WHAT is the purpose of life? Is it just finding food and having sex – survival and reproduction? Maybe not. According to Eric Schneider and James Kay, life is driven by an urge at least as strong as the desire to survive. Life, they say, is a baroque contraption for tearing up energy. It is simply an extreme version of a universal natural tendency to turn concentrated energy into diffuse waste heat.
If Kay and Schneider are right, their idea might explain why ecosystems and organisms are so complex and diverse, and perhaps even why life exists at all. And the two scientists say they now have evidence to back up their claims.
All this follows from one of the most powerful rules in nature, the second law of thermodynamics. The second law encapsulates the irreversibility of nature, the fact that many things can’t happen backwards. For example, heat won’t spontaneously flow from a cool body to a hotter one, only the other way. Open the windows of a warm house on a cold day and the air inside isn’t going to get warmer.
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The original second law doesn’t actually say the house has to cool down after opening the windows, says Kay. But in the mid-1960s, physicist Joseph Kestin of Brown University and MIT engineers George Hatsopoulos and Joseph Keenan showed that closed systems will indeed attain equilibrium when constraints – such as windows, or cell membranes – are taken away. Kestin showed that the equilibrium condition is an “attractor”. No matter how the system starts off, it will always attempt to settle down to that state.
At first sight, this might seem to make life less likely. Life is manifestly out of equilibrium, our molecules awash with fluctuating flows of heat and chemicals. But life isn’t a closed system, it’s sitting in a Universe that is already way out of equilibrium.
In the early 1980s at the University of Waterloo in Ontario, systems engineer James Kay was wondering about this problem, looking for a way to incorporate thermodynamics into a general theory of biological systems. His thinking paralleled that of a long line of physics-inclined biologists, notably Eric Schneider, a former director of the US Environmental Protection Agency’s National Marine Water Quality Laboratory. Decades ago, while observing plants and marine ecosystems, Schneider kept noticing the importance of thermodynamics. “I saw the second law all around.”
Attractive proposition
If the state of equilibrium is an attractor, thought Kay and Schneider, that means that as you push a system out of equilibrium, it will push back. And the harder you push, the stronger the system pushes back.
Tearing up the orthodox view that increasing complexity is an aberration, Schneider and Kay believe that nature will actually create complex systems whenever it can: it will use whatever mechanisms it has at its disposal to move towards equilibrium. They’re not alone in this. “The idea that systems use everything at their disposal to approach equilibrium is pretty intuitive,” says John Collier of the Konrad Lorenz Institute for Evolution and Cognition Research in Altenberg, Austria.
Here’s a simple example. Heat a pan with a very thin layer of oil on the bottom. This sets up a temperature gradient between the top and the bottom of the oil – a sign that it is out of equilibrium. At first, the oil just conducts heat placidly from bottom to top. But if you heat it more strongly, making a steeper gradient, structure emerges. The oil forms small but visible hexagon-shaped flows called Bénard cells. These transfer heat much more rapidly, resisting the attempt to move further from equilibrium.
In many other situations, nature shows this tendency to form more complex set-ups when pushed further from equilibrium. If conditions permit, a tornado or a cyclical chemical reaction will form. Perhaps even life? If so, we are just agents of uniformity, working to bring the Universe into equilibrium as rapidly and thoroughly as possible.
This seems like a bold claim. Life is a precise assemblage of proteins, sugars and other molecules. These make cells, then tissues and organs, whole organisms, and finally sprawling ecosystems. Could all this complexity be driven by a simple physical law? In the realm of ecology, at least, there’s evidence that it is.
To understand why, consider what it means to be out of equilibrium. To describe the “quality” of energy in a system, the portion of some lump of energy that you can extract as useful workis called exergy. At equilibrium, you can’t extract any work, so the exergy is zero. But if, say, you have a container of gas at high temperature and pressure relative to its environment, it has some exergy. If you then let the gas out, you can do work, such as spin a turbine, while at the same time smearing the energy out over space and time. After the gas is released it can’t do as much work as before, so you’ve reduced its exergy.
