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.
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Patrick,
Good point, the Gibbs Free Energy of Formation of products from reactants depends on the chemicals, the reaction path, and the conditions under which the reaction occurs. So if one calculated the Gibbs Free Energy of Formation of methane and oxygen from water and carbon dioxide via the path of breaking down the reactants to individual constituent atoms and then forming the products one would have to provide about 818 kilojoules per mole to make the reaction occur.
The point of the graph is to show that their are other formation processes and paths that are not as unfavorable in terms of the Gibbs Free Energy (e.g. Fischer-Tropsch synthesis). But just because you can release energy along a reaction path still does not mean it is globally favorable in terms of the total entropy of the heat bath (solar radiation) plus system (Earth).
Certainly given the extent of fossil fuel deposits, and limestone sedimentary formations, a good part of the biosphere has been busy sequestering carbon, but why is this entropically favorable?
More wild speculation runs like this:
Without sequestration: Carbon dioxide builds up->Earth warms to optimum 390K->atmospheric gases lost due to gravitational escape->Earth loses insulation->lowers change in entropy of incident photon gas->long run less change in entropy
With biological sequestration carbon dioxide build up is kept in check, so while the Earth never reaches the maximum 390K, it maintains at 290k+-20k, allowing for a greater increase in entropy over longer time scales.
But like I said that is wildly hypothetical.
I guess the poetic way to put it is that life occurs when entropy equivocates.
The other question is whether we capable of seriously disrupting this process in the long run? Probably not. But are we capable of disrupting this process enough in the short run to kill billions of our fellow humans? Probably yes.
The whole picture is described as ‘photon mill’ with high value energy as input and lower value energy as output. There are even opinions saying live may evolve almost instantly under these circumstances. In such an environment live is capable of creating regions of low entropy (e.g. oilfield), leading to an economic opportunity. However, to me it looks like there are not enough resources available for an infinite economic growth and the limit can be computed now.
Summary : the meanin of life is breathing and do crap …
Sean’s question was – which came first the replicating molecule or the capture of free energy?
I asked myself, where does RNA get energy for replication? It’s from breaking bonds when releasing phosphates. So, “metabolism” has to come first. Yet, chemical reactions can happen in the absence of life. It was the segregation of chemical reactions in space that had to happen first. Molecules other then ones that can replicate may take this first step. But, the ones that replicate can evolve. By evolve I mean both be subject to natural selection, and be a program running forward in time.
There is a significant flaw in my hypothesis concerning the atmospheric evolution in the abscence of biological sequestration: the half life for thermal lose of atmosphere is on the order of hundreds of billions of years…so not really applicable.
For a clue to an alternative explanation one can look at Venus. The entropy maximizing thermalization temperature given a solar flux of 2580W/m2 from the Stefan-Boltzmann Law is about 460K, quite a bit cooler than the surface temperature of about 750K, however the overlying cloud cover scatters about 60% of the incoming radiation, giving a Stefan-Boltzmann thermalization temperature of about 370K, which is plausibly close to the thermodynamic range of the sulfuric acid condensation cycle. This also corresponds to an altitude of about 50km in Venuses atmosphere, which is also a plausible altitude for where the atmosphere is nearly completely insulating, maximally convective, and chemically reactive. Succinctly Venus is maintaining the bulk of its thermal budget with incoming radiation at an altitude of 50km. Interestingly this also means that Venus is increasing entropy in the solar flux at about 41% of the maximum possible.
The analogous situation with Earth would require investigating the altitude, thickness, and scattering of the water cloud cover with run away warming, an area of vigorous debate. However if this configuration is thermodynamically unfavorable when compared to the existence of life then the cloud cover would have to scatter more than 66% of the incoming radiation. This is also plausible because a temperature of 273K corresponds to about 76% of the radiation being scattered.
This also leads to hypothesizing that the altitude of maximum convection, phase transition, and chemical activity is the altitude where the thermal budget with the incoming solar radiation is maintained. Which for Earth is near ground level, but for other planetary bodies not so.
On final note of the IR power of Mars. The average is 390 W/m^2, which corresponds to a temperature of 290K, considerably warmer than any of the atmospheric temperatures, indicating that the bulk of Mars thermal budget with incoming solar radiation is maintained by the heat capacity of the ground.
Again adding a little more provocation to the idea that a critical ingredient for life is that the solar thermal budget is maintained near the ground, but not by the ground.
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It seems like a bit of a reiteration of the idea in “Contribution to the energetics of evolution” – Lotka, A. J., 1922. Search for “Survival of the Likeliest” and “Maximum entropy thermodynamics” for modern iterations of the idea.
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I spent some time years ago philosophizing on Life, the Universe, and Everything. It ocurred to me that maybe the purpose of life is to resist the process of entropy. Then I extended the idea to include a scientific definition of good and evil. Good would, in this framework, decrease entropy and evil would increase it. You could also then have relative good and relative evil.
Pretty abstract I know and quite possibly unmeasurable.
