Free Energy and the Meaning of Life

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?

From a slide by Eric Smith

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.)

From a slide by Michael Russell

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.

  1. “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.”

    Wow, guaranteed that quote will show up on some anti-science website to try to vilify how scientists view life. 🙂

    I do find your idea of needing a sufficiently complex system to go around releasing “hard to get to” free energy fascinating. Something interesting to think about for sure.

    I wish I could have been hovering around your conversation with Mike Russell.

  2. When I’m flying, I always wish that someone that interesting would sit next to me. Never happened.

    I’ll be forwarding all people who claim that the second law prevents evolution to this page from now on.

  3. But, but, but … life is built around amino acids, which are are bit more complex than methane. Is an amino acid endo or exothermic? Does it represent a higher or lower entropy state than it’s dissociated atoms? And how do we get to the amino acids before we hydrogenate the CO2?

  4. There are a couple more subtleties, such as where do the acidic conditions come from that can allow for serpentization of CO2 to CH4?

    Presumably proton donation to form acids occurred during the formation of the proto-oceans during an early bombardment, when olivine + water was exothermically converted to serpentine + protons.

    More imaginatively, after that initial kick start of acidification life must have evolved enzymes to acidify the environment of the metabolic process, which should be very early precursors of the modern photosynthetic enzymes.

  5. I was just talking about this concept to my wife a few days ago. I don’t have the educational background to think about it very concretely, but I thought the key idea was of a system bumping up against some maximum rate of entropy. This forces it to find a more efficient way to produce entropy.

    The plot shown here is interesting. It reminds me of a plot in Endless Universe by Steinhardt and Turok, showing the conversion of dark energy from an attractive force to a repulsive one. I’m just thinking out loud – I know trying to tie two speculative theories together with a plot graph isn’t terribly scientific 😉

  6. But it all comes down to the elementary building blocks of Nature doing their best to fulfill the Second Law.

    Sean here gives the elementary building blocks of nature purpose – “they do their best”, etc. But this is religion.

  7. I should add that the above process only works in shallow bodies of water laying on olivine formations (which there would have been a lot of in the early Earth). But once the accessible olivine had been converted, and the oceans became deep enough, then one has the same problem of being at a false minimum of entropy. Presumably one of the chief activities of life is then to access normally inaccessible olivine minerals and make them available for serpentization. Which begs the question: exactly how deeply into the Earth’s crust has bacterial life infiltrated, has it gotten beneath the sedimentary layers?

  8. Sean makes it sound a little dramatic by saying Nature “found a way” to use all the free energy that was just lying around. I think it would sound a little less wild if we acknowledge that it’s not “Nature” per se that’s doing anything but the energy itself. That is, energy enables molecules to sample many configurations, combinations, and reactions. This allows systems from emerge from a local minimum and try alternate, more efficient configurations (like a marble moving rapidly in a bowl, eventually it will fall off the edge). Heat from the sun and from the early earth allowed many combinations of molecules to emerge, providing the “push” that led to complexity and to life.

  9. The German philosopher Friedrich Nietzsche described life and the “world” in terms of what he called the “will to power.” I have developed a thermodynamic interpretation of this concept (“Nietzsche’s Physics,” in International Studies in Philosophy XXXI/3 (1999): 5-17). One of the most controversial quotes from his unpublished notes is “This world is the will to power–and nothing besides! And you yourselves are also this will to power–and nothing besides!”(The Will to Power, 1067) Nietzsche was aware of the early formulations of the second law and I believe he had prescient view of its implications.

  10. If you live your life in fear of being quote-mined by anti-science types, you will have less fun and still be quote-mined by anti-science types.

  11. Arun, it’s called poetic license. He was putting the idea in terms that are easier to understand. He’s not seriously proposing that there is a conscious intent there. If you have to be intentionally obtuse just to make your point, then your point is wrong.

  12. Though which chemical pathway was favored on prebiotic Earth is still quite open to debate, the notion that self-organizing systems decrease entropy locally, but are still dissipative on the scale of an open system is not news or controversial. Yes, catalysis of some sort certainly played a role in bringing prebiotic chemistry anywhere near the levels of complexity we associate with even the simplest organisms, but that’s a historical subject, not really a fundamental one. I think the most interesting question about life is not “How” (in the specific case of Earth, though that’s still an immensely interesting question), but more related to “If”. I.e., if we observe self-organization everywhere, does the efficient dissipative nature of such systems indicate a kind of “attractor” that makes life inevitable in any environment that can sustain some threshold level of complexity for a long enough span of time. For this argument, I think the the order of emergence of replicators or metabolomes is pretty much irrelevant, since you wind up with one or the other eventually. On Earth, people keep finding clever ways to support RNA chemistry in hypothetical prebiotic environments, and since RNA makes for a decent information storage medium as well as raw material for enzymes (ribozymes), in the RNA world, you’re almost getting both for the price of one. And lets not forget some of these building blocks may have come directly from space. Perhaps the fact that it’s so hard to get over that hump in the Gibbs free-energy plot on Earth is tells us it didn’t happen on Earth.

