The Equations of Life How Physics Shapes Evolution

Starting your book blurb by asking why gazelles have legs rather than wheels is a suitably out-there question to immediately grab a reader’s attention. A more pertinent question then; why is all life based on carbon rather than silicon? In The Equations of Life, Charles Cockell takes the reader on a giddy tour down the organisational hierarchy of life – from sociobiology to subatomic particles – to show that nature is far more predictable and understandable than it might appear at first blush. His eloquent answer to above and other why questions? “Because physics is life’s silent commander”.

That physics imposes hard limits on life is not necessarily a novel idea. D’Arcy Wentworth Thompson’s 1917 book On Growth and Form already put forth the idea that morphology and growth are dictated by physical laws and can be described mathematically. And Vogel’s 1989 book Life’s Devices: The Physical World of Animals and Plants is a great, if slightly dated, introduction to biomechanics. Cockell starts off at the level of animal groups such as insect communities and flocks of birds to show how simple mathematical rules can lead to the emergence of complex behaviours. And, like above books, he discusses how the form of organisms is dictated by physics, touching on some of the power laws and network-like properties that George West discussed at length in Scale: The Universal Laws of Life and Death in Organisms, Cities and Companies. There are plenty of equations, some simple, some complex, although they are really only there to illustrate his point. I was expecting the book to stay at this level and discuss how various laws of physics limit the possible outcomes of evolution. What I wasn’t quite prepared for was the giddy tumble down the hierarchies of life in the rest of the book, but was I ever so pleasantly surprised.

Starting with cells and how physics explains why they developed, the book then considers DNA and proteins, offering an explanation of why we have four different base pairs in DNA instead of six or more. On a smaller scale, life is about acquiring energy which is achieved by transferring electrons across cell membranes (In The Vital Question: Why is Life the Way it is?, Nick Lane considered this to be life’s ultimate linchpin). Further down still, water (and not another solvent) is used as the medium in which to carry out biochemical reactions, and life uses a very limited set of atoms as its predominant building blocks. At every turn, Cockell shows how physics explains why life has preferred the solutions it has preferred, and how proposed alternative solutions (such as life forms based on silicon instead of carbon) would not work as well.

Cockell is a professor of astrobiology, a discipline that studies early biology and origins of life in the universe, as well as the possibility of extraterrestrial life and what it might look like. Amongst the many books he has written on the subject is the undergraduate textbook Astrobiology: Understanding Life in the Universe. The abovementioned descent down the hierarchies of life is at the same time an excuse for him to speculate what these findings mean for life elsewhere in the universe. Cockell clearly enjoys his forays into this topic, throwing in a short overview of exoplanets as a bonus. Based on the fact that many principles of physics apply throughout the universe, he makes a strong case for extraterrestrial life likely resembling life forms as we know them on earth, speculative zoology and science fiction notwithstanding.

The question of the predictability of evolution might sound familiar. Indeed, it was also explored in Losos’s book Improbable Destinies: How Predictable is Evolution?. Cockell acknowledges this book several times in his chapter notes, and throughout the book he considers whether what we see in living organisms is a consequence of convergent evolution (life independently evolving the same solution repeatedly) or contingency (the result of chance events that could work out completely different if we could repeat the evolution of life). He recaps Losos’s discussion of the main players in this debate, namely Stephen Jay Gould, who was a proponent of contingency (see amongst others his book Wonderful Life: The Burgess Shale and the Nature of History), and Simon Conway Morris, who was a proponent of convergent evolution (see amongst others his books The Crucible of Creation: The Burgess Shale and the Rise of the Animals and The Runes of Evolution: How the Universe Became Self-Aware).

In the end, your answer to this question depends on your perspective. Cockell is willing to concede that, yes, if you focus on the details, the variety of life seems bewildering and unpredictable, and there seems to be a lot of contingency. But to him this is just icing on the cake. When you zoom out and consider life on a more fundamental level, the outcomes of evolution are highly predictable, and convergent evolution is the rule. From his vantage point, that of physics circumscribing the limits of life, life has almost no other choices. Maybe, just maybe, if we discover life elsewhere in the universe, we might find some very exotic solutions to life’s basic needs, but Cockell convincingly argues that these will the very unlikely exceptions occurring under the confluence of very exceptional circumstances.

Written with enthusiasm, in turns lyrical and infused with a dash of humour, The Equations of Life is a pleasure to read. In a few places, Cockell assumes perhaps just a little too much background knowledge of physics on the part of the reader, such as when he casually talks about the various orbitals and suborbitals that electrons can occupy in an atom. I also think that for a popular science work of this calibre, the book could have used illustrations of basic concepts in places. Overall, however, I think Cockell does a marvellous job of presenting complicated and abstract concepts in an understandable way. Both convergent evolution and the role of physics in biology have been written about before, but with his background in astrobiology, Cockell presents a unique take on the subject that comes highly recommended.