#39…but very thoughtful and I can assure you that you are not the first person to have had that idea occur to you!…Life as a servomechanism to tilt the observed universe (the only one which empirically- and scientifically- exists) away from the chaos of the Second Law of Theromodynamics- and keep a closed universe of finite mass and marginally closed space in a state of perpetual existence- and gradually increasing order and complexity…is a profound idea.
Don’t assume that concept is un-measurable either! Not now, maybe- but…
I have never read a satisfactory explanation why so many scientists strongly assert that space is open and flat when EVERY major experiment has indicated an Omega total of 1.01-1.02 something.
I have the greatest respect for Ned Wright…he said that the cosmological constant (whatever that is…depends on your conceptual framework), if included would produce perfectly flat space. Recent developments have revived talk of a cosmological constant but, wait a minute!
The cosmological constant was developed by Einstein to render the universe static! It is an experimentally determined fact that, cosmological constant or no cosmological constant, the universe is NOT static! Given that fact, we would do well to watch our step when we discuss “the cosmological constant”. Yes, there may be something happening there, true, but whatever it is, it is NOT a cosmological constant!…not what Einstein was thinking about, anyway.
Most criticism’s of quasi-static cosmologies are based on their poor fit with experimental results- ASSUMING a FOUR dimensional universe. That is a big assumption and, also in the light of recent experimental results, an untenable assumption.
A great thread….
The problem with this idea is that the assumption that there are no abiogenic ways to complete this reaction is demonstrably false. There are many methane-producing reactions, that occur in all kinds of environments from cooling magma chambers to laboratory experiments.
An additional problem is that there this energy “minimum” for for the H-C-O system in the absence of rock. Once you add an actual planet into the equation, there are more energentically favorable compositions which include things like carbonates and hydrous minerals.
Also, hydrogen escapes the earth’s atmosphere,, so you can’t keep it around long enough to react with your CO2.
And finally, the chemical potential energy (in the form of oxygen) in the atmosphere now is much greater than when life began, which is the opposite effect than what you would expect from a system designed to dissipate free energy.
My own curiosity about the anti entropic contradiction of life versus thermodynamics was quenched by a law from non linear thermodynamics: A flow of energy creates order. Thus the flow of energy from the sun to earth and into space requires the creation of ordered forms such as life.
http://nobelprize.org/nobel_prizes/chemistry/laureates/1977/presentation-speech.html
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Reconceive Cosmic and Life Evolution:
Pre-History Of History?
Life’s Genesis Was Not Cells, But
First Gene’s Self Reproduction
A. From “Pre-History of Life: Elegantly Simple Organizing Principles Seen in Ribosomes”
http://www.sciencedaily.com/releases/2010/04/100412151823.htm
– Hints from relics of…evolution left behind in MODERN cells.
– Evolution of the MODERN genetic code likely followed a long period of chemical evolution.
– Before the last universal common ancestor the ribosome emerged from an early evolutionary stage of life to help with the translation of the genetic code.
– We believe:
– that the genetic code was established in two different stages. Our data does not shed much light on the early code, consisting of prebiotically available amino acids.
– once some primitive translational mechanism had been established, new amino acids were added to the mix and started infiltrating the genetic code based on specific amino acid/anticodon interactions.
B. This comment is NOT re the genetic mechanism of specifying amino acids compositions of proteins. It is only re the origin and scenario of life’s genesis.
I have been proposing:
* Life’s genesis was not cell(s), but the self reproduction of yet uncelled ungenomed gene(s).
* There was NOT any “Pre-History Of Life” evolving in an archaic pre-modern life cell.
* Cells were definitely NOT life’s genesis. Cells were products of evolution of Earth’s primal organisms, of Earth’s first stratum organisms, the RNA genes that have always been and still are running the show of life, the energy-storing biosphere survival, since Earth life’s day one.
* A gene’s self reproduction was distinctly an evolutionary, enhanced energy constraint event, above the earlier, random, radiated-energy-induced genes formations.
* Every evolutionary step is inherently an event of an enhanced energy constraint.
* Genomes, RNA and DNA, are functional organs evolved by the primary RNA genes. Cell membranes are also functional organs evolved by the primary RNA gene.
* Life is but one of the many many mass formats in the universe, and its evolution is driven as the evolution of all cosmic mass formats, to gain temporary enhanced energy constraint, i.e. to survive as long as possible.
Dov Henis
(Comments From The 22nd Century)
03.2010 Updated Life Manifest
http://www.the-scientist.com/community/posts/list/54.page#5065
Cosmic Evolution Simplified
http://www.the-scientist.com/community/posts/list/240/122.page#4427
“Gravity Is The Monotheism Of The Cosmos”
http://www.the-scientist.com/community/posts/list/260/122.page#4887
It’s an incredible idea, and it really does make sense in explaining why, in a world where rising entropy seems to go against the notion of complex life, we have evermore complex life forms evolving.
I read this post when it was first posted but returned again today to re-read it. It’s still an amazingly simple solution to what might seem an incredible complex problem 🙂