Can we define “life” in just three words? Carl Zimmer of Loom fame has written a piece for Txchnologist in which he reports on an interesting attempt: biologist Edward Trifonov looked at other people’s definitions, rather than thinking about life itself. Sifting through over a hundred suggested definitions, Trifonov looked for what they had in common, and boiled life down to “self-reproduction with variations.” Just three words, although one of them is compound so I would argue that morally it’s really four.
We’ve discussed this question before, and 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. (And no, those lawsuits were frivolous and the court records were sealed.)
It’s the yellow-taxi problem: in a city where all cars are blue except for taxis, which are yellow, it’s tempting to define “taxi” as “a yellow car.” But that doesn’t get anywhere near the essence of taxi-ness. Likewise, living species generally reproduce themselves; but that’s not really what makes them alive. Not that I have the one true definition (and maybe there shouldn’t be one). But any such definition better capture the idea of an ongoing complex material process far from equilibrium, or it’s barking up the wrong Tree.
Self-sustaining metabolic process is suitable enough for me.
Only three words? That’s tough.
Maybe, “Pushing against entropy”?
Longer: “Life is a property of a system whereby it increases the entropy of its environment so as to lower the entropy within the system itself.”
Best visualized as a system in an entropy flow propelling itself upstream by pushing entropy even faster downstream.
No way! You can’t take the reproduction out of life! You have the capability to reproduce, you’re just choosing not to do so.
When it boils down to it, every single living thing is just a little twisty self-replicating molecule with a lot of complex machinery around it. Yours is so complex that you’re able to choose not to let those twisty molecules replicate.
I assume (having not read the post!) that by self-reproduction they may not necessarily just mean that a complex living thing has to reproduce another entire similar organism, but does at least have to have a capacity for maintenance and self-repair by reproducing some component parts (it would be these component parts that might also have the maximum variation). I would consider you alive without you having to have a child as parts of you are reproducing all the time, both on the level of cells and proteins within cells.
You might not have plans for reproduction, but your cells are reproducing themselves continuously.
When they stop reproducing, your continued existence will be exceedingly brief.
It is fine to imagine a complex system, constructed by an engineer, that could achieve some semblance of metabolism. But all such systems that have been observed (that is living systems) are imperfect and destined to fail.
It is only through the capacity for self-replication, that overcomes the failure rate, and the process of natural selection that results in more favorable self-replication/failure ratios that we have any examples to study. And that we are here to study them.
Biochemists for decades have studied cell extracts that carry out “ongoing complex material process far from equilibrium”. Typically not considered to be alive.
Mammalian red blood cells, that lack a nucleus and the capacity for reproduction are limited in their life spans to a few weeks. It could be argued, that like your fingernails, and outer layer of skin that these are the vestiges of life, functionally important for the organism as a whole, but no longer alive.
Before this post, I would have gone with ‘adaptive reproduction’. Sounds moronic, since reproduction itself is not ‘adaptive’, but hey, we only have 3 words.
One of my teachers, a very theatrical guy, once stated that total thermodynamical equilibrium is DEATH ITSELF, we could reverse this idea as you suggest and try something like ‘a system that resists equilibrium in thermodynamical quantities’, but we must be careful not to include refrigerators. Maybe systems that resist equilibrium with the ability to collect the energy needed by themselves. Too long, but my poor imagination says life cannot exsist deprived of the concept of ‘species’, or at least adaptation. Meh 🙂
But you ARE self-reproducing! Some of the cells in your body are dividing right now; even the ones not actively dividing are producing new proteins and nucleic acids, and repairing cellular damage. These processes will stop only when your body is no longer “alive”, by conventional definitions.
…okay, looks like Matt said it first. So I’ll second.
If you want to get it in fewer words, you can just go with “von Neumann machine.”
Where I come from, “self-repairing” seems pretty obviously distinct from “self-reproducing.” I’d be much more willing to grant the former quality as characterizing “life.”
Self-repair is miniscule compared to replication (on the cellular level) in living systems.
