Life, the universe, and blurry boundaries

2007.03.10 - Pinyon pine

Over at the New York Times, science writer Ferris Jabr wrestles with the difficulty of differentiating living things from non-living things—viruses can reproduce themselves and evolve, but need host cells to do it; inorganic crystals can grow and (sort of) reproduce. He concludes that although “life” as we know it is a useful concept, it’s just that—a concept: “We find it useful to think of some things as alive and others as inanimate, but this division exists only in our heads.”

From there, Jabr goes on to a conclusion that (judging from my Twitter stream) has made a lot of people very angry and been widely regarded as a bad idea:

Not only is defining life futile, but it is also unnecessary to understanding how living things work. All observable matter is, at its most fundamental level, an arrangement of atoms and their constituent particles. These associations range in complexity from something as simple as, say, a single molecule of water to something as astonishingly intricate as an ant colony. All the proposed features of life — metabolism, reproduction, evolution — are in fact processes that appear at many different regions of this great spectrum of matter. There is no precise threshold.

Some things we regard as inanimate are capable of some of the processes we want to make exclusive to life. And some things we say are alive get along just fine without some of those processes. Yet we have insisted that all matter naturally segregates into two categories — life and nonlife — and have searched in vain for the dividing line.

It’s not there. We must accept that the concept of life sometimes has its pragmatic value for our particular human purposes, but it does not reflect the reality of the universe outside the mind.

I’ve joined in the discussion on Twitter, a little bit, but I felt like I needed more space for my reaction to this thinking, so here goes: The fact that “life” is a human concept doesn’t mean it has no basis in physical reality.

My initial reaction to the whole essay was to think about biologists’ debates over species concepts—the rules by which we decide that one population of critters is a different collective biological entity than another. It’s easy to say that species are populations of “actually or potentially interbreeding individuals,” per the biological species concept—but there are plenty of cases where things that seem to be different species turn out to interbreed all the time, or populations that look very similar turn out to be completely reproductively isolated. Most biologists would, I think, agree that “species” are, indeed, concepts—ultimately just ways for us to ensure that we’re talking about more or less the same critters when we use a particular Linnaean binomial.

But that doesn’t mean that species have no basis in biology! The genetic diversity of living things is not a uniform mush; it’s possible to trace lineages of ancestry that do not intersect going back millions of years, and members of such different lineages generally are, in fact, very different in appearance and in lifestyle.

Similarly, although there are cases where it’s truly difficult to decide whether a particular system is “alive” or not, the concepts of “life” and “non-life” capture real differences which exist independently of human observation and description. Living things generally engage in processes—conversion of unlike matter into copies of themselves, social interaction with other living things, reshaping their environments to make them more useful—that are not accessible, or even comprehensible, to nonliving things. The existence of twilight doesn’t mean that there aren’t real differences between day and night. (Even if, yes, those are all just concepts for our internal human experiences of ambient light levels!)

I’m not necessarily sure that Jabr is holding that there is no such thing as life at all—although his essay seems to get pretty close to saying that, and a lot of folks (including whatever editor wrote the essay’s headline) seem to have taken that to be his meaning. But regardless, I think it’s pretty clear that the fact that reality isn’t divided by sharp, binary boundaries does not mean that binary concepts can’t describe it pretty well—we just have to be willing to allow some margin for error.

4 comments on “Life, the universe, and blurry boundaries

  1. Ferris Jabr says:

    Dear Jeremy,

    Thank you for your thoughtful reply.

    I am not arguing that the concept of life has no *basis* in biology or the physical universe. Indeed, every concept humans invent is necessarily based on what we observe about our universe. That’s what a concept is: a way of *organizing* observations we make about our world. But concepts themselves have no physical existence outside the brain/mind.

    “Species” is a useful concept because it allows us to think coherently about many very real properties of the arrangements of atoms, molecules and cells we collectively call living things. DNA has a physical reality independent of the human mind. There are physical changes in DNA over time in groups of “living things.” We can measure and predict those changes. All of that is real. But the concept of “species” itself has no physical reality independent of the mind, because it is not an individual property inherent to matter – or even a coherent “phenomenon” of matter (which I do not think is a well-defined term, anyhow) – but rather **a way of thinking** about what all that matter is doing.

