Some like to define living organisms as that which
1) reproduces,
2) has inheritance, and
3) has variation.
In other words, living things would be those which evolve by natural selection. Rosie Redfield (blog) espoused this view in a recent and otherwise really goo talk at the Evolution 2012 conference in Ottawa (#evol2012 Twitter feed). Jerry Joyce (lab page) did the same at the 74th symposium of Quantitative Biology at Cold Spring Harbor Labs in 2009.
But this is folly.
First of all, I can easily give an hypothetical example of something that must clearly be alive, but which does not evolve. I'll defer that to the end of this post.
But I can also give an example of something that most people will not agree is alive, namely languages. Metaphorically, I can accept that languages are alive. "Danish is such a beautiful language, alive with raunchy adjectives and verbs that sing." Or something. But not actually alive in a literal sense. It is spoken by beings that are alive, but is no more alive than thoughts or books, even if it does evolve (note that languages evolution really isn't of the Darwinian kind, either, just like memes aren't).
We can of course define for our own purposes life (or living things) as anything we want. Doing that sensibly, however, is key, since science is all about communication. I could define life as anything that grows, anything that moves, anything that catalyzes chemical reactions, etc. Those are all things that most things we would call life do in some way or other. But it would not be sensible, because there are things that are not alive that do those things, too. Fires grow, the wind moves, earth catalyzes. Defining something sensibly means that it should conform to daily use of the term, or in the cases where it does not, it should make sense to refine the vernacular.
So in defining life as something that evolves by natural selection, we would both include things that clearly are not alive in the sense that most people understand it (language), and we would also exclude some things that are clearly alive, but does not evolve.
This latter thing that is alive but does not evolve - what is it, then? It's true that no living organisms that we know of do not evolve, right?
Well, both true and false. First of all, individual organisms do not evolve at all. Populations evolve. Lineages evolve. Individuals develop - from a single cell to an adult human, for example. Organisms are collections of cells, and does not evolve. Does that mean I am not alive? Clearly I am. This definition does not work. It is true that all living things descend* with modification from ancestors that were different from themselves. But what if we one day discovered an organism, looking quite like any other, but which does not die and does not reproduce? Would it not be alive?
Suppose we go to another planet and find one being there, looking exactly like a human being. Everything we can measure about this being confirms that it is just as much alive as you and me. It eats, moves, heals, replenishes, communicates, feels, defecates. Learning more about this being, though, we find that it has no ancestors, and that it does not age. It does not reproduce, and it is the only such being on the planet. Thus, there is no lineage of descent and no population that can evolve. So this being is then not alive? Of course it is. This definition does not work.
For those of you who would object that this example is irrelevant, because no such being that is alive but does not evolve has ever been found: definitions must encompass such thought experiments, or they are useless. If our definitions can not guide us when we are in doubt - in situations where something new is encountered, them they are useless. In that case we might as well just define life as the things that we already know are alive, which just amounts to bookkeeping.
Lastly, see what I did there? I demolished something without providing a solution. Tough luck! I am not required to put forth another definition that I think is better than this one. Just as well as I am not required to come up with some alternative to religion just because I am an atheist.
Is R2-D2 alive? Is it evolving?
* Actually, I prefer to say 'ascend', like the twigs on a tree grows up, and not down. Idiosyncrasies.
Perhaps, like the iconic SCOTUS ruling on pornography, we know it when we see it?
ReplyDeleteYes, that is of course the definition we currently use, but which may not work in the extremes. Many people seem to confuse prescriptive and descriptive definitions. Above I am talking about prescriptive, while Rosie use a descriptive definition.
ReplyDelete"Suppose we go to another planet and find one being there, looking exactly like a human being. Everything we can measure about this being confirms that it is just as much alive as you and me. It eats, moves, heals, replenishes, communicates, feels, defecates. Learning more about this being, though, we find that it has no ancestors, and that it does not age. It does not reproduce, and it is the only such being on the planet. Thus, there is no lineage of descent and no population that can evolve. So this being is then not alive? Of course it is. This definition does not work."
ReplyDeleteDid you read Solaris by Stanislav Lem (1961)? Lem's being on Solaris is not like a human, however, but like an ocean with metabolism and one issue IIRC was whether such a being would have a mind. The Hollywood movie Solaris (2002) did not transport these philosophical issues.
No, I've never read that. That's perfect. (For the record I should state that whether this being is conscious of course doesn't matter for the life definition.)
ReplyDeleteSure, mind and conscience are not required. It would have been more appropriate of me to call Solaris sentient. Whether Solaris has a mind or conscience remains an open question or not even addressed question. Schätzing's The Swarm seems to have been inspired by it. Surely, many scientists have been inspired by it for I got the recommendation to read it back in the 90s from a physics professor.
ReplyDeleteYou say that "of course" a being which moves, eats, defecates etc is alive despite not reproducing - you have listed characteristics you believe indicate life. In other words, the definition of life as reproducing and evolving only fails because you have decided it fails - if life is defined as something which reproduces, how can the being you describe be considered 'alive'?
ReplyDeleteThere is certainly a need to descriptively define abstract concepts, biological examples of which include 'species' and, in the case of some organisms, 'individual', otherwise using such terms is meaningless. However, we need to look more carefully at the purpose of developing a prescriptive distinction between life and non-life; is such a firm distinction even useful? Does whether or not the hypothetical organism is alive matter, or is it only relevant to note whether it reproduces, metabolises, thinks etc?
Physics does not distinguish between life and non-life; 'alive' is as artificial and human a construct as days of the week. Is midnight part of Monday or Tuesday? We can define it as either without changing its characteristics. Is something alive or not alive? Again, we can define it as either without changing its characteristics. There is no 'right' answer, and so in encountering a new organism we would essentially be asking, with definition or without, "Do we consider this thing to be similar enough to Earth life to also be alive?" It doesn't matter whether we decide it is or not; it will remain whatever it is.
Okay, Samuel, if you don't care what we define it as, then what does it all matter? I am not saying there is right and wrong in defining life - you may define however you want and that is then your that. But I think it is useful to define it sensibly, an in doing that, we have to take into consideration what people in general consider life. That was my point.
ReplyDeleteIf I had to define life, I'd at least want to have individuals like myself included among the living. Now, individuals are not evolving and many human individuals also choose to not reproduce either (even in nature many individuals never make it though they'd want to). I think it would be really foolish to count them out for that matter.
ReplyDeleteHey Bjorn, you are now officially referred to anonymously as a "blogging post-doc", and your words are being used as ammunition by creationists!
ReplyDeletehttp://www.philly.com/philly/blogs/evolution/Is-Darwinian-Evolution-Universal-.html
Egads! :D
For the records, creationists, that it is possible to imagine life that doesn't evolve doesn't mean that life on Earth doesn't evolve. It demonstrably does..
ReplyDeleteYou're also getting (anonymously) beat up in the comments here, because the Faye Flam article referred to your thought experiment in a confusing way. I'm trying to clarify to people that the thought experiment itself (which I thought was very provocative and worthwhile) is NOT a creationist thought experiment. Oy...
ReplyDeleteJames, thanks for relaying.
ReplyDeleteIf you need some more amunition against inane misconceptions, Erwin Schrödinger (yes, the Nobel prize winning physicist) once wrote a book called What is Life?. You'll find arguments about negative entropy in there and more.
ReplyDelete