The Equations of Life

by Charles S. Cockell

  This fascination with physical principles is not bland reductionism. In the finale to his seminal book on convergent evolution, Simon Conway-Morris laments the “dreary reductionism” of those who would incessantly try to simplify the complex structure of biology to genetic determinism.

  Reductionism need not be dreary. There is beauty in physical simplicity. Is there not a stunning elegance and maybe even charm in physical equations manifested in the living form? An equation as unemotive and fundamental as P = F/A reflected in the twitching nose and hurried digging of a mole, or the cold, hard rigor of buoyancy, B = ρVg, alive in the slender shapes and darting dances of a fish?

  Around us is a biosphere of limitless and wonderful detail, but in its forms most simple. We see not a dreadful menagerie of three-and five-legged beasts, grotesquely fashioned in irregular shapes, a collection of creatures whose outlines bewilder and appall, a ghastly farrago of unbridled evolutionary contingency and experimentation. This is a biosphere of symmetry, of predictable scales and pleasing ratios, a pattern in form and construction that runs deep from the very core of biochemical architecture to families of ants and birds. It is the immutable and unbreakable marriage of physics and life.

  ACKNOWLEDGMENTS

  I HAVE BEEN VERY lucky over the years to move across a number of scientific disciplines without let or hindrance and to have observed some of the ferment occurring between them. I owe a debt of thanks to many institutions and people who during these years provided the impetus for exploring the ideas in this book. My undergraduate degree in biochemistry and molecular biology at the University of Bristol in the late 1980s exposed me to ideas on the underlying structure of life at a time when these sciences were getting into their stride, exploring their full reach with the new tools of molecular methods. A doctorate in molecular biophysics at the University of Oxford allowed me to see the newly emerging links between biology and physics, a synthesis of our understanding of living things and the principles of physics—two scientific directions that have tended to remain separate until only recently. The work going on at this interface provides a particularly thrilling vista on the basic principles that guide the assembly of life.

  My transition into microbiology happened during my postdoctoral position at the NASA Ames Research Center in California, and it gave me a view of the microscopic world of life. I was especially fortunate to witness the birth of astrobiology when this science was taking on a new vigor in the form of the NASA Astrobiology Institute. As a great enthusiast of human space exploration and settlement, I could indulge my fascination with biology and space sciences at the same time. At Ames, I was influenced by many fine people who provided me with the opportunity to see the astronomical perspective on life, which one might say underpins the question of whether the structure of terrestrial biology can be described as universal, a question I pick up in these pages.

  In moving to the British Antarctic Survey in Cambridge as a microbiologist to work among scientists whose focus was anything from penguins to seals, I developed a more ecological and evolutionary perspective on life. It was during afternoon walks among the nests of the South Polar skuas in the hills above Rothera Research Station on Adelaide Island, Antarctica, in what felt like an alien continent, when I began to wonder about the principles that shape the similarity between birds. The otherworldly backdrop of the “White Continent” provided the perfect environment to contemplate the extent to which all organisms are constrained into narrowly circumscribed forms, regardless of the place they call their home. I suppose this book is a development and a more in-depth reflection of some of these polar thoughts. Then, in moving to the Open University in Milton Keynes to work in a planetary sciences institute, I learned much about planetary-scale processes and their link with life. It is in this context that one is forced to think about how planetary conditions shape and channel the products of evolution. Now I am among physicists and astronomers at the University of Edinburgh, where a more reductionist view on life is a powerful undercurrent. All these places have been a pleasure to work in and a rich source of ideas. It has been immensely enjoyable to write a book that draws on research going on at the various levels of life’s architecture that I have been privileged to observe firsthand.

  In whatever way this book is received, I hope that minimally it might encourage a few additional evolutionary biologists and physicists to share a common interest in the remarkable products of evolution and inspire everyone else to look in awe at the beautiful simplicity of something that often seems so hopelessly complex.

