John Wilkins at Evolving Thoughts has a great post about the development of the modern definition of “Life” (which, one strongly suspects, is by no means fully developed). Once we break free of the most parochial definitions involving carbon-based chemistry, we’re left with the general ideas that life is something complex, something that processes information, something that can evolve, something that takes advantage of local entropy gradients to make records and build structures. (Probably quantum computation does not play a crucial role, but who knows?) One of the first people to think in these physical terms was none other than Erwin Schrödinger, who was mostly famous for other things, but did write an influential little book called What Is Life? that explored the connections between life and thermodynamics.
Searching for a definition of “Life” is a great reminder of the crucial lesson that we do not find definitions lying out there in the world; we find stuff out there in the world, and it’s our job to choose definitions that help us make sense of it, carving up the world into useful categories. When it comes to life, it’s not so easy to find a definition that includes everything that we would like to think of as living, but excludes the things we don’t.
For example: is the Milky Way galaxy alive? Probably not, so find a good definition that unambiguously excludes it. Keep in mind that the Milky Way, like any good galaxy, metabolizes raw materials (turning hydrogen and helium into heavier elements) and creates complexity out of simplicity, and does so by taking advantage of a dramatic departure from thermal equilibrium (of which CV readers are well aware) to build organization via an entropy gradient.
Update: Unbeknownst to me, Carl Zimmer had just written about this exact topic in Seed. Hat tip to 3QD.
Quasar, I’m the Jolly Bloger, not Blogger 😉
B, I agree it isn’t meaningless to say that a quality of life is the ability to replicate itself, but I still say you can’t include a word in its own definition. That’s just basic lexicography. To say that the definition of life is “something that produces life” is certainly meaningless because you still haven’t said what life is!
Alan, your definition has some of the same problems as more in-depth ones above as it excludes non-reproducing organisms like childless humans or worker ants. In fact, humans don’t replicate at all. Replication implies a clone, asexual reproduction. Also, your definition would include any kind of Turing machine (a robot programed to build robots just like itself). Some would claim those do constitute life, but I think most people have a gut feeling that there’s something more involved.
blog threads replicate themselves all the time.
See also: Meme.
Hehe
Again, one further comment. It’s one thing to say that life is that which reproduces ITSELF and quite another to say “life is that which produces life.” To use your exponential function analogy, its valid to define it as “F reproduces F under derivation” but you can’t define an exponential function as “anything that makes an exponential function.”
If you built a machine that would assemble molecules to make lemmings, so you have a big device with perfect fully formed lemmings popping out, the lemmings are life but the machine is not.
Hi Jolly: I was about to say the same as you wrote in #78. You can include ‘itself’ in a definition. I.e. take 0 = x* 0 for all x as a definition for 0. Whether using a word in its own definition or not is good praxis is arguable. but you can’t define an exponential function as “anything that makes an exponential function.” No, but the problem is not the second use of the word ‘exponential function’ but the verb ‘make’. You could very well say an exponential function is defined through it’s derivative also being an exponential function, i.e. f’ = f. Zero is a number that yields again zero under multiplication with an arbitrary other number etc.
Is it possible that some of the difficulty in this exercise is that we are attempting to give a “physical” definition to life and that life as we know it is biological (2 levels of complexity away from pure physics via chemistry) is generating some of the confusion. Just a thought.
elliot
Nono, the sentence “an exponential function is defined through it’s derivative also being an exponential function” is not the same as f’=f. With the latter, you’re already taking as a given the definition of a function and the definition of a derivative, so there’s a very strict relationship between f and f’.
You still couldn’t say “an exponential function is something that produces an exponential function under derivation” because who knows what an exponential function is? Armed with only that definition you might think that f=x^2 is exponential. You cannot exclude it using only your definition. As long as you have a differentiable function, your definition will be logically valid, so its just as useful on f=x as it is on a real exponential function defined the usual way as f’=f.
It’s like defining ‘green’ as “the colour of green things.”
We’re getting away from the actual discussion with all this philosophical talk about what is a definition. I’d like to move back to the issue.
Elliot, thats just the thing – we’re trying to come up with a definition that goes beyond biology. Biology itself is defined as the science of life, so that’s only as good as we’re able to identify what life is. Life “as we know it” (i.e. anything described by biology) is a small subset of what we can conceive of as life. All life that we know is carbon based, for example, and most is written in the exact same language of DNA. A comprehensive definition that includes all possible forms of life must go beyond this.
