When it comes to life, how do we draw the border between biology and chemistry?
Addy Pross. What Is Life? How Chemistry Becomes Biology. Oxford: Oxford University Press, 2012. 224 pp. £16.99.
Animal, vegetable, or mineral? Anyone who has played 20 Questions on a long car trip knows these are the most fundamental questions that can be asked about an object. Is it alive? Does it think? Unless faced with a particularly crafty opponent, the answer rarely leads a player down the wrong path. The tomato in the salad? Vegetable. The stainless-steel mug? Clearly mineral.
But how did the dividing line we see so sharply first arise? What makes one molecular assembly living and another not? What enables a living thing to behave with intention, to be an ant rather than a carrot? In What Is Life? How Chemistry Becomes Biology organic chemist Addy Pross tackles these questions from a primarily chemical perspective. How, Pross wonders, do purposeful systems arise in a universe that favors disorder over order and that (objectively at least) lacks purpose? What are the chemical principles underlying natural selection and evolutionary fitness?
Pross begins by exploring the boundary between the fields of biology and chemistry. The once undisputed line between chemists and biologists—between those who study matter for a living and those who study living matter—has been blurring since at least 1828, when Friedrich Wöhler succeeded in synthesizing a biological material, urea, from purely mineral materials. But despite our intimate molecular-level understanding of the structure and function of living cells, we have yet to achieve quite the same feat at the cellular level. Craig Venter’s creation of a semisynthetic cell notwithstanding, no one has yet breathed life into a crowd of molecules, creating a living organism capable of metabolism and reproduction. Yet Pross concludes that there is no essential difference between the two fields: biology is a peculiar and particular form of chemistry, and life is no mystery at all to a chemist with the proper perspective. I suspect many chemists will delight in the idea that the secret of life is ultimately in their purview, while some biologists will be vexed by this wholesale assimilation of their field.
The title of Pross’s book deliberately echoes quantum physicist Erwin Schrödinger’s What Is Life? The Physical Aspect of the Living Cell. Schrödinger, writing before the discovery of DNA and its pivotal role in storing the information necessary to replicate living organisms, begins by asking the broader question, “How can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry?” In other words, Schrödinger wondered, how might thermodynamics and quantum physics explain how living systems behave. Using quantum states as a loose model, Schrödinger hypothesized the existence of a code within the chromosomes, a suggestion that kindled both James Watson’s and Francis Crick’s interest in a molecular basis for inheritance.
Pross’s book is more developed and sharply focused than Schrödinger’s and is grounded in familiar notions of physical chemistry, particularly chemical equilibrium and the relative rates of chemical reactions, concepts encountered by students in introductory chemistry courses. Everything goes to equilibrium; diamonds, for example, will all eventually turn into lumps of graphite. But how fast systems reach equilibria can vary, from picoseconds to millions of years. Pross frames the basic problem—the existence of life—by asking how a far-from-equilibrium system, that is, life, can naturally arise when equilibrium is the destiny of all molecular systems.
While the second law of thermodynamics controls the ultimate fate of the universe—equilibrium—Pross argues that the spark of life lies in what he terms systems chemistry and, in particular, with autocatalytic systems—interconnected systems of reactions with built-in feedback loops. A classic example of these reactions is the Belousov-Zhabotinsky reaction, which produces colored patterns within a mixture of clear liquids. In Pross’s view natural selection of the biological variety arises from the ability of a complex chemical system, such as a cell, to be trapped for a while in a more ordered state, as carbon can be trapped in the form of diamonds. As in the Belousov-Zhabotinsky reaction, a system can be dynamically stable for a significant period, but it will eventually wind down to a muddy blur.
I do wish Pross had made one thing clearer in setting up his question of life: that the Earth itself is a system far from equilibrium. Not inconsequential amounts of energy flow into and out of the terrestrial biosphere, suggesting that the question of how order is fueled is less mysterious than Pross makes out. However, Pross does an excellent job of succinctly conveying the difficulty in crafting an unambiguous general definition of life and provides a road map to much of the work on the origin of life done by chemists in the past 50 years. The book is worth the read for these discussions alone.
Schrödinger’s book grew from a series of lectures at Trinity College in Dublin in February 1943. He characterized the task of putting together this work as complex not because “the physicist’s most dreaded weapon, mathematical deduction,” would be deployed but because of the need to pull from the language of both physics and biology. Likewise those unfamiliar with the philosophical map of biology may find Pross’s initial exposition on teleonomy and teleology and the subtle distinction between these two ways of characterizing “purpose” in biological systems unnecessary and distracting.
Pross suggests that biology faces an identity crisis, that even large organisms and ecosystems can be better understood in the light of his systems chemistry. I’m not sure I agree that biology will be subsumed entirely into chemistry. I suspect that scientists will continue to organize fields around common methods and equipment and less around core theories. The borders between the two fields might shift substantially again, as they did after the discovery of DNA’s structure; there might be a wide swath claimed by both fields, but I suspect there will always be intellectual spaces tagged chemistry and biology.
The book has not changed how I look at my own research on structural chemistry, but it will absolutely change how and what I teach. For years I’ve taught about autocatalytic reactions in upper-division chemistry courses, a topic that is still on the edge of the standard chemistry major curriculum, situated in a course that few biologists would take. Pross’s framework has encouraged me to pull a serious explication of autocatalysis into the introductory chemistry course required of biologists.
Scientists can sometimes find the philosophy of science a shade too philosophical and too far removed from the testable practicalities of their field for comfort. But the questions Pross poses in What Is Life? engage chemists in a serious philosophical conversation to which they can bring significant expertise and where the outcomes have the potential to drive chemical research. Nonexperts may well find long-held notions of what constitutes life expanded and enriched. Pross pushes us all to consider the deeper implications of our molecular understanding of biology. Animal, vegetable, or mineral? It’s all chemistry.