Life Emerges from the Second Law of Thermodynamics

Every species of living thing can make a copy of itself by exchanging energy and matter with its surroundings. One feature common to all such examples of spontaneous “self-replication” is their statistical irreversibility: clearly, it is much more likely that one bacterium should turn into two than that two should somehow spontaneously revert back into one. From the standpoint of physics, this observation contains an intriguing hint of how the properties of self-replicators must be constrained by thermodynamic laws, which dictate that irreversibility is always accompanied by an increase of entropy. Nevertheless, it has long been considered challenging to speak in universal terms about the statistical physics of living systems because they invariably operate very far from thermodynamic equilibrium, and therefore need not obey a simple Boltzmann probability distribution over microscopic arrangements. Faced with such unconstrained diversity of organization, it is quite reasonable to worry that the particular mechanistic complexity of each given process of biological self-replication might overwhelm our ability to say much in terms of a general theory.

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The process of cellular division, even in a creature as ancient and streamlined as a bacterium, is so bewilderingly complex that it may come as some surprise that physics can make any binding pronouncements about how fast it all can happen. The reason this becomes possible is that nonequilibrium processes in constant temperature baths obey general laws that relate forward and reverse transition probabilities to heat production. 2 Previously, such laws had been applied successfully in understanding thermodynamics of copying “informational” molecules such as nucleic acids. 8 In those cases, however, the information content of the system's molecular structure could more easily be taken for granted, in light of the clear role played by DNA in the production of RNA and protein.

What we have glimpsed here is that the underlying connection between entropy production and transition probability has a much more general applicability, so long as we recognize that “self-replication” is only visible once an observer decides how to classify the “self” in the system: only once a coarse-graining scheme determines how many copies of some object are present for each microstate can we talk in probabilistic terms about the general tendency for that type of object to affect its own reproduction, and the same system's microstates can be coarse-grained using any number of different schemes. Whatever the scheme, however, the resulting stochastic population dynamics must obey the same general relationship entwining heat, organization, and durability. We may hope that this insight spurs future work that will clarify the general physical constraints obeyed by natural selection in nonequilibrium systems.

Hypothesis that life is the result of needing to dissipate energy that builds up.