Schneider and Kay’s idea then is that organisms and ecosystems will tend to evolve into more intricate machines for wiping out exergy. Is there any way of proving this?
Life runs on light from the Sun, which has a lot of exergy. Because the star is so hot, each solar photon has a high energy, and high energy per photon equates to high exergy. Once you know how much exergy is present in solar energy, you can begin to analyse the energy flowing through biological systems.
When sunlight gets absorbed and turned into other forms of energy, that exergy is reduced. What Kay and Schneider say is that life does this more thoroughly than would happen otherwise. For example, plants spread out the solar energy they absorb by allowing water to evaporate from tiny pores in their leaves. The lukewarm water vapour and the cool plant it leaves behind emit only low-grade infrared, so ultimately the radiation coming off the Earth is less capable of doing work than if it had just bounced off bare rock.
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Animals play their part by further degrading the chemical exergy stored in plants, and doing it better than, say, burning the plant instead. A fire radiates relatively high-energy photons, which still carry quite a lot of exergy—less than sunlight, but more than the infrared coming off a ruminating wildebeest or a sated lion.
Schneider, who now works from his Montana ranch, says ecosystems that are more mature or healthier will capture and dissipate more of the Sun’s rays than younger or more decrepit ones. The measurements seem to back him up. Jeffrey Luvall and his colleagues at NASA’s Global Hydrology and Climate Center in Huntsville, Alabama, have been taking satellite measurements of radiation from ecosystems for more than a decade. Their results suggest that ecosystems emit cooler radiation the more mature they are. It turns out that a 400-year-old Douglas fir forest is cooler than a younger forest, which is cooler than a clear-cut forest. And there’s a gap of 25 °C between the mature ecosystem and a rock quarry.
Another of Kay and Schneider’s predictions is confirmed by experiments in a more controlled environment. If plants are driven by the urge to consume exergy, they should do it best, and therefore be coolest, under the conditions they are used to. In 1998, Timothy Allen, Tanya Havlicek and John Norman at the University of Wisconsin in Madison grew soybean plants in a wind tunnel. Plants grown in high wind were cooler in high wind; plants grown in low wind were cooler in low wind.
At the moment, the only good way to assess an ecosystem’s diversity is to go in there and count the different species. But if diversity turns out to be reflected by temperature, then an aeroplane or satellite fly-by could provide a cheap way to assess long-term ecological changes and find out whether a system’s resource base is wasting away.
Another application could be farming. In one of Luvall’s studies, conducted with researchers at Auburn University in Alabama, the distribution of cooler zones across a farm correlated well with higher yield. The farmer, Don Glenn of Hillsboro, Alabama, says that if he’d had a baseline for comparison, he might have used the information to sell crop futures in advance of the harvesting season, before prices fell. Another experiment by Kay and a group at the University of Guelph in Canada showed that crop plants got cooler as they were given more fertiliser, but that this cooling eventually levelled off. This may mean that farmers could use infrared data to judge how much fertiliser to apply for maximum yields.
Far more controversial is the claim that exergy drives evolution. “Evolution does have a direction,” says Schneider. “Capturing and degrading energy is selected for.” They’re not saying that exergy is the only driving force. “There’s a bunch of things that organisms have to do that aren’t about thermodynamics,” Kay emphasises. But whenever faced with two options equally favourable to survival, life will choose the one that eats more exergy. “This is one part of the story of evolution. It’s turning out that it’s a really important part,” says Kay, who originally thought the temperature difference between ecosystems would be swamped by other factors.
In the 1970s and 1980s, the Russian physiologist Alexander Zotin, then at the Soviet Academy of Sciences’ Institute of Evolutionary Biology, compared the metabolism of a wide range of organisms. He found that crustaceans and molluscs, part of a group which appeared earlier in the geological record, had metabolisms four times slower than birds and mammals, which appeared hundreds of millions of years later. Schneider interprets this to mean that as evolution progressed, it “invented” newer and better ways to degrade energy.