Jolly,
My point is that life may be an emergent phenomenon not subject to an all or nothing physical definition. In other words it pops up here and there not necessarily carbon based. If life is to be given a definition or a “physical life test” developed (this thing is/is not alive) doesn’t that imply that life “may” reflect some as yet undiscovered physical law that predisposes it’s development?
Jolly, I have tried to explain you why an auto-reference of the object-to-be-defined can indeed make sense in a definition (what you doubted in #71). Whether or not an object with a given definition actually exists, or whether it is unique is another question (and one that needs to be shown in both examples I’ve given above). As I’ve said earlier, if you have a problem with my use of words, just do whatever you like, there’s a reason why I like maths better than philosophy. I agree that my above wording isn’t satisfactory, as the word ‘an’ should have more precisely referred to the same function (otherwise you could define a class of function whose derivative is also in that class). Sorry about that. Since all of this is irrelevant for the discussion anyhow, and it seems you’ve gotten the point with the self-referral, I’ll drop the issue here.
Do galaxies reproduce? We’ve not been able to observe one through its life cycle, so we can’t know.
Emergence, complexity, self-organization, and so forth are transparent revivals of the discredited doctrine of Vitalism. Not that there’s anything wrong with that . . . .
And so on. For endless amusements on the topic of life’s origin and scope, be sure to meander through http://www.starlarvae.org.
Self-Replicators play a much more fundamental role in physics than most people realize.
http://arxiv.org/abs/0706.3379
For the record, emergence, complexity, and self-organization are not at all tied to vitalism, they are all involved in perfectly scientific formulations of life through evolution without invoking some hocus pocus 17th century life energy. Those concepts are also important in economics and computer science.
B, I agree it looks like we sort of agreed all along, we just misunderstood each other. I still maintain that it is strictly improper, according to the laws of language, to use a word in its own definition. You never see this in a dictionary 🙂 (unless the word is referring to an above definition of the same word).
Old but not dead thread in a fascinating subject. Most of my 2c has been said, but I can amplify:
I agree with Jason Dick that a functional description is better. My own motivation is that it fits established theory better. Also, in biology distinctions are fuzzy. For example, we are forced to use several species concepts. So we can and probably must use several life concepts.
[For living sexual populations we can use the “biological species” concept. (Different populations who can’t or won’t mate belong to different species.) But that doesn’t describe asexual populations or fossils.]
Therefore I propose the following modern concept of life (in analogy with “concept of species”):
The model for the definition is an organism as the current slice in a continous process. Thus combining the idea of life as individual and life as process with evolution and an implicit assumption of am inheritance mechanism, and a robust definition of organism. Quite a few birds knocked down with one stone.
This definition excludes organelles and such replicators as prions because they have entered dependent niches, as they are subsumed into an organism. But viruses are individual organisms under the definition, as they coevolve instead.
Similarly such things as memes, stars and galaxies are excluded, as they have no inheritance. While software/hardware such as implementations of genetic algorithms are included. (As noted, such populations may or may not inhabit other environments than we do.)
Finally, to amplify:
Viruses and cells from different extant and extinct domains could have crossed the Darwinian threshold several times from a progenotic state. Also, early total extinctions are consistent with the observation of early and so likely easy abiogenesis.
These crossings are initiated by diverse selfish elements, a mechanism that is often observed in genes. Evolution, especially past a progenotic state, is a competitive [sic!] and robust process which we should expect to be common elsewhere.
Maybe we will meet singular existences, biological or mechanical, that aren’t described thusly. But I believe the way to bet is that they are rare and bound on an extinction path. We could probably approximate “life” with “populations that obey evolution” as above. Other characteristics are coincidental.
Heresiarch:
Acute characterization – I haven’t seen it expressed so sharply before.
Evolution isn’t a “ladder of progress” or other misplaced Victorian concepts, but a symmetric process that can go both ways. The initial conditions was asymmetric, but after 3-4 Ga the majority of biomass is still simple Virus and Bacteria. Simple is better, and I believe biologists think that it is energy requirements that constrain size.
There is a coincidental tail diffusing towards complexity in lieu of the constraint pushing back. Simpler and faster reproducing Bacteria have outcompeted Archaea to inhabit extreme environments and Eukaryota to exhibit complex phenomes. As these things go, we are the scum of Earth. 😛
I’d say even the least complex prokaryotes and viruses are not at all simple compared to your average glob of abiotic matter.