  13. This is an interesting idea, but I’m not sure I buy it quantitatively.

    The total atmospheric content of CO2 today is about 3e15 kg, or 7e16 moles. Using the ~ 120 kJ/mol free energy release from CO2 to CH4 according to Russell’s plot, that implies the total stored free energy in the Earth’s atmosphere is about 8e21 J.

    Well, the Earth receives that much energy from incident solar radiation in about a day and a half (factoring in the current mean albedo of 0.4; the raw incident solar power is 1.7e17 W). So the amount of energy stored in CO2 is just utterly negligible in terms of the total energy balance of the biosphere. Even if you factor in a massively higher CO2 bath (say, a mass equal the entire atmosphere of Venus, 5e20 kg) then solar energy still equals the CO2 stored free energy after a mere 400 years.

    So it’s a cute story, but I don’t think it describes well the energy balance of the real biosphere we live in.

    (P.S. It was nice to meet you after your talk here at UCLA last week, Sean. Thanks for a very stimulating and thought-provoking afternoon!)

  14. All of the comments above are interesting. I appreciated Charles’ thoughts especially…they have profound implications.

    In biology we reflect that “ontogeny recapitulates phylogeny”…we can observe the processes of organic evolution over cosmological time by observing the short-term development of life from pre-conception through maturity in a given individual of the species. I won’t get into an involved analysis of the implications of that process…believe anyone can evaluate those for themselves. However, let me say that….

    there is something analogous between organic evolution and classical Newtonian physics, and that the 4D “processes” we observe probably are not cosmologically significant. They are significant to US for the simple reason that the universe we live in is defined by space, time, motion and change…entropy- stuff life that.

    We all know that Newtonian physics is a very good approximation of the real universe, a nice model. Relativity is an even closer approximation of reality. In the biological realm, the processes of organic evolution are indisputable.

    I think one of the things which makes me edgy when these creationists assert their bizarre ideas is that I realize that organic evolution may, like newtonian physics or relativity be a close approximation of reality, a very good model, but far from the cosmological truth…biological evolution “happens” when we observe the universe from certain coordinates in a manifold of space and time.

    However, we live in a quantum universe. That is why I thought Charles reflections were so appropriate.

    A quantum universe has a certain singular, instataneous quality about it, and Quantum Mechanics is our best model of reality…better than Newton or Einstein.

    To understand where life “comes from” we need to understand what energy really is, what invariant frames of reference are and how all information and complexity from one side of the universe to the other is eternally and near instantly entangled.

    When we realize that the living material in our bodies has never known death as we define death…the border between the inorganic and organic universe….for 3.2 billion years…a time span of cosmological significance, we get a serious hint that life and consciousness are not some universal after-thought…these things are fundimental to existence itself.

    If I am a 4D object occupying a fixed set of coordinates in a manifold, in all probablity I really developed as a person as a complex molecule develops from less complex molecures in an organic soup. That REALLY makes investigating the nature of energy important to a knowledge of what I am, where I REALLY came from and my purpose in the universe.

    It makes some people kind of angry when a scientist starts to investigate “purpose”. My only comment on that is that my grandchildren and great grandchildren would not exist if I had not existed EXACTLY the way I have, nor would I exist, had not my forebears existed EXACTLY as they existed. My purpose, the purpose of all life, is support; we are a part of the living whole. Like the “keystones” in a cathedral, if one is removed, everything comes down…the structure disintegrates. This means that the time when I “come down”…when everything “comes down” is important to the universal structure. Biblical intuition says it this way: “Are not two sparrows sold for a farthing? Yet none of them falls to the ground without your father. Fear not…you are worth much more than sparrows!”

    I originated in the universe exactly where I “am”. Organic evolution only hints at the extent of my entanglement with the entire inorganic and organic whole of reality. Unquestionably the developent of organization and complexity in evolution is a reflection of the phylogenic development of our individual selves over eternity…Ontogeny recapitulates Phylogieny.

    Everybody has a problem avoiding personification of the inorganic. The reason is that the inorganic and organic universe of information and complexity are completely entangled. Where, REALLY can we draw the line between the inorganic and organic worlds?

    I’m reminded once again of Fred Hoyle. When he found that, in the center of stars, the exact conditions demanded for the existence of the necessary proportion of carbon to permit life in the universe, were duplicated…he exclaimed that “life monkeys around”. Life sure does “monkey around”! How it “monkeys around” will be determined when we understand what energy really is….

  15. Here I’ve always thought that the meaning of the universe was to produce iron and black holes, with any intermediate step of ‘life’ just acting as a distraction.

  16. Of course in the modern era the with the biosphere’s ability to convert a part of an incident 5500K black body spectrum to a 300k black body spectrum results in about a 17 fold increase in entropy per mole of converted photons, calculated through conservation of the Gibbs Free Energy (scattering by itself results in a negligible increase in entropy). The same logic has Venus increasing entropy per mole of converted photons by only 6 fold.

