From Inert to Living

The Same Elementary Building Blocks

With all the objects in the universe, we share the same elementary building blocks, the same particles, the same stardust. The particles that make us up have assembled into amino acids, clusters of atoms, monomers, polymers, macromolecules, and proteins that are the basis of cellular function in living organisms.

The structures and conformations of these proteins will determine their functions. Based on our current knowledge, is there a conceptual definition of life? It seems not, yet this question is fundamental. Everyone has the impression of knowing what life is, but no one can really answer it, neither biologists, nor doctors, nor biochemists, nor physicists, nor exobiologists, and even less philosophers. Even more surprising, they do not like this question at all because behind the term life, there is a metaphysical resonance.

It is very difficult to define a fundamental concept like life, time, or matter. Let's talk about the living that we observe on Earth. What is the current definition of the living or living matter? Once again, experts do not give an indisputable conceptual definition but enumerate a series of properties that define the living. Often these definitions contain the term "life" and therefore presuppose the idea or existence of life. In other words, these are not definitions of the living but metaphors of the living.

Examples of definitions of life or the living:

• Life is the mode of existence of living bodies.
• Living beings include all organisms that inhabit our planet and are endowed with life.
• Life is a chemical process from which living organisms emerge.
• An organism is said to be living when it exchanges matter and energy with its environment while maintaining its autonomy, when it reproduces and evolves through natural selection.
• According to NASA, a living system is any system spatially delimited by a semi-permeable membrane of its own making and capable of self-maintenance, as well as reproduction by manufacturing its own constituents, from energy and/or from external elements.

In these definitions, it is understood that life would be a self-organized system, a mysterious mechanism linked to matter. But this shows that we still do not know the exact nature of life and where the division between the living and the non-living lies.

What is the Mechanism that Creates the Conditions for Life to Start?

Compared to inanimate objects, a living being is a chemical system that forms its own substance from what it draws from the environment. Living matter will procure for itself the energy it needs to self-replicate; this is where the mystery lies.

The living and the non-living are made of the same matter, the matter that created stars, galaxies, nebulae, and planets. On Earth, the transition from inert matter to life likely occurred in water about ≈4 billion years ago when the first organic molecules reproduced. Rapidly, a proto-bacterial life appeared, less than a billion years after the formation of the Earth. A proto-bacterium is already an advanced stage of life because in this very simple organism, all functions reveal remarkable complexity, particularly that of replication.

Observation of life on Earth shows the ability of matter to gradually climb, by chance, the rungs of complexity. But where is the breaking point between the living and the non-living? In other words, what is the assembly of molecules that allows life to start?

Carbon is produced by nucleosynthesis (fusion of 3 helium nuclei) in the core of massive stars and then released into space when they explode. Our chemistry began with a small molecular structure built on a skeleton of carbon atoms, amino acids.

Scientists are trying in vain to create life based on carbon molecules, particularly 12C, which is a biological signature. The other avenue explored is molecular archaeology, the goal being to find the primordial fossilized molecules, but the oldest traces of life (fossilized bacteria) stop at ≈3.5 billion years. In the oldest sediments, archaeobiologists have found organic molecules enriched in 12C dating back to ≈3.8 billion years.

Life hides its secrets from us, so scientists are turning to space and the planet Mars in the hope of finding life elsewhere. Finding a fossilized living system as simple as possible is already a gigantic challenge.

The Living Carries Replicable Information!

If we consider the problem from the primordial building blocks of ordinary matter, we observe that the original matter, quarks, nucleons, atoms agitate with temperature (heat directly translates the agitation of particles).

At absolute zero (−273.15 °C), matter is in a state of minimal energy, its entropy is zero, and this translates to a "total immobility of particles," although in quantum physics, particles always possess a non-zero amount of momentum according to Heisenberg's uncertainty principle.

As soon as absolute zero is exceeded, particles agitate, organize, and assemble into pure atoms (i.e., a single chemical species, H, N, Fe…) then into simple molecular bodies (H2, N2, Fe…) before mixing by chance with other species to form simple electrically neutral molecules, atomic aggregates with particular shapes that share electrons (covalent bonds).

So far, the activity of matter is relatively well understood, and at this stage, matter cannot be considered living; it simply assembles thanks to the electromagnetic force according to migrations in the environment.

Matter has no design, intelligence, intention, or particular project. It is easily manipulated by the environment and also by humans when they manufacture nanoparticles. Molecular aggregates, under the effect of thermal agitation and the random jumps of atoms from one position to a neighboring position, form various and stable chemical compounds.

The more atoms transported by the environment, the more the fortuitous movement of atoms and molecules will order itself. This is "order from disorder" as Erwin Schrödinger (1887 - 1961) said in his 1944 essay.

Among all these naturally stable molecules, there will be solid molecules with crystalline structures (which do not repeat) and disordered molecules. From a very small number of atoms, there will be an almost infinite number of assembly possibilities. Each encounter between two atoms will depend on their electronegativity, i.e., their reciprocal ability to create an electromagnetic bond.

Chemical compounds are molecules of several different chemical elements linked together by chemical bonds. From time to time, by chance, chemical compounds self-organize into covalent chemical bonds.

The assemblies between atoms are of 3 types:

• Ionic bonds: these are bonds resulting from the loss of one or more valence electrons (electrons of the peripheral shell involved in chemical bonds) by an atom and the capture of these electrons by another atom. This type of bond is generally established between elements with a very high difference in electronegativity ≥ 1.7 on the Pauling scale, between fluorine = 4 and francium = 0.7).
• Covalent bonds: the bonded atoms each contribute a valence electron to create a shared electron pair between two entities. The elements linked in this type of bond have a difference in electronegativity of zero or <1.7 on the Pauling scale.
• Coordinate covalent bonds: these are covalent bonds between two atoms for which the two shared electrons in the bond come from the same atom.

In Summary

It seems that it is from certain chemical compounds built on a skeleton of carbon atoms rolled up in gigantic molecular structures that the activity of matter changes in nature.

The assembly and shape of these structures determine the chemical functions that will be part of a manufacturing plan (the most parsimonious and simplest way to self-replicate). This manufacturing plan possesses all the information that will allow the system to be reconstructed.

It is from this moment that life starts and the system is considered living.

Hermann von Helmholtz (1821 − 1894) "If all our efforts to provoke the birth of organisms from inert matter fail, it seems to me that we are proceeding in a perfectly justified manner by asking whether life ever had an origin, whether it is not as old as matter, and whether its germs, transported from one celestial body to another, have not developed wherever they found favorable soil?"

N.B.:
The electronegativity of an element is a quantity that characterizes its ability to attract electrons during the formation of a chemical bond with another element.