Chemical Economics of Life

One challenge for the theory of evolution: it needs to encompass both creativity and predictivity. Creativity, because new species come into being, never having been observed before. Predictivity, because underlying causal mechanisms come into play as organisms with certain traits are selected for instead of their less adaptable relatives. In addition, biological arguments often invoke the notion of ‘function’ – which is an overlying rather than an underlying cause hierarchically. Biochemical arguments, excising macroscopic ecology from the picture, attempt to explain the function in terms of ‘structure’ to emphasize underlying causes.

 

In his treatise, Jeffrey Wicken incorporates thermodynamics and information to broaden the theory of evolution beyond the neo-Darwinian. Chapter 12 of his book is titled “The Economics of Selection”. How do organisms grow and reproduce? By funneling energy through themselves and degrading it to build new structures. Why do they so? Well, to grow and reproduce. Note the chicken-and-egg argument I just made with the how and why questions, which has led to the idea of biological relativity – no particular level within the hierarchy can be easily singled out as the primary cause.

 

The economic currency here is energy. In our neighborhood of the solar system, an energy gradient exists between the sun (shooting out photons) and the ‘coldness’ of the vacuum of space. In between sit the planets, full of matter and some (okay, one) with the potential for supporting living organisms. Translating the energy gradient in chemical terms, Wicken writes: “As energy penetration increased the free energy of the emerging biosphere, it built into it proximate free energy gradients that could be exploitatively tapped. These involve both sources with high-energy electrons and oxidative sinks for accepting those electrons. The fundamental economic boundaries of a system are set by its source-sink relationships and the kinetic mechanisms at its disposal for exploiting them.”

 

My chemistry students will hopefully recognize that some sort of redox reaction must be involved. Electrons are moving from a source (the reducing agent) to the sink (the oxidizing agent). Hydrogen is the prototypical reducing agent. Oxygen is the prototypical oxidizing agent. Using handwaving (or bookkeeping) arguments involving ‘oxidation numbers’, students learn in introductory chemistry that hydrogen transfers its electron to oxygen and H₂O is formed from elemental hydrogen and oxygen. The formation of water is energetically very favorable – there’s a gradient from the higher (potential) energy reactants, H₂ and O₂, to the lower (potential) energy product H₂O. More subtly, and I emphasize this point in my upper division P-Chem thermodynamics class, O₂ is the “high-energy” species from the perspective of the relative ‘strengths’ of chemical bonds. And why is carbon central to life? I’d say it’s along for the ride thanks to its ability to expand chemical space to create new structures. This notion of expanding chemical space will be important to Wicken’s thermodynamic argument.

 

While our present atmosphere has lots of free O₂, and heterotrophic organisms (we can’t make our own food, we have to eat it!) utilize O₂ for metabolism, this was not so at life’s origin on our planet. Oxygen was locked into both minerals (many ores are metal oxides) and carbon (as carbon dioxide in the gas phase, bicarbonate in aqueous solution, and carbonate both dissolved and in ionic solids). The reason why we have lots of O₂ today is because, at some point in the distant past, photosynthesis was ‘invented’ thereby allowing autotrophic organisms (that make their own food) to directly utilize the sun’s high-energy photons to reduce CO₂ essentially by adding hydrogen to it. That’s why living organisms are full of compounds containing carbon, hydrogen, and oxygen. Carbohydrates and lipids are in this category. (Proteins and nucleic acids also contain some nitrogen, sulfur and phosphorus.)

 

As pathways to utilize energy emerge, there will be competition. This is where thermoeconomics comes in. Wicken writes: “Competition among thermodynamic flow patterns expresses itself in ecosystem dynamics by imposing a condition of selection on community structures that in turn imposes selective conditions on adaptive strategies of populations. No community organization is uniquely able to process the energy made available by a given abiotic matrix. Any community is but one of many alternative solutions to the business of energy processing, and is therefore in competition with these alternative solutions.”

 

And how do we expect evolution to proceed in this case? Here’s what Wicken has to say: “Since the biosphere is a closed system, and since irreversible flows of energy through closed systems require cyclic movements of matter… ultimately all [autocatalytic pathways] must contribute to the cyclic movement of matter through the biosphere. As open systems, organisms fulfill their thermodynamic destinies by fitting into higher-order systems that express greater degrees of closure. The ecosystem, with its community structure for processing energy, is the first level of cyclic closure – cycling scarce nutrients and unidirectionally processing others, such as carbon and oxygen, which are in greater supply. Then come systems of ecosystems, in which the outflow of one is utilized as input by another. The biosphere as a whole is the ultimate unit of cyclic closure.”

 

This makes sense to me as a physical chemist. Using the language of thermodynamics allows us to deal with macroscopic-level variables without being bogged down by microscopic causal explanations. We just need to follow the energy trail! Wicken’s distinguishing the fates of C and O (and implicitly H, the most abundant element in the universe) from N, S, P, and trace metals is, I find, a refreshing way to look at the interplay of the different elements in chemistry. That’s why we have a nitrogen cycle! Cycles are fundamental to life.