  I thank my research group at the time this book was written (Rosie Cane, Andy Dickinson, Hanna Landenmark, Claire Loudon, Tasha Nicholson, Sam Payler, Liam Perera, Petra Schwendner, Adam Stevens, and Jenn Wadsworth) for their forbearance. I also thank countless friends and colleagues who emailed suggestions or papers that in one way or another have influenced the thoughts expressed here. Thank you, Harriet Jones, Hanna Landenmark, Sydney Leach, and Rebecca Siddall, all of whom provided detailed comments on the manuscript. I am grateful to the University of Edinburgh, an excellent intellectual home for the last five years.

  I am very grateful to my agent, Antony Topping, for his advice and guidance in developing this project. I thank T. J. Kelleher at Basic Books and Mike Harpley at Atlantic Books for their editorial suggestions and for guiding this book to its final publication.

  Charles Cockell is Professor of astrobiology at the University of Edinburgh and director of the UK Center for Astrobiology. He received his BSc in biochemistry and molecular biology from the University of Bristol and his doctorate in molecular biophysics from the University of Oxford. His interests encompass life in extreme environments, the habitability of extraterrestrial environments, and the exploration and settlement of space. He held a National Academy of Sciences associateship at NASA and then worked at the British Antarctic Survey and the Open University. He has published over three hundred scientific papers and numerous books, including a series on the conditions for liberty beyond Earth. His teaching has ranged from undergraduate education to Life Beyond, a program he established in prisons to engage prisoners in designs of space settlements. He received the Chancellor’s Award for Teaching, the highest award for teaching at the University of Edinburgh. He is Chair of the Earth and Space Foundation, a nonprofit organization he established in 1994; the foundation supports expeditions that link space exploration with environmentalism.

  Photograph courtesy of Johnny Watson.

  ALSO BY CHARLES S. COCKELL:

  Astrobiology: Understanding Life in the Universe

  The Meaning of Liberty Beyond Earth

  Human Governance Beyond Earth: Implications for Freedom

  Dissent, Revolution and Liberty Beyond Earth

  Extra-Terrestrial Liberty: An Enquiry into the Nature and Causes of Tyrannical Government Beyond the Earth

  Space on Earth: Saving Our World by Seeking Others

  Impossible Extinction: Natural Catastrophes and the Supremacy of the Microbial World

  An Introduction to the Earth-Life System

  Biological Processes Associated with Impact Events

  Ecosystems, Evolution, and Ultraviolet Radiation

  NOTES

  CHAPTER 1

  I once heard a distinguished: This was an observation made by Martin Rees, Astronomer Royal, in a public lecture, but he also made a similar observation in print: “Even the smallest insect, with its intricate structure, is far more complex than either an atom or a star.” Rees M. (2012) The limits of science. New Statesman 141 (May), 35.

  Other helium atoms: Lequeux J. (2013) Birth, Evolution and Death of Stars. World Scientific, Paris.

  Like modern birds: Witton MP, Martill DM, Loveridge RF. (2010) Clipping the wings of giant pterosaurs: Comments on wingspan estimations and diversity. Acta Geoscientica Sinica 31 Supp. 1, 79–81.

  Scurrying among their short knobbly: Edwards D, Feehan J. (1980) Records of Cooksonia-type sporangia from late Wenlock strata in Ireland. Na
ture 287, 41–42; and Garwood RJ, Dunlop JA. (2010) Fossils explained: Trigonotarbids. Geology Today 26, 34–37. Indeed, the type specimens of many early plants and invertebrates were found first in Scotland.

  Return just a few: Pederpes: Clack JA. (2002) An early tetrapod from “Romer’s Gap.” Nature 418, 72–76.

  Rather, in observing: For folding of proteins, see Denton MJ, Marshall CJ, Legge M. (2002) The protein folds as Platonic forms: New support for the pre-Darwinian conception of evolution by natural law. Journal of Theoretical Biology 219, 325–342. Working out how proteins fold is not a simple matter, a point raised with great clarity in Lesk AM. (2000) The unreasonable effectiveness of mathematics in molecular biology. Mathematical Intelligencer 22, 28–37.

