Distinguishing a rock from a rabbit is simple, but the line blurs at microscopic and theoretical levels. Since no single definition of life is universally accepted, science uses a collection of shared characteristics to form a working definition. This framework is challenged by entities on the edge of biology and is expanding to accommodate the search for life beyond Earth and within the digital world.
The Core Biological Consensus
Living things are structurally coordinated entities composed of one or more cells, which are the basic units of life. While a consensus definition remains elusive, biologists identify several properties that collectively characterize a living entity. These characteristics provide a framework for identifying and studying life as we know it.
- Organization: Life exhibits a high degree of organization. In the simplest organisms, atoms form molecules that assemble into complex cellular components. In multicellular organisms, this hierarchy continues as cells form tissues, tissues create organs, and organs work together in organ systems.
- Metabolism: Organisms process energy through a sum of chemical reactions. They take in energy and matter from their environment to fuel their activities. This includes catabolism, the breakdown of molecules to release energy, and anabolism, the use of energy to construct components like proteins.
- Homeostasis: Living organisms maintain a stable internal environment despite external fluctuations. Cells require specific conditions, such as a stable temperature and pH, and organisms have regulatory mechanisms to keep these variables within a narrow range.
- Growth and Development: Living things grow and develop according to genetic instructions encoded in their DNA. Growth is an increase in size, while development is the transformation an organism undergoes through its life cycle.
- Response to Stimuli: Organisms react to changes in their environment. This can range from a plant turning toward light to a bacterium moving away from a toxin. This ability allows organisms to adapt and improve their chances of survival.
- Reproduction: Organisms produce new individuals, passing their genetic material to their offspring. This occurs through asexual reproduction, creating genetically identical offspring, or sexual reproduction, which involves two parents and results in genetically unique offspring.
- Evolutionary Adaptation: Populations of organisms adapt over time. Through natural selection, individuals with traits better suited to their environment are more likely to survive and reproduce, passing those traits to the next generation.
The Chemical and Thermodynamic Basis of Life
Principles of chemistry and physics govern life’s existence. The chemistry of all known life is built upon carbon. Carbon’s ability to form four stable covalent bonds allows it to create the vast array of complex molecules, like proteins and nucleic acids, that provide the structural and functional foundation for cells.
Water serves as the solvent in which the biochemical reactions of life occur. Its polarity makes it an excellent medium for dissolving the salts, sugars, and other molecules necessary for metabolism. Water’s thermal properties also help organisms regulate temperature and maintain homeostasis.
Thermodynamically, life is a system that maintains a state of high order, or low internal entropy, by constantly working against the natural tendency toward disorder. To achieve this, organisms must continuously take in energy from their environment. An organism maintains its internal order by converting energy from sources like food or sunlight. It does this by releasing heat and waste into its surroundings, thereby increasing the total entropy of the universe, allowing life to persist as a localized pocket of order.
Challenging the Boundaries
Certain biological entities challenge the consensus characteristics of life by existing in a gray area between living and non-living. These boundary cases test the rigidity of our definitions and reveal that the line between complex chemistry and simple biology is not always clear.
Viruses
Viruses are a classic example of this ambiguity. They possess genetic material (DNA or RNA) and evolve through natural selection. However, they lack a cellular structure and have no metabolic machinery of their own. To reproduce, a virus must infect a host cell and hijack its biochemical processes. Outside of a host, a virus is an inert chemical complex, fueling the debate about whether it is a simple life form or a non-living replicator.
Prions
Prions are infectious agents composed solely of misfolded proteins. They contain no genetic material, yet they “replicate” by inducing normally folded proteins to adopt their own misfolded shape, which can lead to neurodegenerative diseases. Prions demonstrate that replication of a structural state is possible without DNA or RNA, showing that propagation alone does not equate to being alive.
Viroids
Viroids further blur the line between chemistry and biology. Simpler than viruses, they consist only of a short, circular strand of RNA without a protective protein coat. Like viruses, they can only replicate inside a host cell. Viroids and other subviral agents underscore that processes like replication and evolution can be carried out by simple molecular structures that lack most other characteristics of life.
Expanding the Definition for Astrobiology
Astrobiology, the search for life beyond Earth, requires re-evaluating our definition of life. Because all known biology is terrestrial, our criteria are based on a single example. Our definition may be too narrow and Earth-centric, potentially causing us to overlook life that does not conform to our familiar biological model.
A primary consideration is the potential for alternative chemistries. While life on Earth is carbon-based, life could arise from different chemical foundations. Scientists have speculated about life based on silicon, an element with chemical properties similar to carbon, which could form the backbone of biological molecules in different environments.
Solvents other than water could also support life. On frigid moons like Saturn’s Titan, vast lakes of liquid methane and ethane exist. In such an environment, a different kind of biology could thrive, using these hydrocarbons as a solvent. Exploring these possibilities requires focusing on the functional capabilities a chemical system would need to be considered alive.
This has led to searching for “agnostic biosignatures”—signs of life not tied to a specific molecular framework like DNA. Instead of looking for specific molecules, scientists might search for evidence of complex structures unlikely to form through geological processes. Another approach is to look for signs of thermodynamic disequilibrium in a planet’s atmosphere, which could point to the presence of a metabolizing biosphere.
The Frontier of Artificial Life and Intelligence
Computing and robotics introduce new entities that challenge our understanding of life from a different perspective. These frontiers raise questions about whether life must be biological or if the same processes could exist in a digital or mechanical form. This forces a re-examination of life’s foundational criteria.
Artificial life (A-life) creates software simulations and robotic systems that exhibit life-like behaviors. These digital organisms can demonstrate self-replication, adaptation, and a form of metabolism where they process information to maintain their existence. Some A-life programs even undergo evolution, with their code mutating and being subjected to selection pressures.
Artificial General Intelligence (AGI) presents a more complex case. If an advanced AI were to achieve consciousness and demonstrate all the functional characteristics of life—using electricity for metabolism, learning, and reproducing its code—it would be difficult to argue that it is not alive. Such a scenario would compel a shift in our definition away from specific physical structures and toward the processes themselves.
These developments question if a cellular body is a prerequisite for life, or if the defining characteristic is the complex processing of information and energy to sustain order against entropy. The frontier of artificial intelligence suggests that life might be a property of organization and process, independent of its physical substrate. This reinforces the idea that defining life is a dynamic challenge that evolves with our scientific knowledge.