Williams, G. R. 1996. The Molecular Biology of Gaia. Columbia University Press. 210 pp.
This is a book I wish I had read when it was first published. Williams lays out so many interesting scientific problems so clearly that I would have expected that it would have considerable influence on subsequent research, somewhat as Schrodinger’s What is Life? the subject of the first post in this series. I was somewhat surprised that Google Scholar only finds a few citations of this book. Perhaps William’s scholarly papers have been more extensively cited.
William’s goal is to see why the famous Gaia hypothesis has attracted so much popular interest, while receiving little positive notice from practicing biologists. He wants to determine whether the hypothesis is actually useful, either as a metaphor or a verifiable model of the function of the biosphere. The central question is whether it can explain why the Earth has remained habitable throughout the several billion-year history of the biosphere. That it has is not in question: all evidence points to the occupation of Earth continuously by the descendants of the first living things, which originated 3.5 billion years ago. This strongly implies that the earth has not frozen or boiled and that life has not otherwise been poisoned or starved during that time. Some factor or factors has kept the conditions on at least some of the Earth within the ranges essential to living organisms of some kind. In fact the conditions have not become intolerable to land plants and metazoans at least for hundreds of millions of years. The concept of the continuity of descent, expressed beautifully by Loren Eisley’s image of each of us trailing a long chain of ghostly ancestors, stretching back to those first living things, is to me one of the most useful ways to imagine what evolution is all about. If there had ever been a break in that chain, you and I would simply not exist.
The Gaia hypothesis states that this stability is the result of homeostasis: the regulation by negative feedback (like a thermostat) of a living super organism, Gaia. In its strongest form, the hypothesis is that life on the planet, the biosphere, regulates itself just as a single organism, whether a single cell or a multicellular individual, does. This idea has an obvious appeal: just as networks of interacting macromolecules make up a cell, which is capable of regulating its internal environment, so do networks of interacting cells make up tissues, organs and whole organisms that are able to regulate their internal environment. At least some organisms, like ants and bees, live in self-regulating colonies. Why shouldn’t all the organisms on earth form a self-regulating system?
Williams answers that for biologists the problem is how such a self-regulated super organism could be put together in the first place. Natural selection can explain how self-replicating systems can evolve, because natural laws can discriminate among multiple variant copies that compete for limited resources. The Earth is not self-replicating. There are no variants among which nature can select. There is only one. This problem led Lynn Margulis to argue that Darwinian evolution was not really that important, and that symbiogenesis was the true explanation. Margulis’s great contribution was the discovery that certain cellular organelles, chloroplasts and mitochondria, were once free-living organisms. More broadly, she showed that evolutionary advances by the incorporation and integration of separate living parts were behind the origin of the eukaryotes and that similar processes continue to operate in the form of horizontal gene transfer. The trouble with claiming that symbiogenesis is a replacement for Darwinian natural selection is that it appears obvious that all such new combinations remain subject to survival of the fittest.
Would it be possible for a Gaia-like system to arise in part of the biosphere and then spread, supplanting the less effective parts? Only if it’s self-regulating effects were confined to where it first existed, as might work for something like the terrestrial nitrogen cycle. It seems less likely where the atmosphere and oceans are involved, since they carry the products all over the planet.
Williams also points out that there is more than one possible explanation for the continuous suitability of the Earth for living things. He lists four: luck, inertia, equilibrium, and homeostasis. He analyzes each possibility in turn, and shows how each may contribute to the persistence of habitable conditions. In the case of homeostasis, he distinguishes between negative feedbacks from purely physical and chemical forces involving the lithosphere, atmosphere and hydrosphere and ones that require the biosphere. It is possible that even if there were no life on Earth, the temperature would stay within habitable limits (basically the range where liquid water can exist) just because of feedback among the temperature and the release and sequestration of carbon from air, ocean and rocks.
According to Williams, if you try to assess this possibility, the difficulty is that today the rates of almost all steps in this process, except volcanism, are under catalysis by organisms. We don’t know what an abiotic planet would be like. As of the time he wrote this book, not enough was known about the global chemical cycles at the molecular level to settle the question how much life matters. He gives an example of what was known about the molecular biology of nitrogen to show how complex the regulation of these cycles is likely to be. Nutrients move among four pools: inorganic forms in the lithosphere, hydrosphere and atmosphere; nutrients in forms available for uptake by organisms in the same three spheres and the biosphere itself as accumulated by organisms; nutrients incorporated into living cells and tissues; and bio products, from the cellulose of wood in trees to dead plants and animals to dissolved organic compounds to fossil fuels. All these are connected by flows and many of those flows (mobilization, assimilation, regeneration, sequestration and excretion) are controlled by living organisms, via enzyme-catalyzed, energy-requiring reactions.
I like this book because Williams thinks about Earth and ecology very much as I do. I learned from my professors at Cornell in the early 1970s about five processes of ecology: population dynamics, natural selection, energy flow, nutrient cycling and cultural evolution. These are closely interrelated ways of looking at the overall phenomenon of life on earth, or as I like to define ecology, the structure and function of the biosphere. Is the function of the biosphere to regulate the habitability of the planet, or does the planet have the property of remaining a stable habitat for life without life being involved? You can’t really answer that question with only one habitable planet and one biosphere to study.
I will add that I tried to read another account of the same problem of why the Gaia hypothesis had been largely criticized by biologists while being so well received by non-biologists: The Gaia Hypothesis: Science on a Pagan Planet by Michael Ruse (University of Chicago Press, 2013) I did not find it helpful, being mostly a historical narrative, with a focus on a wide variety of –isms, such as Platonism, Mechanism, Organicism, Hylozoism (the belief that all matter possesses life) and Paganism. I have never been much interested in –isms or cultural explanations for why people accept of don’t accept given ideas. Williams gives us a scientific way of thinking about the problem.