What is life? And how did it get started here on Earth? These are two of the thorniest questions humans have been asking themselves through the ages. They are the basis of all religions, and even when a deity is not invoked to answer them, until fairly recently almost all answers to these questions involved some force outside the realm of the purely physical.
This was all well and good, because once you remove some sort of élan vital from the equation and insist on a purely physicochemical explanation for life’s origin, things get very tricky. It’s hard to get from an abiotic world governed by the laws of thermodynamics to a world teeming with living organisms, each of which is an entity of organized matter far from equilibrium that maintains its existence by constantly tapping external energy sources.
In “What Is Life?: How Chemistry Becomes Biology,” chemist Addy Pross attempts to answer these fundamental questions strictly within the framework of physics and, especially, chemistry. Pross believes that a relatively recent subdiscipline of chemistry—systems chemistry, which focuses on replicating systems—holds the answer to these age-old questions and provides a framework for unifying biology and chemistry as a single discipline.
Does Pross accomplish his ambitious goal in this book? I’m not sure. But he certainly leads his readers on an intellectually lively exploration of some fundamental issues in life’s origins and posits an intriguing theory about how inanimate matter can become animate within the constraints of thermodynamics.
Pross is an organic chemistry professor at Ben-Gurion University of the Negev, in Israel. The book “What Is Life?” grew out of a paper, “Toward a General Theory of Evolution: Extending Darwinian Theory to Inanimate Matter,” Pross published in the Journal of Systems Chemistry in 2011 (DOI: 10.1186/1759-2208-2-1).
Pross succinctly states the problem facing theories of the origin of life early in the book. “It is not just common sense that tells us that highly organized entities don’t just spontaneously come about,” he writes. “Certain basic laws of physics preach the same sermon—systems tend toward chaos and disorder, not toward order and function. No wonder several of the great physicists of the twentieth century, among them Eugene Wigner, Niels Bohr, and Erwin Schrödinger, found the issue highly troublesome. Biology and physics seem contradictory, quite incompatible.”
The title of Pross’s book is, in fact, a direct tribute to a book entitled “What Is Life” that Schrödinger wrote in 1944. In this book the great physicist admitted that, at that point in time, physics and chemistry could not account for the existence of life. Pross writes that the purpose of his book “is to reassess this enthralling subject and demonstrate that a general law that underlies the emergence, existence, and nature of all living things can now be outlined. I will argue that thanks to a newly defined area of chemistry … the existing chasm separating chemistry and biology can now be bridged, and that the central biological paradigm, Darwinism, is just the biological manifestation of a broader physicochemical description of natural forces.”
In the first three chapters of “What Is Life?” Pross explores the scientific and philosophic challenges of understanding life. “Living and non-living entities are strikingly different, yet somehow the precise manner in which these two material forms relate to one another has remained provocatively out of reach,” he writes. He points to the complexity of organisms: their purposeful character—dubbed teleonomy, as distinguishable from teleology—their dynamism, resilience, diversity, and other unique characteristics. He briefly discusses previous efforts to develop a “theory of life,” most of which he finds unsatisfactory. He probes what it means to “understand” a phenomenon, focusing especially on the distinction between reduction and holism.
These are deeply philosophical issues, and Pross is adept at exploring them in clear and concise prose. He has obviously read widely on these topics and given them deep thought. He usefully quotes other scientists and philosophers to explain how thinking on the topics has evolved over time.
Pross then engages in a discussion of chemical stability and instability and introduces a concept he calls “dynamic kinetic stability,” which applies to replicative systems. Dynamic kinetic stability, or DKS, is central to Pross’s attempt to develop a chemical explanation of life. “In the context of chemical systems,” he writes, “static and dynamic forms of stability are very different. In the ‘regular’ chemical world a system is stable if it does not react. … In the world of replicating systems, however, a system is stable (in the sense of being persistent and maintaining a presence) if it does react—to make more of itself, and those replicating entities that are more reactive, in that they are better at making more of themselves, are more stable (in the sense of being persistent) than those that aren’t. This is almost a paradox—greater stability is associated with greater reactivity.”
Throughout the remainder of “What Is Life?” Pross makes the case that DKS, although entirely consistent with the second law of thermodynamics, is sort of the dodge that allowed living systems to arise from inanimate matter. Replicating entities are, he maintains, a different sort of matter governed by a different sort of chemistry from nonreplicating entities. In the long, penultimate chapter of the book, entitled “Biology Is Chemistry,” Pross insists that “Darwinian theory can be integrated into a more general chemical theory of matter, and that biology is just chemistry, or to be more precise, a sub-branch of chemistry—replicative chemistry.”
Pross explicitly equates evolutionary concepts with concepts drawn from systems chemistry. Natural selection, for example, is “kinetic selection”; fitness is “dynamic kinetic stability”; and maximizing fitness is “maximizing DKS.” I’m not sure I buy all of these connections. Pross doesn’t provide a chemical mechanism that underpins natural selection, for example. His argument that natural selection equals kinetic selection seems to be based entirely on the fact that the two forms of selection lead to the same result, but that doesn’t make them equivalent.
Nevertheless, equating fitness and DKS is critical to Pross’s complex argument. “The immediate consequence of relating fitness and DKS is that it indicates more explicitly that fitness is best viewed as a population characteristic, not an individual one,” he writes. “The concept of DKS has no real meaning at the individual level. A stable population of some replicating system is the reality that comes about through individual replicators being formed and then decaying,” and “if you focus on the individual entity … you are missing the essence of what defines life—its dynamic nature, the continual turnover of the individual entities that make up a particular replicating population.” This line of argument even leads Pross to question whether individual living entities actually exist.
All of this leads Pross to conclude that “biology then is just a particularly complex kind of replicative chemistry and the living state can be thought of as a new state of matter, the replicative state of matter, whose properties derive from the special kind of stability that characterizes replicating entities—DKS. That leads to the following working definition of life: a self-sustaining kinetically stable dynamic reaction network derived from the replication reaction.” Complexity, he argues, is not the essence of life but a consequence of life—replication induces complexity. And the teleonomic, or purposeful, aspect of life can be accounted for by the energy-gathering capability of living organisms.
Does Pross’s argument hold water? I’m not entirely convinced. Pross attributes almost mystical power to the dynamic kinetic stability of replicating systems. He freely admits that his ideas get us no closer to knowing precisely what occurred chemically to produce the first living organisms and that they probably won’t help chemists synthesize artificial life. He devotes much of “What Is Life?” to developing the ideas behind DKS and replicating systems but only a couple of pages to the critical corollary of how those replicating systems latched onto energy-gathering systems to fuel their continued growth. Without access to a source of external energy, the replicators could never be viewed as “alive.”
But I’m also not sure that it matters all that much. “What Is Life?” is a lively, intellectually stimulating examination of profound scientific and philosophic questions. It provides an intriguing and possibly plausible way to think about life and its origins. It provides much food for constructive thought, and that’s probably enough.
Rudy Baum is C&EN editor-at-large.