DNA replication fidelity may be enhanced several orders of magnitude by repair processes.
But other “repair” processes (proteins, RNA, membrane components) are really replacement functions and generally indistinguishable from growth that is necessary for replication.
I am not trying to be intentionally obscure here. My cells reproducing themselves is different from me reproducing myself. The question is whether I am alive, not my cells.
A short discussion of What is the meaning of life? By James Lovelock.
He seems to put a lot more emphasis on metabolism than reproduction.
http://www.webofstories.com/play/51904?o=MS
Three words? Not sure I can do that. But the one I like is: “Member of a population whose change through time can be described by evolution through natural selection of random variation of heritable characteristics.”
Viruses replicate themselves and are considered by some scientists as non living organisms (I am not a biologist, so if I am wrong please correct me). My point is that self reproduction may not be the only right criteria to define life.
Pingback: Defining Life in Three Words « Physicalism
Self Organising Matter seems the best 3 word definition I’ve ever heard. Consider the Prion that only reproduces a protein configuation but if it gets into the brain, it corrupts the system as in Mad Cow Disease. The meaning of reproduction varies though all are a part of the self organising concept.
My attempt is ‘Reproducing sentient being’.
Go look at the discussion by David Hillis, and the associated comments, in Sean’s link above (“shouldn’t be one”). For me this is the best discussion I’ve seen of this issue. Briefly, Hillis argues for an ostensive definition of Life, the taxon “(by pointing to it, noting when and where it began, and following its lineages from there)” rather than by arguing about definitions (for which we can always find exceptions). It’s a version of “it is what it is.”
Since what you set out to explore was a minimal definition of life, it makes sense to consider the proper unit for that definition.
Cells are the units of life as we know it. There are many control mechanisms established over the eons to coordinate their behavior in complex organisms. But individual cells are the units of life. You are alive because your cells are alive, if they die you die.
Whether cells in the cutting from a plant, before or after grafting or a cells from a human tumor that can be maintained as an immortal cell line, the life in the thing is not defined in terms of the whole but in minimal units.
When you consider human reproduction, is it you that reproduces or one particular sperm that finds a receptive ovum, to resurrect a program that was paused in a terminally differentiated state.
Cells aren’t the units of consciousness certainly, so if that is what you think of as “me” then we are off into philosophy or something else — not biology.
Independence and temporary self-sustenance?
An example on the macro level: all male mules are infertile, but nobody would deny that they are nevertheless alive.
Which leads to Sean’s question (#11), which by the way as been asked for millennia: “what makes up the “I”?
I think the requirement that reproduction be included is debatable, at least at the organism level. Mules are certainly alive.
I think “sustained local lowering of entropy” is closer to the nub. That said, it does seem that some level of reproduction is required for that… even if it is sub-cellular in some cases (thinking of “zombie” or “lazarus” bacteria.) Which actually brings up such basic organisms that can hibernate or enter some kind of spore state.
So, that leads to something like “local baseline entropy preserving structure via entropy export.” Certainly death, on whatever level you choose, is when the biochemical machinery is no longer able to maintain it’s particular disequilibrium; that holds for large, multicellular organisms (even if cell death does not occur uniformly across the organism) or small.
even if you don’t want to reproduce you are still a product of reproduction….so it would be “capability of self-reproduction with variations” instead….what’s the difference??…
Second “local entropic reversal”.
I think it’s pretty obvious that “Natural Life” is self-reproducing, and the fact that nature produces aberrant individuals which fail to or are unable to reproduce themselves doesn’t change that – these very individuals are themselves the end product of an unbroken line of reproducers that goes all the way back to the very first organism.
Or, to put it another way, mules are still self-reproducers, though they are failed ones. (BTW, per Wikipedia, “Since 1527 there have been more than 60 documented cases of foals born to female mules around the world”).
Artificial life might not have to be self-reproducing, but then we’re not avidly searching the universe for artificial life, and if we do run across it, we’ll have uncovered something a lot more interesting anyway – artifice requires an artificer.