacteria' contained only some imaginary particles claimed to contaminate our precious bodily fluids. But this new evidence for very small bacteria looks pretty convincing, though I haven't yet carefully read the paper.