Over at Cosmic Variance, Sean asks whether we can define “life” in three words.
Our concept of life (like most of our concepts) is actually a cluster of concepts that pick out related but distinct natural properties. So the only way we could actually come up with a simple definition would be to stipulate that henceforth we shall take the word “life” to mean such-and-so.
But it’s still worthwhile to try to figure out what the key features of life are. Obviously reproduction plays a very prominent role, but I – like Sean – am not inclined to place this at the core of the concept. (Maybe that’s because we focus on physics and not biology.) After all, worker ants and bees are obviously alive, but they can’t reproduce.
Here’s my stab at a three-word definition: “Pushing against entropy”
A slightly longer version to clarify what I’m getting at: “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.”
The picture I have in mind is that life can only exist in a situation where there is an entropy flow. We need the low-entropy energy coming from the sun (or the Earth, in deep ocean thermal vents) and then a high-entropy place for it to flow into (like the cold of space) in order to have life on Earth. And then what living systems do is speed up the flow from the low-entropy source to the high-entropy sink, and this allows them to lower the entropy in some small region (inside their skin, or cell walls).
I visualize this as being like a fish swimming upstream. The overall flow of the river has to go downhill. The only way a fish can go uphill is to push against the water, which causes the water to flow even faster downstream (not that the change would be noticeable, of course). Similarly, a living thing “propels” itself against the flow of increasing energy by taking in low-entropy energy sources and controlling when, where, and how those sources transition into high-entropy energy. This allows it move against the flow of entropy.
I think this is right as far as it goes, but it doesn’t seem this characterization is narrow enough to differentiate life from non-life. After all, refrigerators decrease entropy locally by increasing the entropy of their environment, but we’re not inclined to say th at they’re alive. And likewise for pretty much any energy-consuming machine. The gasoline goes into the car, and heat energy goes out.
So maybe here we should appeal to something like reproduction. It’s remarkable that there is reproduction of information that enables systems to swim against entropy. And it is also remarkable that the blueprints for organisms are contained within the organisms themselves, while the blueprints for refrigerators are external (unless you stick them in the freezer — but you get what I mean).
But what if we build a long-lived multicellular organism that isn’t able to reproduce (even at the cellular level), but that is able to take in food and behave like a standard living organism? Is it “living” or is it a “mere machine”? Perhaps the distinction just breaks down. Indeed, we should expect any naturalistic criterion to get fuzzy at certain points; the emergence of higher-level properties requires it. (More on that at some later date.)