‘What is not just a thing’: an alternative inquiry into ‘what is life’
Our understanding of visible matters we called things on Earth, in terms of invisible atoms and molecules, comes from the theory of equilibrium statistical thermodynamics, a part of physics and physical chemistry. Distinguishing things from beings, or dead matters from living organisms, is the next fundamental question that puzzles people. In English language, "living things and their activity" is called "life" (Oxford Dictionary), which is itself "a characteristic that distinguish physical entities or physical body that have biological processes from those that do not" (Wikipedia). The effort to find answers to "what is life" has focused for decades on identifying the "physical body" with the biological characteristics or the characteristic unique to the "living matters". Following and narrating the laws of physics and chemistry, two key notions that have emerged as essential are E. Schrödinger's neg-entropy and I. Prigogine's dissipative structure. These concepts, together with many others, explained some facets of "life". But the line that distinguishes things from beings is always vague, or maybe impossible to draw.
S. Bai and H. Ge at Peking University, together with H. Qian at University of Washington, proposed to ask a different question: Instead of "what is life", they tried to answer what could be an entity that is not just a thing, the matters defined in the equilibrium statistical thermodynamics. Their line of reasoning yielded a new hypothesis on how a "live" process could spontaneously emerge, and they identified one of the earliest hallmarks along the evolution path from things to beings.
Extending the notion of neg-entropy and dissipative structure, Bai et. al. introduced a chemical kinetic view of individual molecules and stable molecular complexes and their populations called kinetic species. It is known from physical chemistry that the individual entities, e.g., molecules and molecular complexes, come and go with different lifetimes, but the populations can exist through a dynamic balance called nonequilibrium steady state (NESS). They identified that the earliest "live" process is likely embedded in a special interaction between a pair of carbon-based components under a particular, corresponding environmental conditions. The interaction leads to an inter-molecular-force-bond complex (IMFBC) that couples two separate chemical processes: One is the spontaneous formation of the IMFBC driven by a decrease of Gibbs free energy as a dissipative process; while the other is the disassembly of the IMFBC driven thermodynamically by free energy input from the environment. The two chemical processes coupled by the IMFBC were originated independently and considered non-living on Earth, but the IMFBC coupling of the two can be considered as the earliest form of metabolism; it is no longer a "thing": This might be the first landmark on the path from things to a being (Figure 1). The dynamic formation and disassembly of the IMFBC, as a composite individual in a population, follows a principle designated as "… structure for energy for structure for energy …", the cycle continues, so is life as a phenomenon. More importantly, by introducing the notion of individual kinetic entities with variations within a population, an important ingredient of Darwinian thinking, evolutionary narratives can start even at a molecular level. The paper suggests that with additional features derived from this starting point, the IMFBC-centered "live" process spontaneously evolved into more complex living organisms with the characteristics currently known.
The authors pointed out three key points among many differences between their view and other conceptual frameworks: The first is that the essence of a living system is specific interactions of the specific components under particular environment; the second is that the intermolecular force(s) are most important; and the third is that the environment is an indispensable component of a living system. Interestingly, these simple thoughts have a logic consistence and continuity with the epic scenario presented in a recent investigation by E. Smith and H. J. Morowitz (The Origin and Nature of Life on Earth: The Emergence of the Fourth Geosphere. Cambridge University Press, 2016): Based on a sophisticated analysis of geochemical status and the chemical synthesis feasibility, they arrived at the conclusion that carbon metabolic cycle is the origin of emergent life on Earth.
See the article:
Bai SN, Ge H, Qian H (2018) Structure for Energy Cycle: A unique status of Second Law of Thermodynamics for living systems. Sci. China Life Sci. doi: 10.1007/s11427-018-9362-y