From Matter to Life: The Blurred Frontier of Biological Emergence

A blurred threshold between the inert and the living

A progressive complexification of matter

Life is not a trivial property of matter. It does not emerge abruptly, but seems to appear at the end of a progressive complexification of chemical structures. The origin of the living lies in a grey zone, between chemistry and biology, thermodynamics and information.

From viruses to self-organized mineral structures

If a living cell is unambiguously a living system, what about viruses, prions or certain self-organized mineral structures that mimic biological growth? These systems possess certain properties of the living—growth, reproduction, transmission of structural defects—but none of them combines them all. They illustrate the gradual nature of the boundary between inertia and life.

The physicochemical definition of the living

A set of criteria rather than a single definition

Rather than a single, clear-cut definition, biologists today favor a set of criteria. A living being must exhibit a certain number of fundamental characteristics: the ability to metabolize energy and to reproduce, but also to react to its environment and to evolve. More precisely, six operational criteria make it possible to characterize the living: compartmentalization (delimitation of an interior and an exterior), metabolism (flow of energy and matter), transmissible information (genome or copyable structure), replication with variation, Darwinian evolution (natural selection), and maintenance far from thermodynamic equilibrium. These criteria are not all present from the outset: they may have emerged successively, in stages.

The ambiguity of criteria at the frontiers of the living

However, these criteria remain ambiguous at the frontier of the living. A virus, for example, does not possess its own metabolism but can, once inside a host cell, reproduce and evolve. Viruses are generally placed at the frontier of the living: they possess genetic information and evolve, but depend entirely on a host cell to replicate and have no autonomous metabolism. Conversely, a crystal can grow and replicate, without ever being alive.

Thermodynamic frontiers: improbable order

Life, a dissipative system far from equilibrium

Life is a dissipative system far from equilibrium that locally maintains order by consuming free energy and exporting entropy to its environment. It does not contradict the second law of thermodynamics: it exploits its laws. Just as a refrigerator extracts heat from a cold compartment by consuming electricity, a cell maintains its internal order by dissipating heat and waste to the outside. The physicist Ilya Prigogine showed that dissipative structures far from equilibrium are a general phenomenon, of which the living is only a particularly complex manifestation.

Highly improbable macromolecules

This dynamics rests on a highly improbable chemical structure: macromolecules capable of self-replication. Without this property, no self-organized system can claim the status of living. Biological order is not a negation of entropy, but its corollary: the living is a machine for producing disorder around itself in order to preserve order within itself.

Comparison of structures close to the living

The following table compares several entities situated at the frontier of the living, according to their capacity for reproduction, their metabolism, their internal organization and their biological status.

The hypothesis of intermediate worlds

Systems neither truly living nor totally inert

Modern hypotheses, such as that of prebiotic life or the RNA world, postulate the existence of intermediate systems: neither truly living nor totally inert. These systems may have possessed a rudimentary capacity for self-replication, evolution, or even catalysis, opening the way to a phase transition toward the living cell.

The Awakening of Matter: Information at the Heart of the Living

Information, a fundamental property of the living

One of the most fundamental characteristics of the living is its capacity to store, process and transmit information. DNA, often compared to a code, is a molecule but also a vector of instructions. This duality between physical support (nitrogenous base, phosphate, sugar) and informational content (coding sequences, regulations) is specific to biology and absent in inert matter.

An algorithmic and evolutionary logic

The living rests on an algorithmic logic: transcription, translation, feedback control, intracellular signaling. These operations do not merely transform energy or matter, they also manage the continuity of an evolutionary program. Even viruses, although at the frontier of the living, use genetic information as a vector of replication and evolution.

Informed matter and evolutionary memory

Life could thus be defined as informed matter capable of self-maintenance and self-replication with variation. Unlike a crystal, whose order is static and without memory, a living cell possesses a genome that encodes functions, which can mutate, be corrected or selected. This informational approach to the living links biology, thermodynamics and computation theory.

In this sense, the appearance of life on Earth marks a critical transition: that from a blind chemistry to a chemistry endowed with evolutionary memory. The living is therefore not only matter that reacts, but matter that projects itself into the future while preserving the past.

Protocells: compartmentalization, energy and information

Lipid vesicles: spontaneous membranes

Protocells are self-organized structures that concentrate biomolecules and isolate them from the environment. In water, amphiphilic molecules such as fatty acids spontaneously assemble into bilayers forming vesicles, without any enzyme. These prebiotic membranes may have appeared from simple fatty acids, synthesized under prebiotic conditions and detected on meteorites.

Coacervates: compartments without membranes

Coacervates are droplets resulting from liquid-liquid phase separation, formed of peptides or nucleic acids. Described as early as 1929 by Oparin, they concentrate certain molecules, grow and divide mechanically. Without complete heredity, they offer a microenvironment favorable to chemical reactions.

Mineral catalysts and membrane formation

In 2024, Neal Devaraj's team showed that cysteine and a thioester spontaneously form lipids in the presence of silica. These lipids then assemble into protocell-type vesicles, stable enough to shelter biochemical reactions. Silica acts as a mineral catalyst, suggesting a plausible path from mineral chemistry to biological compartmentalization.

