Life as Physics
Questions about life and its purpose concern all of us. There are recurring debates on this by Scientists, Philosophers and Religious thinkers.
What is life is a question that does not fetch unique and unambiguous answer. We can only describe the essential features common to all living organisms. All living organisms form out of a few elements. Hydrogen, Carbon, Oxygen, Nitrogen and Phosphorus are fundamental to all living matter on this planet.
A second feature is structural organization. All living organisms are embedded in a very definite structure called a cell. The composition and complexity of these cells can vary among organisms; they range from single-celled bacteria to humans who have trillions of cells.
A third feature is the capacity of all living organisms to maintain this structure very efficiently. They achieve this by self-replication. Living cells continuously synthesize their specific molecules to grow and divide.
The living things capture energy from the environment and dissipate it as heat, which is another way of saying they want to stay away from equilibrium. Finally, a unique property of life is that it undergoes evolution. Trace the characteristics of any present form of plant or animal life back in time, and we see them converging to one or a few primitive organisms, which first appeared 3.5 billion years ago. Therefore life probably had an origin in such primitive microorganisms. Archaea are uni-cellular organisms and early life on Earth have many common features. The earliest evidence for life may be 3.5-billion-year-old sedimentary structures.
Science must self-consistently explain how all these features came about to explain life credibly. How did inert molecules like Hydrogen and Carbon combine to form biologically significant molecules like amino acids, etc.?
Attempts to synthesize biological molecules from basic elements started with the work of Stanley Miller and Harold Urey in 1953 at the University of Chicago. They exposed a gaseous mixture of water vapour, Hydrogen, Methane and Ammonia to an electrical discharge. In the residue, they found amino acids, which are biological molecules. Amino acids are biologically significant organic compounds composed of amines and carboxylic acid functional groups, along with a side chain specific to each amino acid (1).
Many similar experiments followed. A refinement of this classic work was reported a few months back by John Sutherland from the U.K. He exposed a mixture of hydrogen cyanide and hydrogen sulphide to ultraviolet (UV) light and created nucleic acid precursors (2). In addition, he also made the precursors to synthesize natural amino acids and lipids. These results indicate that a single set of reactions may have produced most of life’s building blocks at one go.
Earth had conditions favouring these reactions. HCN may have come from comets, abundant in the early history of Earth. H2S came from volcanoes. Sunlight contained UV radiation that could power the reactions. Rainwater would then wash the product compounds into a shared pool.
The next problem is to explain how the structural organization characteristic of life comes about. Then, finally, we should explain how the simple biomolecules formed into more complex molecules.
Order is not the characteristic feature of nature — perfume in a bottle in a room is a pattern, an order, in fragrance. But perfume diffuses through the air, and the order disappears. Energy and matter tend to disperse as time progresses, as enunciated by the Second Law of Thermodynamics. Entropy is a measure of how the particles in a system share the energy and how spatially distributed those particles are. Entropy always increases, which result from probability: more possibilities for energy to spread out than to localise exist. Eventually, the system degrades to a state of thermodynamic equilibrium, which is characterised by uniform distribution of energy, which is equivalent to a state of maximum entropy.
The spreading of perfume is irreversible as long as the bottle and the room are left alone, a closed system. The fragrance would never spontaneously concentrate because the probability of this concentration is quite low. However, in a a room where energy can flow from outside, pushing the molecules of the perfume to go back into the bottle is possible. An open system can lower its entropy by enhancing the entropy of the surroundings. A plant, for example, uses the energy absorbed from sunlight to synthesise sugar. It ejects heat, a degraded form of energy. Thus, photosynthesis increases the overall entropy of the universe by dissipating solar energy, while the plant maintains its ordered structure, a state of lower entropy. So living things increase order and decrease entropy.
Our understanding of how life increases local entropy is based on the work of the Belgian physicist Ilya Prigogine who first predicted the behaviour of open systems absorbing external energy. Prigogine won the 1977 Nobel Prize in chemistry for proving that open systems can be displaced from equilibrium. External sources of energy strongly drive them. Unfortunately, we did not know how they behaved until the late 1990.
Two scientists, Chris Jarzynski (University of Maryland}, and Gavin Crooks, (Lawrence Berkeley National Laboratory) showed that the entropy produced by a thermodynamic process, such as spreading of perfume, corresponds to a ratio between the probability that the atoms will undergo the spreading process divided by their probability of the reverse process happening (3). The Jarzynski equality (JE) and its basis, the Crooks fluctuation theorem have evoked intense interest among physical and biological scientists (4). As entropy increases, this ratio also increases and the behaviour of the system increasingly irreversible.
Jeremy England from MIT developed this idea further for systems that are strongly driven by an external energy source and dump heat into its surroundings (5). All living things are like this. England found out how such systems evolve as they increase their irreversibility. He showed that the group would gradually restructure to dissipate more energy. A plant is more capable of capturing and using solar energy than a collection of carbon atoms. Thus under certain conditions, matter undergoes self-organization and inexorably acquires a key physical attribute associated with life.
A system might dissipate a large amount of energy over time (3) by the process of biological reproduction. More copies will dissipate more energy. Thus the underlying principle of increasing self-organization, complexity and replication is a dissipation-driven adaptation of matter.
Can non-living system replicate itself? Eddies in turbulent fluids undergo replication driven by the shear in the surrounding fluid. Snowflakes, sand dunes and turbulent eddies all possess intricate structures that appear in many-particle systems driven by dissipative process (3). Thus the same physics responsible for forming patterned structures in the universe can also explain the origin of living things.
A broad principle governing life and evolution gives us a better perspective on how structure and function emerge in living things. England’s approach could free biology from the need to explain every adaptation on a Darwinian basis. They will be freed to think of mechanisms based on the general process of dissipation-driven organization (3).
An organism develops into X and not Y primarily because physical constraints preferred X to evolve than for Y to evolve. Nigel Goldenfeld and Carl Woese at the University of Illinois suggest that we think of biology as a branch of condensed matter physics (6). Their fundamental conjecture is that life is an emergent phenomenon in systems far out of equilibrium.
Emergence refers to the appearance of higher-level behaviour due to the collective dynamics of a system’s components (7). The higher-level phenomena are more than the sum of the components or the behaviour or properties of the components. A classic example of emergence is the flocking of birds, which is the formation of patterns when a a large group of birds fly together. Hurricanes, sand dunes, termite mounds, and superconductivity are other examples of emergence.
Are the laws that describe such systems? According to Goldenfeld and Woese, only a discipline like physics can reveal such laws. Biology, in its nature, cannot. That’s a bold and provocative idea that can change our perception of life.
Everything in the universe evolved by the self-organization of matter towards more and more complex structures. Quarks, protons and atoms self-assembled out of the fundamental particles produced by the Big Bang. Earth, stars and galaxies are self-organized through gravity.
Why should life be an exception to this?
1. Pranab Basuchaudhuri, Amino Acids, 1st Edition, Published 2016, CRC Press, page 62
2. eBook ISBN 9780429154140Origin-of-Life Claims: Triple Header or Strike Three? Evolution News @DiscoveryCSC, March 19, 2015
4. Liao Y. Chen, The Journal of Chemical Physics Oct 2008:091101DOI:10. 1063/1. 2978949