The question of “What is life?” has challenged scientists and philosophers for centuries, resisting a simple, concise answer. Biology operates with an understanding of what life does, rather than a single, universally accepted definition of what it is. Any characterization of life must therefore be an imperfect list of properties observed in terrestrial organisms. The complexity of the natural world ensures that any strict set of rules will inevitably be challenged by exceptions and entities existing in a transitional space between living and non-living matter. Setting the criteria for life is a continuous process of observation and refinement.
The Core Characteristics of Living Systems
Living systems are characterized by a set of observable properties that, when considered collectively, distinguish them from inanimate matter. A fundamental characteristic is organization, as all known life is composed of one or more cells, the basic structural and functional units of an organism. Within these cells, molecules are arranged into highly structured organelles, forming a coordinated system that maintains an ordered state.
Organisms exhibit homeostasis, the ability to regulate an internal environment to maintain a stable, constant condition. This internal balance, such as maintaining a specific body temperature or pH level, is achieved despite fluctuating external conditions. Related to this stability is the capacity for growth and development, involving an organism increasing in size and complexity through controlled cell division and differentiation.
All living systems possess the ability to reproduce, passing on hereditary material to offspring, ensuring the continuation of the species. Reproduction can be sexual, involving the combination of genetic material, or asexual, creating genetically identical copies. Organisms also demonstrate sensitivity, or a response to stimuli, reacting to changes in their environment, such as a plant growing toward light. Finally, adaptation and evolution are defining features, where populations change over generations to become better suited to their environment through natural selection.
When the Definition Fails: Borderline Cases
The complexity of nature reveals entities that possess some, but not all, of life’s fundamental characteristics, complicating any strict definition. Viruses are the most frequently cited example; they contain genetic material (DNA or RNA) and are capable of evolution, aligning with criteria for adaptation and inheritance. However, a virus is an acellular particle, existing as an inert virion outside a host cell and lacking the machinery for independent metabolism.
Viruses cannot generate their own energy or synthesize the proteins required for replication. Instead, they must hijack the metabolic pathways of a host cell to force the production of new viral particles. Because they lack cellular structure and self-sustaining energy processing, they fail to meet fundamental requirements for life. This obligate parasitism places them squarely on the boundary, exhibiting replication and evolution only when they commandeer a living system.
A more extreme example is the prion, a proteinaceous infectious particle that causes neurodegenerative diseases such as Creutzfeldt-Jakob disease. Prions are misfolded versions of a normal cellular protein that contain no nucleic acid (no DNA or RNA), carrying no genetic instructions. The infectious particle propagates by inducing normally folded proteins to change their shape into the abnormal, disease-causing conformation. This process acts as a form of self-propagation without genetic material, metabolism, or cellular structure, challenging the idea that reproduction requires nucleic acids.
The Engine of Life: Energy and Metabolism
The ability to maintain the high degree of organization characteristic of life is directly dependent on a continuous input of energy, a process known as metabolism. Living organisms function as open thermodynamic systems, constantly exchanging both energy and matter with their surroundings. According to the second law of thermodynamics, all systems naturally tend toward increased disorder, or entropy.
Life successfully counteracts this universal tendency by constantly acquiring energy from an external source to maintain a state of low internal entropy. This energy is processed through complex, highly regulated chemical reactions, often resulting in the production of adenosine triphosphate (ATP), the universal energy currency of the cell. The continuous use of ATP fuels the cellular work required for homeostasis, growth, and movement, maintaining a dynamic disequilibrium with the environment.
Organisms employ two main strategies for energy acquisition, which broadly define metabolic types. Autotrophs, such as plants and certain bacteria, capture energy directly from non-living sources, primarily sunlight through photosynthesis, converting it into chemical energy. Conversely, heterotrophs, including animals and fungi, obtain their energy by consuming other organisms or organic matter, breaking down complex molecules to release stored chemical energy.
Defining Life for Astrobiology
The search for life beyond Earth requires a definition not rigidly based on the specific, carbon-based biochemistry of terrestrial organisms. NASA’s working definition for astrobiology is that life is “a self-sustaining chemical system capable of Darwinian evolution.” This definition intentionally focuses on the abstract processes of self-maintenance and heritable change, making it independent of Earth’s specific molecular building blocks, such as DNA or liquid water.
Astrobiologists look for evidence of life’s processes, rather than its specific forms, recognizing that extraterrestrial life may be built from different elements or solvents. Concepts like “dynamic disequilibrium” are considered powerful indicators, suggesting that a system is actively working to maintain a difference in chemical or physical concentration with its environment. This non-Earth-centric approach allows for the possibility of life based on silicon chemistry or life that utilizes solvents other than water, expanding the potential targets for exploration.