When we think about the "meaning of life," we tend to conjure ideas such as love, or self-actualization, or justice, or human progress. It's an anthropocentric view; try to convince blue-green algae that self-actualization is some sort of virtue. Let's ask instead why "life," as a biological concept, actually exists. That is to say: we know that entropy increases as the universe evolves. But why, on the road from the simple and low-entropy early universe to the simple and high-entropy late universe, do we pass through our present era of marvelous complexity and organization, culminating in the intricate chemical reactions we know as life?
Yesterday's book club post referred to a somewhat-whimsical vision of Maxwell's Demon as a paradigm for life. The Demon takes in free energy and uses it to maintain a separation between hot and cold sides of a box of gas -- a sustained departure from thermal equilibrium. But what if we reversed the story? Instead of thinking that the Demon takes advantage free energy to help advance its nefarious anti-thermodynamic agenda, what if we imagine that the free energy is simply using the Demon -- that is, the out-of-equilibrium configurations labeled "life" -- for its own pro-thermodynamic purposes?
Energy is conserved, if we put aside some subtleties associated with general relativity. But there's useful energy, and useless energy. When you burn gasoline in your car engine, the amount of energy doesn't really change; some of it gets converted into the motion of your car, while some gets dissipated into useless forms such as noise, heat, and exhaust, increasing entropy along the way. That's why it's helpful to invent the concept of "free energy" to keep track of how much energy is actually available for doing useful work, like accelerating a car. Roughly speaking, the free energy is the total energy minus entropy times temperature, so free energy is used up as entropy increases.
Because the Second Law of Thermodynamics tells us that entropy increases, the history of the universe is the story of dissipation of free energy. Energy wants to be converted from useful forms to useless forms. But it might not happen automatically; sometimes a configuration with excess free energy can last a long time before something comes along to nudge it into a higher-entropy form. Gasoline and oxygen are a combustible mixture, but you still need a spark to set the fire.
This is where life comes in, at least according to one view. Apparently (I'm certainly not an expert in this stuff) there are two competing theories that attempt to explain the first steps taken toward life on Earth. One is a "replicator-first" picture, in which the key jump from chemistry to life was taken by a molecule such as RNA that was able to reproduce itself, passing information on to subsequent generations. The competitor is a "metabolism-first" picture, where the important step was a set of interactions that helped release free energy in the atmosphere of the young Earth. You can read some background about these two options in this profile of Mike Russell (pdf), one of the leading advocates of the metabolism-first view.
I was reading a bit about this stuff because I wanted to move beyond the fairly simplistic sketch I presented in my book about the relationship between entropy and life. So I did a little research and found some papers by Eric Smith at the Santa Fe Institute. Smith has taken quite an academic path; his Ph.D. was in string theory, working with Joe Polchinski, and now he applies ideas from complexity to questions as diverse as economics and the origin of life.
On Saturday I was on a long plane ride from LA to Bozeman, Montana, via Denver. So I had pulled out one of Smith's papers and started to read it. A couple sat down next to me, and the husband said "Oh yes, Eric Smith. I know his work well." This well-read person turned out to be none other than Mike Russell, featured in the profile above. Here I was trying to learn about entropy and the origin of life, and one of the world's experts sits down right next to me. (Not completely a coincidence; Russell is at JPL, and we were both headed to give plenary talks at the annual IEEE Aerospace Conference.)
So I explained a little to Mike (now we are buddies) what I was trying to understand, and he immediately said "Ah, that's easy. The purpose of life is to hydrogenate carbon dioxide." (See figure above, taken from one of Eric Smith's talks.)
That might be something of a colorful exaggeration, but there’s something fascinating and provocative behind the idea. An extremely simplified version of the story is that the Earth was quite a bit hotter in its early days than it is today, and the atmosphere was full of carbon dioxide. At high temperatures that’s a stable situation; but once the Earth cools, it would be energetically favorable for that CO2 to react with hydrogen to make methane (and other hydrocarbons) and water. That is to say, there is a lot of free energy in that CO2, just waiting to be released.
The problem is that there is a chemical barrier to actually releasing the energy. In physicist-speak: the Earth’s atmosphere was caught in a false vacuum. There’s no reaction that takes you directly from CO2 and hydrogen to methane (CH4) and water; you have to go through a series of reactions to get there. And the first steps along the way constitute a potential barrier: they consume energy rather than releasing it. Here’s a plot from one of Russell’s talks of the free energy per carbon atom of various steps along the way; it looks for all the world like a particle physicist’s plot of the potential energy of a field caught in a metastable vacuum. (Different curves represent different environments.)
Here is the bold hypothesis: life is Nature’s way of opening up a chemical channel to release all of that free energy bottled up in carbon dioxide in the atmosphere of the young Earth. My own understanding gets a little fuzzy at this point, but the basic idea seems intelligible. While there is no simple reaction that takes CO2 directly to hydrocarbons, there are complicated series of reactions that do so. Some sort of membrane (e.g. a cell wall) helps to segregate out the relevant chemicals; various inorganic compounds act as enzymes to speed the reactions along. The reason for the complexity of life, which is low entropy considered all by itself, is that it helps the bigger picture increase in entropy.
In ordinary statistical mechanics, we say that high-entropy configurations are more likely than low-entropy ones because there are simply more of them. But that logic doesn't quite go through if you can't get to the high-entropy configurations in any straightforward way. Nevertheless, a sufficiently complicated system can bounce around in configuration space, trying various different possibilities, until it hits on something that looks quite complex and unlikely, but is in fact very useful in helping the system as a whole evolve to a higher-entropy state. That's life (as it were). It's not so different from other cases like hurricanes or turbulence where apparent complexity arises in the natural course of events; it's all about using up that free energy.
Obviously there is a lot missing to this story, and much of it is an absence of complete understanding on my part, although some of it is that we simply don’t know everything about life as yet. For one thing, even if you are a metabolism-first sympathizer, at some point you have to explain the origin of replication and information processing, which plays a crucial role how we think about life. For another, it’s a long road from explaining the origin of life to getting to the present day. It’s true that we know of very primitive organisms whose goal in life seems to be the conversion of CO2 into methane and acetate — methanogens and acetogens, respectively. But animals tend to produce CO2 rather than consume it, so it’s obviously not the whole story.
No surprise, really; whatever the story of life might be, there's no question it's a complicated one. But it all comes down to the elementary building blocks of Nature doing their best to fulfill the Second Law.