 

But there’s more. Wicken writes: “Yet ecosystems are quite vulnerable thermodynamically to invasive dissipative modes – whether nurtured within the relational structure of the community or by endogenously generated parasites.” There’s a reason why life is dynamic: the second law of thermodynamics. Entropy must continue to increase in the biosphere (approximated as a closed thermodynamic system) if it is not stuck in chemical equilibrium ‘death’. This thermodynamic vulnerability of an ecosystem is a feature, not a bug, in the evolution of life. I expect this principle of vulnerability to be dominant at life’s origins, before the establishment of complex control mechanisms in organisms to stay alive.

 

As an ecosystem matures, the flows change. Wicken argues: “That mature ecosystems have lower specific dissipations [i.e., lower specific entropy production] than immature ones (higher biomass/throughput ratios) reflects the selective premium on using resources efficiently. That evolution occurs under conditions of limited resources means that it occurs under economic boundaries with limiting kinetic means of degrading energy.” What this means is that in the early immature stage, there tend to be multiple kinetically accessible pathways to dissipate energy. New pathways (the “invasive modes”) continue to emerge, essentially to expand the energy economy. But gobbling up resources leads to shortages, leading to the selection of more efficient routes – and the closing down of some pathways. This interplay leads to specialization and mutualism. Wicken writes: “In ecosystem development, founder populations of energy-intensive generalists give way to more complexly organized communities of interacting specialist populations that get more biomass for their energy dollar.” This also applies to a complex multicellular organism such as humans! Wicken argues that our natural life span corresponds to the decreasing specific entropy production from early stages of the zygote to eventual death of the organism.

 

Wicken subdivides this into three stages: (1) “In early development, an organism’s high dissipative rate and low efficiency provide thermodynamic force for the generation of organization according to its coded information.” (2) “Maturation is accompanied by decreasing specific dissipation as body size peaks and organizational networks reach their full, mutualistic complexification.” (3) “The decline of the dissipation during the aging process expresses a gradual yielding of homeostatic function… systematic losses of the behavioral and metabolic plasticity that allow organisms to accommodate environmental stresses.”

 

But why do organisms eventually lose homeostatic ability? Why can’t we stay immortal keeping ourselves at steady state and away from equilibrium? Why is death built into life? Wicken writes: “The wearout of energy-processing pathways inheres to organisms by virtue of their particular condition of closure, which is to information rather than energy.” We’ll need to unpack this statement. First, the environment surrounding an organism is ever-changing. To maintain itself, an organism needs to buffer itself against many of these changes. An energy-inefficient generalist (with lower information content) has a variety of mechanisms it can employ to respond to external perturbations. An energy-efficient specialist within a mutualist community (with higher information content) has fewer response mechanisms. Don’t forget that specializing and mutualism is to some extent inevitable because of the second law. Wicken writes: “Since success in the competition for energy flows is predicated on current adaptive payoff rather than the future stabilization of community structures, the survival-reproductive fruits of minimizing metabolic and behavioral burdens make the trend toward increasingly specialization inexorable… This essential conflict between adaptive commitment and environmental change mandates eventual extinction for most species.”

 

So on the one hand, thermodynamics drives the opening up of chemical space and encourages a diversity of structures to form. The more different kinds or types, the higher the entropy produced. And entropy must be produced for anything to proceed. Otherwise you’re stuck in chemical equilibrium – the stasis of death (or I suppose an unchanging immortality). You’re down in the (potential energy) well and there’s nowhere to go. But as you open up the chemical space and increase in complexity, specialization and mutualism kick in for increased efficiency in a climate of ever-changing resources. But specialization is also an eventual death-knell in a dynamic environment. All those command-and-control systems that emerged allowing an organism to grow (and possibly reproduce) become limiting because they are selected here-and-now and not for the future. We’re all short-termers at heart. To build robustness for the long-term requires an efficiency trade-off, and the possibility of losing the fight for resources in the here-and-now. A Catch-22.

 

Why are we in this situation? Here’s Wicken’s overaching view: “The driving force [for this evolutionary trend, in both the organic and socioeconomic realms, has been the fact that energy resources have existed in excess of their mechanisms of utilization. Evolution has been more kinetically than energetically limited. Prior to the emergence of technology, this kinetic limitation was guaranteed. Relatively small fractions of influx in solar radiation are autotrophically fixed. Of that fraction, not all can be heterotrophically utilized – the residue sinking into fossil fuel. Socioeconomic evolution has powerfully accentuated the trend toward the invention of kinetic mechanisms to create new patterns of dissipation. Econodynamics is inescapably evolutionary in this way. A caveat is that these dynamics have promoted the evolution of more highly dissipative economies by exploiting the fossil fuels that resulted from eons of surplus production of autotrophic fixation. As future socioeconomic evolution becomes more energy-limited, selection for efficiencies will inevitably reverse the historical relationship between organizational complexity and dissipation.” Humans – we’ve really changed things on Planet Earth!

 

I close this long meandering blog with closing thoughts by Wicken on the relationship between parts and the whole: “The whole is not exactly more than the sum of its parts, since parts are relationally constituted by the wholes in which they evolved… The organic world consists of just such mutually constituting part-whole relationships between individuals and higher-order flow patterns. The success of any whole is predicated on the coordination of its parts; conversely, adaptive strategies of parts are conditioned by the requirement that they participate in higher-order webs of processing energy. The information content of an ecosystem expresses this hierarchically codefining relationship between part and whole.”