  Evolution is just: This simple observation is compatible with the important role of natural selection in shaping life, but also with many factors that shape organisms that are not linked directly to primary selective effects. The multifarious ways in which organisms are evolutionarily shaped, explored for example by Gould and Lewontin, are entirely compatible with those same mechanisms being narrowly circumscribed and limited by physical principles. See Gould SJ, Lewontin RC. (1979) The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proceedings of the Royal Society of London. Series B, Biological Sciences 205, 581–598. Some of those factors, particularly many “architectural” ones, are fundamentally physical constraints. For instance, Gould and Lewontin’s spandrels are the physical consequence of joining two arches.

  The limited number: For a remarkable discussion on the limitations and effectiveness of mathematics in describing physical processes, see Wigner E. (1960) The unreasonable effectiveness of mathematics in the natural sciences. Communications in Pure and Applied Mathematics 13, 1–14. See Lesk (2000), above, for a more modern take on this classic essay.

  That the laws of physics: A wonderful technical summary of some of the physical principles that are instantiated into life at the level of the whole organism is Vogel S. (1988) Life’s Devices: The Physical World of Animals and Plants. Princeton University Press, Princeton, NJ. Steven Vogel also wrote a range of interesting papers examining fluid mechanics in life and other observations. The bibliography of his book, although dated, contains several excellent papers about physical measurements in organisms. A more popular exposition (although replete with detail and beautiful comparisons with human technology) is to be found in Vogel S. (1999) Cats’ Paws and Catapults: Mechanical Worlds of Nature and People. Penguin Books, Ltd., London.

  At the molecular level: Autumn K et al. (2002) Evidence for van der Waals adhesion in gecko setae. Proceedings of the National Academy of Sciences 99, 12,252–12,256.

  The forces involved: Alberts B et al. (2002) Molecular Biology of the Cell (4th ed.). Garland Science, New York.

  Put simply, when water is frozen: Smith R. (2004) Conquering Chemistry (4th ed.) McGraw-Hill, Sydney.

  When clarifying how large: Not all fish flex their bodies. Electric fish depend on keeping their bodies rigid so that they can generate stable electric fields with which to sense the world. These fish have evolved a long, continuous fin along their body; the fin uses wavelike oscillations to drive the fish forward.

  And despite the inherent uncertainty: Schrödinger’s cat is a thought experiment in quantum mechanics elaborated by Erwin Schrödinger in 1935. The scenario involves a cat that may be simultaneously both alive and dead, made possible by a state known as a quantum superposition. It results from the cat’s life being linked to a random subatomic event that may or may not occur. Werner Karl Heisenberg was a German theoretical physicist and one of the pioneers of quantum mechanics.

  The idea of organisms: Or “fitness” landscapes. An elegant exposition of this concept, first developed by Sewall Wright, can be found in McGhee G. (2007) The Geometry of Evolution. Cambridge University Press, Cambridge.

  All these adaptations: I am not dismissing the role of developmental constraints in evolution. See, for example, Smith JM. et al. (1985) Developmental constraints and evolution. The Quarterly Review of Biology 60, 263–287; or Jacob F. (1977) Evolution and tinkering. Science 196, 1161–1166. Indeed, very complex interactions can exist between physiology and evolution. See Laland KN et al. (2011) Cause and effect in biology revisited: Is Mayr’s proximate-ultimate dichotomy still useful? Science 33, 1512–1516. However, as will become apparent throughout this book, life seems to have more flexibility to overcome these prior historical quirks and “fixed accidents” than is typically assumed, whether they be in the genetic code or in macroscopic forms of creatures. That is not to say that we cannot find plentiful evidence of history in animals—such as the four legs of land-dwelling animals derived from pectoral and pelvic fins of fishes. This history may restrict the options open to life within what are the dominant physical principles that shape it.