But looking at modern proxies for ancient organisms isn’t good enough, says Daniel McShea of Duke University in North Carolina, who studies macroevolutionary trends and complexity. Modern crustaceans may be part of an old group, but that doesn’t mean they’re identical to their forebears.What you really need is some fossil property that correlated with metabolism, perhaps complexity of form. If such a connection could be found, the trend could be tested more directly and more questions could be posed. Is the improvement steady or intermittent? What ecological and environmental factors affect it? How is it affected by mass extinctions? But people working on the thermodynamics of life don’t often seem inclined to ask these kinds of risky empirical questions, says McShea.
The strongest criticism comes from Darwinists, who say that there’s simply no need for Kay and Schneider’s new “life force”. Natural selection can already explain the increasing diversity of life.
Peter Corning of the Institute for the Study of Complex Systems in Palo Alto, California, thinks that energy flows cannot determine the direction of evolution, or the design of complex systems, because the struggle for survival is too multifaceted and sensitive to details. The second law describes the way energy is wasted, when what’s important is how energy is captured and utilised. “Horse manure does not explain a horse,” as Corning’s late Stanford University collaborator Steven Kline liked to say.
Corning thinks that the pay offs from improving energy capture and use are the key factor, because natural selection has favoured these improvements. So thermodynamics brings us back to Darwin, not to an extra-Darwinian tendency to degrade energy.
Beyond Darwin
Schneider and Kay don’t dispute that natural selection is important, but say that ecosystems in particular show the need for new, general laws to account for the fact that they capture more energy as they mature, make better use of it and recycle it more. Darwinism has a tough time with ecosystems because they can’t be selected for in the sense that organism can. Kay points out that ecosystems are very bad at turning solar energy into biomass, which seems strange if the goal of life is to reproduce itself, but would be expected if life is trying to use up energy by evaporating water.
If we are really in thrall to the second law, could even the origin of life have been legislated by thermodynamics? “I would expect biology to emerge, given the reformulated second law,” says Kay. Indeed, Schneider sees life as a Bénard cell that learned how to reproduce itself. It’s nowhere near as simple as heating a pan of oil, of course. Schneider imagines pools of self-catalysing chemical reactions going on, perhaps near hydrothermal vents in the ocean. These pools started out simple, then new structures and processes developed to degrade more exergy. At first, these chemical structures would fall apart whenever the gradients went away for a moment, and the whole process would have to start from scratch. But then some self-replicating system, perhaps DNA or RNA, appeared to preserve the knowledge of how to eat exergy. The reactions could then build up complexity and cycle on forever.
Some responses to Kay and Schneider’s work are more enthusiastic than Corning’s and McShea’s. According to ecologist James Brown of the University of New Mexico, they are asking really important questions about the physical underpinnings of life at a time when looking for general principles of biology is frowned upon. Rod Swenson at the University of Connecticut’s Center for the Ecological Study of Perception and Action agrees that life is responding to a thermodynamic imperative, but says that the same or better ideas have been out there for a while. He thinks life works to degrade heat energy – generating the maximum possible entropy. Kay disputes this on the grounds that heat isn’t the only possible source of energy for life. Exergy, on the other hand, includes all sources of available energy.
So there is still a lot of convincing to be done. The next stage of experiments will have to be more rigorous, to work out the total energy budgets for living systems – everything from cells to global ecosystems. Finding the money for such research is a challenge. Luvall, Kay and Schneider all did their work on shoestring budgets, and Norman and Allen have tried repeatedly to get even a small grant to test further how plants’ temperature-regulating abilities might evolve over time.
Allen might have an even harder time getting a grant to test his wildest idea. He has speculated that exergy is fundamental to human civilisation too. Our history could be seen as a series of leaps towards better ways of extracting energy: first wood, then coal and oil, and on to nuclear power. The marketplace and the slaughterhouse are effectively the Bénard cells of farming and hunting: more efficient ways of degrading useful energy. Now, if we just had some rocket ships, we could take life’s energy crusade to the stars.