Coupling between compartment and replicative molecules

Compartmentalization alone is not enough: there must also be molecules capable of storing information and replicating, such as RNA. Jack Szostak's work has shown that fatty acid vesicles can encapsulate RNA, while montmorillonite greatly increases vesicle formation and helps RNA polymerization. It is in this coupling between compartment, energy and information that the true threshold of the living may lie.

From the RNA world to the DNA-protein world

Ribozymes: RNA that catalyzes

The RNA world hypothesis proposes that before DNA and proteins, RNA served both as an information carrier and as a catalyst. The discovery of ribozymes, awarded the 1989 Nobel Prize in Chemistry, confirmed that a single molecule can carry information and catalyze reactions.

The ribosome: a living fossil of the RNA world

In the ribosome, it is ribosomal RNA, not proteins, that catalyzes the formation of peptide bonds. The ribosome is therefore in part a ribozyme, a vestige of the RNA world. The transition to proteins occurred through progressive delegation, with RNA retaining the oldest function.

The transition to DNA

DNA replaced RNA as the carrier of genetic information because it is chemically more stable. RNA, more fragile, remains a temporary intermediate, while DNA ensures long-term memory. This separation of roles—DNA for storage, RNA for the message, proteins for catalysis—characterizes all modern biology.

LUCA: the last universal common ancestor

All current forms of life descend from a last universal common ancestor, LUCA. A 2024 study estimates that it lived about 4.2 billion years ago, with a genome of about 2.75 Mb encoding nearly 2,600 proteins. Anaerobic and probably acetogenic, LUCA was already part of an established ecosystem, engaged in an arms race with viruses. Its very early appearance suggests that several steps of the living rest on plausible physicochemical mechanisms, even if the cosmic frequency of this emergence remains unknown.

Since life is chemical, where is the boundary with the inanimate?

An elusive boundary

One might think that, as a physicochemical phenomenon, life should emerge at a clearly identifiable threshold of molecular complexity. Yet this boundary remains elusive. The living does not emerge by the brutal addition of a "magic molecule," but by a progressive transition in which self-organization, catalytic reactions, compartmentalization, and information processing are interwoven.

An emergent property of a network of functions

This absence of a clear threshold is explained by the continuous nature of chemical self-organization processes. In a universe governed by thermodynamics and the laws of complexity, certain structures can temporarily self-maintain, without being capable of Darwinian evolution. Life is therefore an emergent property, not of a single compound, but of a network of functions: replication, variation, selection.

This blur explains why definitions of the living vary across disciplines: a biologist will insist on reproduction, a chemist on autocatalysis, a physicist on far-from-equilibrium dynamics, a computer scientist on the capacity to process and transmit information. The boundary is therefore not a line, but a transition zone, a space of complexity where matter begins to act upon itself.

Is the tenacity of life not proof that it is present everywhere in the Universe?

From extremophiles to extreme environments

On Earth, life has manifested itself in conditions once thought incompatible with its existence: high-pressure hydrothermal vents, acidic lakes, deep rocks several kilometers below the surface, hyperarid deserts, strongly irradiated environments or those rich in radionuclides. These extremophiles defy our former habitability criteria and considerably broaden the spectrum of potentially livable environments. Among them, thermophiles such as Pyrolobus fumarii survive above 100 °C, acidophiles such as Picrophilus oshimae tolerate pH close to zero, and Deinococcus radiodurans withstands radiation doses of several thousand grays.

Tardigrades: champions of survival

Tardigrades, small microscopic animals, are not technically extremophiles in the strict sense—they tolerate extreme conditions without thriving in them—but they hold survival records. By entering a state of cryptobiosis, they can suspend their metabolism and survive temperatures ranging from -272 °C to +149 °C, the vacuum of space, ionizing radiation and prolonged dehydration. Tardigrades were exposed to the vacuum of space in 2007 (FOTON-M3 mission) and survived, although exposure to solar ultraviolet proved much more lethal.

A robust phenomenon, but not universal

This resilience suggests that life is not a fragile accident, but a robust phenomenon, capable of adapting to extreme chemical and thermal gradients, provided a source of energy and complex molecules are available. However, the robustness of terrestrial extremophiles demonstrates that life, once it appeared, can adapt to extreme conditions, but this does not prove that the appearance of life itself is a frequent phenomenon in the Universe. The distinction between the adaptability of the living and the probability of its emergence remains one of the most fascinating open questions in astrobiology.

Toward extraterrestrial life?

On Mars, the Perseverance rover (Mars 2020 mission) is actively searching for traces of ancient biosignatures, dating from the time when the red planet harbored liquid water on its surface. Farther away, the Europa Clipper mission, launched by NASA in October 2024, is heading toward Europa, Jupiter's icy moon, whose subsurface ocean could offer the conditions necessary for life. The probe will perform 49 close flybys starting in 2030 to determine whether habitable environments exist beneath the surface. However, the universality of life remains a hypothesis: no biosignature confirmed by consensus has been detected on an exoplanet to date. The tenacity of life on Earth constitutes strong evidence, but not yet direct proof of its cosmic generality.