Life time.
The most striking feature of biological systems is their irreversibility. Molecules decompose. Organs fail. People die. In evolutionary terms, the combination of natural selection acting on individual variation produced by preceding rounds of selection means that evolutionary history is essentially irreversible. Evolutionary biologists have long recognized the statistical improbability of following the same evolutionary history, even when environmental conditions may be similar, which of course itself is virtually impossible. The same irreversibility pervades all our lives, from the development of our bodies, to cell division, to molecular interactions and biochemistry that sustain living things.
Ultimately, the temporal irreversibility of life is related to the directionality of time itself. This seems like a trivial notion. However, this is also fundamental to the realization of the physical basis of biological function. If time did not always flow from the past to the future, then the mechanics of living things could in principle be reversed. The physical basis of this phenomenon is the foundation of evolutionary physics of living matter.
What is time? For Greek philosophy, time was the product of motion—a kind of a natural counting process—encapsulated in the etymology of the word itself, which originally meant to divide. In Newtonian mechanics, time was an idealized property, one of the absolute laws of the natural world. Einstein and twentieth century physics transformed time into a relative observation, by establishing its dependence on gravity. Today, the fundamental nature of time is subject to vigorous attempts to unify quantum gravity as a physical theory of everything. This debate is ongoing, and remains unresolved because of the experimental inability to test many of the proposed models.
Whatever conceptual or mathematical form this unified theory will take, natural phenomena cannot escape the inherent tendency of all systems to increase in disorder over time. In fact, this may be the most fundamental property of the natural world. This is true for molecular systems like the ones originally studied by Boltzmann to derive the second law of thermodynamics (billionth of a meter). As pointed out by Rovelli, this is also true for systems that exist on the Planck scale, the smallest observable distance where quantum gravity is thought to originate (100 million billion billion times smaller). The transfer of energy from one particle to another—be they molecules or elementary particles, with the accompanying increase in entropy and disorder—is the physical basis of time. If there were no such transformations, time would stand still. The natural universe would be static, with no motion of matter or time.
This physics of time also orients all molecular and cellular processes in our bodies. However, we must make a distinction between the irreversible physics of time and the reversibility of equilibrium thermodynamics. For example, consider oxygen molecules diffusing into the core of hemoglobin for delivery from the lungs to deep tissues. It is intuitive to see how oxygen molecules cannot follow the same path every time, given that their diffusion is enabled by the essentially random thermal fluctuations of the hemoglobin polypeptide chain. The statistical improbability of this is analogous to the impossibility of a group of organisms following the same evolutionary path in history. However, under the physiologic conditions of oxygen delivery in our bodies, many oxygen molecules reliably bind and dissociate from hemoglobin to sustain cells in all our organs. While the dynamics of any individual molecule vary, they exhibit similar and often indistinguishable properties in aggregate.
On some scales, this process is conservative and reversible. Ultimately, this requires continued input of energy, as every binding even dissipates some of the energy as heat, which is irretrievably lost. Our molecules, cells, and bodies are organized to conserve these losses, and some have argued that their evolutionary fitness is at least in part derived from their energetic optimality. Nonetheless, these losses are inevitable insofar as molecules interact with each other, and our bodies live in exchanging environments.
How can the irreversibility of physical time and statistical reversibility of thermodynamic ensembles near equilibrium be reconciled with the biologic order of our bodies? And what is so special about the nature of order and biologic function in living things versus other physical systems? We have described biologic order as a self-organizing process guided by hierarchical conformational selection, bound by the energetic fluctuations of physiologic temperature. The power of evolution to explain the diversity of living things ultimately stems from the simplicity of its tautology. Natural selection is the survival of the fittest, and the fittest are those that survive. It may seem as if we are making a similar argument for biologic function, insofar as molecular order arises from conformational selection upon binding, and biologic function emanates from the resultant physical order. This is reasonable for developmental and homeostatic processes, where feedback signaling can stabilize specific interactions that otherwise occur from stochastic sampling of diversified states. What about biologic processes that do not use recursive structures such as signaling feedback?
Schrödinger distinguished biologic order from mechanical order by the propensity of living things to avoid thermodynamic equilibrium. At thermodynamic equilibrium, molecules and bodies have no energy to do physical work, and therefore no capacity for biologic function. Life proceeds by extracting order from the environment. Whereas mechanical systems transfer “order from order,” living things create “order from disorder.” As a result, our bodies counteract the tendency of all things to increase in disorder by consuming energy from the environment and decrease our entropy. This is an intuitive and satisfying concept, with life being a kind of structured disordering.
