Book Review: The Origin and Nature of Life on Earth
( I originally wrote this as an Amazon review, but ideas from this book are closely related to many ideas that have been or will be covered by this blog. You can find the book here! Also, for a video version of many of the ideas in the book, check out Eric Smith’s lecture! )
In this stunning, magisterial, and surprisingly wide-ranging book, Eric Smith and Harold Morowitz approach the origin of life from first principles and bring together knowledge from astrophysics, geochemistry, biochemistry, information theory and statistical mechanics to show how all the hints and constraints we know so far about life’s start and the universal patterns of life we’ve found compel us to adopt a new framework to understand life: life is a complex series of nonequilibrium phase transitions driven by a persistent geochemical redox potential.
First, Smith and Morowitz argue that the ecosystem is the correct level to view the phenomenon of life and how it integrates with the rest of the planet’s geochemistry. It is only there that one can see the closed, robust nature of life. In every ecosystem from hydrothermal vents to rainforests, the ecosystem as a whole takes in inorganic inputs and free energy and builds biomass using a conserved core set of metabolic reactions. These core reactions give us many hints to the context of where and how life began suggesting the primary role of hydrothermal vents and of autocatalytic chemical networks.
Next, Smith and Morowitz examine the physical conditions that were present before life began and ask the question: what stress was present in the non-living world that could have compelled the phenomenon of life into existence? Any non-equilibrium process (of which life is the most complex example) requires energy and a barrier to keep that energy from dissipating away too quickly. Smith and Morowitz take the physicist’s view and list many possible energy sources present on or around the early Earth and the barriers that could have maintained them. For example, they point out that the sun uses gravitational potential energy to drive nuclear fusion and the barrier to dissipation is the low probability of having a fusion collision which is set by the weak force!
They then review the geophysical and geochemical processes that take place in the mantle, the crust and the atmosphere and describe the redox states of minerals in the mantle and how the atmosphere on the surface is held at a more oxidized state by hydrogen escape. (This chapter is the most in-depth and best introduction to the field of geochemistry that I’ve ever read.) The reduced state of the mantle and the oxidized state of the atmosphere turns the Earth into a giant redox battery (which has precisely the right energy difference to run the organic reactions of life) with the barrier being the crust itself and hydrothermal vents acting to concentrate this diffuse redox energy to specific points on Earth.
Smith and Morowitz then present the core metabolic reactions that are universal to all life. This set of reactions creates a closed feedback loop and the whole loop (and most of the steps) are energetically favorable in a reduced hydrothermal vent environment. The core processes of life want to happen spontaneously at vents! Patterns and structure in core metabolism also reveal many more hints as to the key steps life needed to pass through on its journey to “lift itself off the rocks”. The evidence is fascinating and compelling, but there’s too much to adequately summarize here.
Throughout the book, Smith and Morowitz argue that the most important question is not how were the organic precursors to life synthesized, but instead how was the selectivity of chemical reactions robustly maintained? There are many proposed reaction pathways to produce the building blocks of life, but the problem is that these pathways make many more products than the ones life uses and these unwanted products would starve the early reactions of life from getting enough input material. Bringing the questions of selectivity and robustness to center stage immediately suggests that one could use the tools of information theory and phase transitions to make progress.
Information theory studies the storage and communication of information and deals with things like error-correcting codes that help you preserve information if you’re sending it through a noisy channel. Studying phases of matter has yielded a mathematical framework to understand how qualitatively different macroscale behavior (like ice or water) can result from the same fundamental building blocks and interactions (H2O molecules and electromagnetic interactions). Error correction in message transmission and the maintenance of some system in a particular phase require that only certain configurations of message or system can be selected. Furthermore, we know that the stability of a phase and the efficacy of message transmission mean that such selectivity can be robust.
The authors then give an introduction to the broad and successful field of phase transitions and describe how ideas from information theory fit into understanding what a phase really is. This introduction leads to the math necessary to understand dynamic phase transitions in out of equilibrium chemical networks (which is close to what we’d want to understand life!) and includes a fantastic introduction to the modern viewpoint of how the universe formed as a cascade of phase transitions starting with the quark-gluon plasma right after the Big Bang to nucleons to neutral atoms to chemistry to eventually, after every symmetry has been frozen, superconductivity.
Finally, Smith and Morowitz bring all these ideas together and describe how they conceptualize “the nature of the living state”. Central are the ideas of robustness and modularity. They recount a parable about two watchmakers who are trying to build complex watches and are both interrupted every few minutes. One builds the watch in a very complex way so that it only works if all the pieces are in place and falls apart if the watchmaker leaves halfway through. The other watchmaker builds his watch out of self-contained modules which are each stable on their own. His progress remains when he is interrupted. The second watchmaker is able to finish his watch much faster than the first. We can see that modularity gives us a way of preserving a complex system against perturbations and of building more complex structures without the whole system becoming unmanageable or unstable. The modularity of life is evident everywhere one looks and is returned to again and again throughout the book.
Modularity links with another idea that Smith and Morowitz emphasize: why can physics theories work at all if we don’t know the underlying building blocks of the world? The answer actually comes from the study of phases and is closely related to modularity. Matter in a certain phase can be described by an “effective theory” within that phase that allows us to ignore laws of the universe that act outside the regime of that phase. For example, the laws of chemistry don’t really apply when the world is too hot and atoms are ionized into plasma. The cascade of freezing transitions that matter undergoes itself represents a type of modularity and robustness. One phase lets us have molecules, we can then use molecules as the building blocks for minerals, which can then be used as the building blocks for more complex structures. Although much work remains, Smith and Morowitz see similar ideas present in how life self-organized into such a hierarchical and modular form (although in life’s case, it remained fully out-of-equilibrium unlike the phase transitions for matter mentioned above).
Overall, this was one of the most profound and thought-provoking books I’ve ever read. It provides a solid foundation for those who are sufficiently motivated to reach the forefront of the current discussions on the origin of life. It also provides tools and ideas that have shaped how I understand the world more generally. I learned so much from this book! As a warning, the book assumes comfort with several branches of science (geology, chemistry and physics) and is rather dense at parts, but the book is laid out in a modular enough way that one should be able to find those parts of the book they are most interested in. It’s not light reading, but the insights in this book are totally worth the effort for those interested in the origin of life!
(Originally published on my Wordpress blog)