Thursday, September 06, 2007

Defining Life

The August issue of SEED has a wonderful article by Carl Zimmer, the best science writer on the planet. The article, The Meaning of Life, has just appeared on the SEED website so you can all read it [THE MEANING OF LIFE]. You should read it.

Zimmer asks an important question,
We create life, we search for it, we manipulate and revere it. Is it possible that we haven't yet defined the term?
What do you think of the definition(s) in the article?


  1. A very interesting article. Zimmer does good work.

    Chemists can now say water is H2O. However, "'Water is H2O' isn't a definition," says Cleland. "It's a discovery."

    Why can't it be both?

    Knowing how to recognize life is important not just scientifically, they argue, but ethically as well. Scientists need to know when their tinkering with chemistry has crossed over into a tinkering with life itself, says Bedau.

    So what? Is there some problem with 'tinkering with life'? Is agriculture unethical?

  2. I find it odd that Cleland and Chyba rule out a current definition for life, then suggest:

    that finding alien life would allow us to start figuring out what is truly universal about life, rather than just generalizing from life as we know it.

    Is this not a bit of a circular argument? It seems to me that alien life being informative would rely on us being able to recognise
    it as life - which would suggest there is already an implicit definition of life in our heads, which would in turn suggest that we have everything we need already to define life.

    Also, I agree: Water is H20 is a definition and a discovery.

  3. In the first lecture of my first-year biology course I tell students that the most important criterion is not whether something fits a definition of 'life'. Rather, what we (and all biologists) study is entities that evolve by natural selection. This criterion seems to capture everything that we would consider as alive.

  4. In the first lecture of my first-year biology course I tell students that the most important criterion is not whether something fits a definition of 'life'. Rather, what we (and all biologists) study is entities that evolve by natural selection. This criterion seems to capture everything that we would consider as alive.

    Um, so evolutionary computer algorithms are alive?

  5. Um, so evolutionary computer algorithms are alive?

    I'm not familiar with the precise terminology of computer science, but I would think the algorithm is more analagous to the environment, and the program running the algorithm is what is alive.

  6. Tegumai Bopsulai, FCD wrote:

    "Chemists can now say water is H2O. However, 'Water is H2O' isn't a definition,' says Cleland. 'It's a discovery.'

    "Why can't it be both?"

    I agree it can be, but I think the important point is that the discovery preceded the current definition, arguably more useful than any of its predecessors (e.g., those enumerated in the article, such as aqua regia, "noble water," actually a combination of acids, but considered to be a type of water because it dissolved things).

    Thus, I think Cleland is arguing that though we can arrive (and have arrived) at innumerable definitions of "life," a more useful definition awaits the discovery of more fundamental principles of life, equivalent to the fundamental principles of chemistry that allow us to define water as H2O.

    Cleland believes this will only occur if/when we find extraterrestrial entities that are arguably living. I'm not sure I necessarily agree with that, but it does seem clear we haven't settled on a definition of "life" that commands the unanimity of, e.g., the definition of water as H2O.

  7. I think this sort of verbal quibbling should be left to philosophers (is anyone aware of how many trees the likes of Kripke and Putnam have killed arguing about water vs. H2O? You'd be amazed), who unlike scientists have nothing better to do. Definitions are mere matters of convenience and have no fundamental significance.

    The idea that "recognizing" life, when "the line is crossed" in the lab, is "ethically" important is particularly obnoxious (and incoherent), and stinks of a concealed religious agenda.

  8. [Sorry about the technical difficulties. They involve a cup of coffee (missing) and software failures (present).]

    Well, I know love when I see it. ... What, are we discussing life? Oh.

    Why can't it be both?

    This cuts to the chase. I'm sympathetic to helpful descriptions and definitions, but I read Cleland as wishing for the oh so common and content free philosophical coherent view of, say, Aristotle. (Not that I relish the burden of a philosopher that instead is tasked with describing hundreds of different definitions of life.)

    The weight of useful definitions can go from descriptive to a set of formal ones while we accrue data and theories. Why would that be any different here?

    It represents only a single data point, ...

    I don't think this is necessarily true for the case and characteristics considered. Viruses could have crossed the Darwinian threshold several times from the progenotic state, and so could cells from different extant domains, not including possible extinct domains. Also, early total extinctions are consistent with early and easy abiogenesis.

    I get the impression that these crossings are done by the mechanism of selfish elements, a mechanism that is often observed.

    IANAB but FWIW my conclusion is that 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 be is that they are rare and bound for extinction.

    I generally champion definitions that are inspired by theory since they correlate better with it. Therefore I currently try on for size the following conception of life (to use John Wilkins terminology on species):

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

    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 a hereditary mechanism, and a robust definition of organism. Quite a few birds knocked down with one stone.

    As I understand it 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.

    (Btw, again IANAB but I wonder if that definition can't be followed dipping into the "progenotic" state dominated by horizontal transfer. Some lineages of genes can start earlier, perhaps. But maybe I don't get its application yet.)

  9. Wilkins, in typical fashion, has started a long boring series of posts about the subject.

  10. “Life, they decided, was a self-sustaining chemical system capable of Darwinian evolution.”

    Pragmatically I haven’t seen a better definition. What’s yours?

    “Cleland, for example, doubts that Darwinian evolution, the core of the NASA definition of life, is essential. "I think those arguments are weak," she says. She envisions alien microbes filled with enzymes but lacking genes. The enzymes build more enzymes and the microbes split in two. They couldn't evolve through Darwinian evolution, because they wouldn't have genes. But they might still change, as their environment changed.”

    But where did those enzymes come from? That’s a bit of magic that Cleland leaves unexplained. I’m afraid we really are stuck with the Blind Watchmaker. Either that, or God did it.

  11. Computer viruses evolve by selection, but I don't think it could be called 'natural selection' as the system doing the selecting is a human construct.