And even protists share many, many genes with multicellular organisms as large and complex as ourselves. Indeed, biology reveals a remarkable level of recapitulation throughout its putative history. What one sees with multicellular organisms is a new layer of complexity over what can be achieved with a genome surrounded by a membrane, and this arrangement is almost totally constrained by size, most notably the ratio of surface-area to volume.
I don’t think there is a “better” in evolutionary terms. When there is selective pressure (and even in its absence, due to genetic drift), life tends to fill up all the niches, but there’s no reason to expect any kind of symmetry in terms of number of species or mass of a particular species in those niches, and no value judgement we can readily place on such metrics so long as the species in question is viable from generation to generation.
I would hasten to add that if somebody starts in with the vitalistic hocus-pocus of life’s “fundamental theory” or the woolier gestalt concepts, run away screaming, of course. But I think it’s too much to dismiss all talk of complexity and emergence as mysticism. I admit it’s not so well-defined, but IMHO “more is different” is one of the pithiest little scientific observations one is likely to come by, and derserves consideration.
Hmmm. Just read the Seed article. I have a feeling I would have been one of the folks yelling at Cleland if I were there. I find the idea that we should give up on defining life until we find extraterrestrials to be rather vacuous, really. That sounds like opening our minds to the point our brains fall out to me. It’s also not at all clear to me why we need any more theory than natural selection to make a reasonable stab at being comprehensive. What the hell else is supposed to give rise to this thing we refuse to even try to define until we find it under a rock on Titan scratching itself? What are we even looking for, then? I also find the argument against Darwinian evolution’s reliance on genes to be specious, quite frankly. If there is not some information in this magical bag of enzymes (how one gets something as complex as an enzyme as a mere constituent is completely ignored, apparently, which itself is outrageous), of what significance is “change”? If it gets dry the water in the puddle evaporates, and the mud turns to dirt. That’s environmentally-driven change. So fricking what?
It’s a little strange to me, defining “life”. To do so, you have to abstract yourself, or separate yourself from it. We might be running up against that whole “nothingness” problem again, only in a variation.
Part of it seems to depend upon where we choose to look, and on what scale. Cells of the body, and each one alive, composed of un-alive atoms? And we thinkers, an unlikely result of the collective efforts of these cells?
I suppose if a cell in our body might contemplate its existence, it would have to observe the things around it, or better yet, travel outside the body, finding, perhaps, a gargantuan being, beyond comprehension, scratching itself.
Or, if planets could talk to one another, might the Earth brag, oooo, look what I’ve done: look at my hairdo, as all we little critters now go springing out into the space surrounding it.
Everything seems so interdependent upon everything else. It’s all connected, if even just from the Big Bang. It makes me wonder if asking the question, is something alive, is really all that important. What does seem important, though, is a general respect shown toward all our constituency, and whatever constituency we might find ourselves a part, on whatever scale.
We have suns whose component processes manufacture carbon and metals. We have plants whose component processes manufacture oxygen. We have ourselves whose component processes manufacture thinking about stuff like this.
And we’ve likely got big clusters of all this stuff going on everywhere. I wonder, is a delineation between life and non-life anything? Or are we just looking for something we recognize to be something like ourselves — something we can, in some more intimate way, relate to? And if that’s the case, maybe it’s just as good to ask the question, what are we, to wonder such things, or to wonder at all?
Maybe we don’t really need to abstract ourselves from life to define what it is and is not, creating boundaries where none seem, on various scales, to exist.
I just love to death, though, that we really don’t want to be all alone in it all, though.
There is an interesting paper on a computer model for one of the primary nucleosides.
Rainer Glaser, Brian Hodgen, Dean Farrelly, and Elliot McKee
‘Adenine Synthesis in Interstellar Space: Mechanisms of Prebiotic Pyrimidine Ring-Formation in Monocyclic HCN-Pentamers’
Article, Astrobiology 2007, 7, 455-470. PDF. (Publication in Print: June 2007)
Online Visualization: Chime Displays and Reaction Animations
http://web.missouri.edu/~glaserr/vitpub/adenine_ast_2006.0112.pdf
Back to that Vitalism thing: The olde conception at least honored the notion of causality. The vitalist doctrine DID propose (various versions of) a “life energy,” but the contemporary versions (complexity, emergence, self-organization, etc.) don’t EVEN do that. They propose that nature violates entropy with NO cause whatsoever. It just happens.This is rational thinking? Czech it out: http://www.starlarvae.org
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