Therefore, we must envision biologic function occurring within and based on systems of physical interactions that transmit energy, while consuming some of it for self-organization. Brillouin termed the associated decrease in entropy of living things, negentropy. At its grandest, our living planet is a “network of ecosystems” that recycles organic matter, while consuming solar energy to sustain life. Humans form one link in this network, consuming energy captured in the organic matter of plants and animals. The function of this process is bidirectional, as the energy is used for the development and activity of our bodies, while simultaneously recycling carbon, nitrogen and other elements for the growth of other organisms. It is a kind of dissipative system on the planetary scale.
The conception of our bodies as a self-organizing dissipative system has several ramifications. First, just as evolution of life cannot be conceived without symbiosis—the cooperative interactions among organisms that produced eukaryotes, plants, and animals—the function of our bodies must be understood as fundamentally interactive. The heart affects with the kidneys, which interact with the liver, which affects with the intestine, which interacts with the brain, which controls the lungs, which interact with the heart, and so on. The interactions are pervasively multi-scale, occurring among organs, tissues, cells, and molecules. The immune system is integrated into the intestine, where mucosal immune cells regulate self-tolerance and antigen response. Brain immune glial cells regulate neuronal function. Heart immune cells remove cardiac myocytes damaged from injury or disease. Same sets of genes and molecules are expressed in divergent cell lineages, e.g. neurons and immune blood cells, where they regulate both diverse and common cellular functions.
While some local elements of the body operate near thermodynamic equilibrium—oxygen binding to hemoglobin, for example—others are mostly dissipative and irreversible. Insofar as the body as a whole must consume negentropy from the environment to provide the energy for self-organization, we can postulate that all developmental and homeostatic physiologic processes must be dissipative and fundamentally irreversible. Any restoration of prior state must come from new energy consumption.
It is this dissipative physics that also enables the temporal stability of living things. Without the physical irreversibility of time and its entropic constraint on dissipative systems, our bodies would not have sufficient temporal stability required for complex multicellular development. Dissipative processes provide the necessary ratchet mechanisms to move development forward. This applies to molecules, such as for example during chromosome segregation in meiosis and mitosis, where molecular motors use Brownian thermal to separate chromosomes into daughter cells. This applies to cells, such as for example during egg fertilization by sperm, where a series of microscopically reversible interactions between sperm and egg eventually result in irreversible fusion and intracytoplasmic sperm injection. And this applies to complex organs such as our brain, heart, and many others, where a series of fluctuant cell interactions are organized into stable, stereotypic, and ordered structures. This requires that the dynamics of living systems are inherently asymmetric, ultimately imposed by the unidirectional flow of energy through symmetry-breaking structures. The flow of blood by the action of valved hearts illustrates this neatly.
Dawkins defined living things as genetic replicators with their physical bodies as subjects of natural selection and evolution. This process is inherently dissipative. Its evolutionary irreversibility ultimately derives from the irreversibility of physical time and its entropic constraints. In this sense, neutral evolution where genetic variation occurs spontaneously from errors in DNA replication and noise of other mutational processes is analogous to the thermal fluctuations that are inherent in all energetic systems, be they elementary particles, molecules, cells, or bodies. This genetic variation is random, though not necessarily uniform, as it is shaped by the inherent propensities of specific mutational processes. For example, spontaneous deamination of cytosine in our DNA under physiologic conditions leads to the accumulation of C-to-T mutations upon aging; T-to-C mutations are far less frequent. When this occurs in somatic cells, this leads to somatic mosaicism. In germ cells, along with other mutational processes, this leads to the genetic diversification required for natural selection. Recently, this was documented by observations of increasing incidence of de novo mutations in children from older fathers due to the age-dependent accumulation of mutations in sperm cells.
Whereas neutral evolution by genetic fluctuations can be thought of as analogous to thermal noise in physical systems, adaptive evolution is inherently structuring, asymmetric, and requires energy consumption. The irreversibility of evolutionary processes depends on the structuring properties of adaptive evolution. Without the elimination of maladaptive traits in unfit individuals, evolution would either stand still or reverse. The arrow of time starts with the entropic asymmetry of elementary particles, flows through dissipative living bodies, and continues unidirectionally in the evolution of genetic replicators. This irreversibility is the natural determinant of biologic time.
The asymmetric nature of this process is the defining feature of biologic function. This distinguishes it from purely mechanical (non-living) phenomena. Biologic function enables living things to repeatedly and reproducibly carry out specific activities, in spite of changing environmental conditions. While the reversibility of certain mechanical phenomena may appear to fulfill this requirement, they typically lack the high degree of robustness of biologic phenomena.
The irreversibility and asymmetry of biologic function also enables living systems to self-organize. While certain components of biologic systems are largely reversible and operate near thermodynamic equilibrium, as determined by thermal fluctuations, the essential parts of our bodies are fundamentally dissipative. This dissipative nature enables consumption and conversion of environmental energy and negentropy to generate the organization of our bodies. In this way, evolutionary physics provides a self-consistent definition of living things, without invoking animalcules or vital force.