    Similarly, “life” is a way of thinking about different kinds of matter. We can point to many very physical, very real properties of matter that tend to be associated with what we call living things. But the concept itself has no physical reality outside the mind. Here’s how I put it in my SciAm blog, the progenitor of the NYT piece: “It’s not that there’s no material difference between living things and the inanimate; rather, we will never find some clean dividing line between the two because the notion of life and non-life as distinct categories is just that—a notion, not a reality.”

    I know that some people on Twitter have claimed that there does not need to be a clean dividing line because life is a spectrum, but I think that is a flawed argument for several reasons. First of all, most people – and textbooks – treat life as a property or state of matter that is categorically different from non-life. If we are so comfortable with the idea that some things are alive, some not and some inbetween, then why have biologists disagreed for so long about how to classify viruses? And why do biologists regard bacteria, plants, animals and people as equally alive – isn’t it a spectrum? Shouldn’t at least some of those be “less” alive than others? Finally, even if you think of matter as falling on some spectrum from non-life to life, you are still treating “life” as some kind of special property or state of matter distinct from everything else – you still need to define what makes matter at the life end of the spectrum different from the rest of it. And I think those differences all boil down to complexity and to properties of matter we already understand. I’m not proposing that we ditch the concept of life altogether, because it is useful, but we should stop treating it as one coherent thing instrinsic to matter that separates it from supposedly inanimate matter and realize that “life” is really our human way of *thinking* about that matter.


  2. Jeremy Yoder says:

    Hi, Ferris, and thanks for a thoughtful reply! Thanks also for pointing me to the SciAm piece—I think, reading that, I follow your thought process better.

    I think we agree on the distinction you’re making between concepts like “life” or the identity of a particular species, and whatever actual “real” phenomena they try to describe and delineate. I guess that where we differ is that I don’t think that ambiguous cases necessarily undermine the validity (if that’s the word?) of a concept we use to classify things. In many of the cases we’re discussing, I think that’s because of the broader understanding of natural processes in which those concepts are embedded.

    To go back to species as an analogy, the fact that boundaries between some species are ambiguous is a direct result of the evolutionary processes that produce distinct species—gradual divergence due to geographic isolation and/or adaptation to different environments. The ambiguous cases tell us something about the clear-cut cases. They’re the exceptions that prove the rule, in the original sense of “proving.”

    Similarly, if we really do think that (what we call) living things trace their ancestry all the way back to (what we call) non-living matter, why would we be surprised if there are cases that fall in some sort of transitional zone, or even in which life has given rise to things that are less obviously alive? And why would our emerging capabilities for building systems that engage in life-like processes—maybe even what we’d call “low-level” life—undermine the qualitative difference of those processes from what the matter engaged in those processes would otherwise do? So I think I’m perfectly content, as a biologist, to accept “life” as a spectrum, and to say that some things are more alive than other things. (I’m inclined to think most biologists would say the same, but I admit that I haven’t taken a survey.)

    I’m also not sure why the fact that the properties of things we call “alive” are (at least potentially) reducible to more fundamental processes undermines the differentiation between “alive” and “not alive.” Does the existence of quarks undermine the concept of “the atom”? Does the existence of thousands of individual commuters whose various schedules converge on trips across the Hennepin Avenue Bridge into downtown Minneapolis between 5 and 6 p.m. undermine the concept of “rush hour”? These are not entirely rhetorical questions! I’m actually, honestly, not completely sure I think my instinctive “no” is correct.

    Anyway, this, and my original post, are not so much to argue against your conclusions as to try to wrap my head around them and decide what I think!

  3. physiolus says:

    i always think about this picture when the issue ‘boundaries’ comes up:

  4. […] Pollination syndromes were first proposed in the 1870s, and from the start they’ve been controversial in much the same way that many attempts to classify living things often are. […]

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