  For simplicity’s sake: The reader might claim a certain degree of tautology here. Whether evolution is a characteristic of life rather raises the question of how we define life. We can indulge in fantastical ideas of life forms that adapt to their environment and read these adaptations back into their genetic code in a Lamarckian form of evolution. If such a system were powerful enough, the Linnaean system of classification we associate with the hierarchical nature of the phylogeny of life on Earth would not emerge. However, along with Dawkins R. (1992) Universal biology. Nature 360, 25–26, I am going to start with an assumption that evolution in a Darwinian sense is universal in natural things that replicate with a code. Indeed, here I will simply take it as a working assumption of my book that systems of matter that reproduce and exhibit Darwinian evolution are the things that concern my discussion. Even if the reader refutes this universality and can describe a reproducing system that adapts to its environment quite differently, most of the conclusions I draw in this book, particularly regarding the restricting effects of physical processes, are likely to hold. A real example might be very early cells on Earth when life first arose in which genetic information may have passed more fluidly between them just as horizontal gene transfer occurs in microbes today such as discussed in Goldenfeld N, Biancalani T, Jafarpour F. (2017) Universal biology and the statistical mechanics of early life. Philosophical Transactions A 375, 20160341. Some people have argued that such a community of cells has non-Darwinian properties (in that genetic material is added into a primitive genome in a quasi-Lamarckian way). However we situate these ideas in a description of the evolutionary process (even the products of horizontal gene transfer are still subject to environmental selection), the processes are narrowly circumscribed by physics. Dawkins puts a compelling case that Darwinism is not merely part of the definition of life as we know it, but a universal characteristic of replicating things that have adaptive complexity: Dawkins R. (1983) Universal Darwinism. In Evolution from Molecules to Men, edited by DS Bendall, Cambridge University Press, Cambridge, 403–425. For Joyce quote, see Joyce GF. (1994) In Origins of Life: The Central Concepts, edited by DW Deamer and GR Fleischaker, Jones and Bartlett, Boston, xi–xii. Joyce points out informally on the internet that the definition was developed during panel meetings of NASA’s Exobiology Discipline Working Group in the early 1990s.

  We could argue that the word life: A problem explored by Cleland CE, Chyba CF. (2002) Defining “life.” Origins of Life and Evolution of Biospheres 32, 387–393.

  In his engaging 1944 book: Schrödinger E. (1944) What Is Life? Cambridge University Press, Cambridge.

  Mathematical models: Discussed by Baverstock K. (2013) Life as physics and chemistry: A system view of biology. Progress in Biophysics and Molecular Biology 111, 108–115.

  As early as 1894: Wells HG. (1894) Another basis for life. Saturday Review, 676.

  In 1986, Roy Gallant: Gallant R. (1986) Atlas of Our Universe. National Geographic Society, Washington DC.

  What we see on Venus: To emphasize the caveat I make in the main text, someone imaginative could argue that these planets just lac
ked an origin of life or that an origin of life is very rare. However, if life had originated on these planets, we would indeed see these very creatures. It is difficult, in the absence of any probabilities on the origin of life or a certainty about the conditions required for it, to argue against this position. However, as I will discuss in later chapters about the limits to life, there are more-fundamental limits to the possibility of life in hell-like worlds such as Venus, regardless of whether an origin of life could have (or even did) occur there.

  It is apposite: Darwin C. (1859) On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. John Murray, London.

  CHAPTER 2

  Ant civilization: Wilson EO. (1975) Sociobiology: The New Synthesis. Belknap Press, Cambridge, MA. The book Wilson wrote in collaboration with Hölldobler on ants was the first academic work to win the Pulitzer Prize: Hölldobler B, Wilson EO. (1998) The Ants. Springer, Berlin.

  Quickly, we have: A more macabre demonstration of feedback processes in ant societies has been shown in how ants make piles of ant corpses: Theraulaz G et al. (2002) Spatial patterns in ant colonies. Proceedings of the National Academy of Sciences 99, 9645–9649.

  Remarkably, no architect: I have focused on the rules that drive certain collective behaviors in ants. Another question entirely is why ants live together in the first place and how eusociality (the tendency that some groups of animals have to be split into reproductive and nonreproductive groups, the latter merely tending for everyone else) could have arisen in the raw competitive world of evolution. This question can itself be reduced to plausible physical principles and mathematical modeling and is discussed in Nowak MA, Tarnita CE, Wilson EO. (2010) The evolution of eusociality. Nature 466, 1057–1062.