  17. “So it’s a cute story, but I don’t think…”

    Marshall, I’m curious what potential energy barrier you see involving the energy from the Sun. Because if there isn’t one… then you missed the entire point of this discussion.

  18. Aaron,

    Interesting comments, but I had a couple questions, as my knowledge of primordial geochemistry vis prebiotic stews is fairly limited.

    On the subject of acidification, my understanding of the relevance of acidity to photosynthesis is limited to the fact that the low pH in the thylakoid space provides a proton gradient which, long story short, couples ATP synthesis to work done by light. The key is not acidification per se but the potential difference across a membrane, so what’s in it for an enzyme or primordial cell to simply follow the low pH? Some enzymes catalyze reactions better in an acidic environment, e.g. lysosomal, but it takes a lot of work to a) synthesize those enzymes, and b) decrease the pH of the lysosome, and c) protect the rest of the cell from this environment while getting things in and out of it in an orderly fashion. So it’s the differences in pH that matter, not any particular pH. What’s the significance of an overall acid environment to the evolution of early enzymes and life?

    Also, could you expound a little on the conversion of black body spectra and how that relates to life? Is the Earth vs. Venus increase in entropy an indirect measure of photosynthesis? Could the ratio of some range of values explicable by abiotic planetary surface and/or atmospheric conditions and some measured value be used to suss out the presence of life on exoplanets, in principle?

  19. So does that make us a short-term entropic experiment, doomed to die off quickly, as we increase net CO2 no matter what we do?

  20. Actually my comments on the pH wasn’t based on biological reasoning but rather physical chemistry. The graph for the serpentization process Sean reported required donor protons which, don’t quote me because I haven’t worked out the pK’s, wouldn’t be in enough of excess from the equilibrium of dissolved CO2 with carbonic acid. Which got me reading as to where the excess acidity could come from, a natural fit is the conversion of olivine. But this is all a vast stretch of my limited knowledge of undergraduate physical chemistry. This process would be rate limited by diffusion: CO2 from the top of the body of water, and protons from the bottom. The wildly imaginative part is hypothesizing that the earliest proto-enzymes were thermodynamic improvements on this process.

    I don’t know if the fractional increase in entropy is a measure of life; because by the same earlier logic Mars’ black body spectrum gives a 25 fold increase of entropy per mole of converted photon gas. The calculation just shows that thermodynamically life on Earth is not so much out of place, and might plausibly be the optimum for the particular configuration of orbit, incident radiation, chemical composition, etc…

  21. One more thing…the place to look for signals of exo-life is in heat capacity. Someone should sit down and plot a graph of the heat capacities of the solar planets versus orbit. I have a hunch that Earth lies off the curve, specifically Earth has a higher heat capacity then should be expected; because of the more entropically favorable conversion of incident radiation.

  22. I went to that IEEE conference one year at the request of my boss. I don’t ski and immediatly got altitude sick, plus I had to give my paper at 10 pm. I did learn a few things (none as provactive as this) and wish they would relocate to lower ground.

  23. Interesting. From a strict physical viewpoint, it can be shown that there is a free energy availability maximizes as size decreases, i.e. from large to small to tiny. Consider the only two processes, motion and growth. Motion describes change in position, while growth must describe change in “size”, i.e., mass. A link to my chapter in a new book to be published shortly is on the web site,, that goes into such detail.

  24. Help! Brain lock: “…there is a lot of free energy in that CO2, just waiting to be released.” I thought that carbon dioxide is the low energy compound. Doesn’t the reaction (hydrocarbon) + oxygen -> carbon dioxide plus water release energy that was originally supplied from the environment to facilitate the reverse reaction?

  25. Took a couple of moments to do a few napkin calculations.

    If the Earth could perfectly thermalize the 1350 W/m2 5500K incident radiation, then the from the Stefan-Boltzmann Law the Earth would have to be at a temperature of about 390K. This corresponds to about a 13 fold increase in entropy per mole of converted light. As it stands, again from the Stefan-Boltzmann Law, the Earth at 300K is converting about 450W/m2 of the 1400W/m2 incident radiation. To get a rough comparison of the actual total entropy change one can use the ratio of the converted fluxes times the ratio of the ratios of entropy per mole , so with life, Earth is at about 42% of the maximum possible change of entropy if every mole of incident flux was thermalized.

    Unfortunately for everything except thermophilic bacteria 390K is uncomfortably close to the boiling point of water, I suspect there is something optimum about keeping the Earth near the triple point of water in terms of thermal physical feed back (that is hotter configurations of the atmosphere are not stable and result in long term cooling below 300K), then again maybe runaway heating is the inevitable thermodynamic fate of Earth’s atmosphere.

    I haven’t had time to work out the comparisons for the other planetary bodies.

    I’m sure this must all be in the literature somewhere, I’ve just been to lazy to look it up. Some of it is vaguely reminiscent of the problems in my second year modern physics course.