Updated on: Nov 25, 2025
Life
Life is a multi-faceted concept with no simple definition; this article is confined to the primary meanings in biology; articles on life in other senses are included in the article life (disambiguation).
Defining the concept of life
Life has several senses with biological meaning
- “life” may refer to the ongoing process of which living things are a part;
- “life” may refer to the period between birth and death of an organism;
- “life” may refer to the state of something that has been born and has yet to die, i.e., that which makes a living thing alive;
The remainder of this section focuses on the last biological sense — how can one tell when an entity is a living thing?
It would be relatively straightforward to offer a practical set of guidelines if one’s only concern was life on Earth as we know it (see biosphere), but as soon as one considers questions about life’s origin on Earth, or the possibility of extraterrestrial life, or the concept of artificial life, it becomes clear that the question is fundamentally difficult and comparable in many respects to the problem of defining intelligence.
A conventional definition
In biology, an entity has traditionally been considered to be alive if it exhibits all the following phenomena at least once during its existence:
- Growth
- Metabolism, consuming, transforming, and storing energy/mass; growing by absorbing and reorganizing mass; excreting waste
- Motion, either moving itself or having internal motion
- Reproduction,is the ability to create entities that are similar to itself
- Response to stimuli – the ability to measure properties of its surrounding environment, and act upon certain conditions.
These criteria are not without their uses, but their disparate nature makes them unsatisfactory from a number of perspectives; in fact, it is not difficult to find counterexamples and examples that require further elaboration. For example, according to the above definition, one could say:
- fire is alive. (This could be remedied by adding the requirement of locality, where there is an obvious feature that delineates the spatial extension of the living being, such as a cell membrane.)
- male mules are not alive as they are sterile and cannot reproduce.
- viruses are not alive as they do not grow and cannot reproduce outside of a host cell.
Biologists who are content to focus on terrestrial organisms often note some additional signs of a “living organism”, including these:
- Living organisms contain molecular components such as carbohydrates, lipids, nucleic acids, and proteins.
- Living organisms require both energy and matter in order to continue living.
- Living organisms are composed of at least one cell.
- Living organisms maintain homeostasis.
- Species of living organisms will evolve.
All life on Earth is based on the chemistry of carbon compounds. Some assert that this must be the case for all possible forms of life throughout the universe; others describe this position as ‘carbon chauvinism’.
Other definitions
Other definitions include:
- Francisco Varela and Humberto Maturana’s definition of life (also widely used by Lynn Margulis) as an autopoietic (self-producing), water-based, lipid-protein bound, carbon metabolic, nucleic acid replicated, protein readout system
- “a system of inferior negative feedbacks subordinated to a superior positive feedback” (J. theor Biol. 2001)
- Tom Kinch’s definition of life as a highly organized auto-cannibalizing system naturally emerging from conditions common on planetary bodies, and consisting of a population of replicators capable of mutation, around each set of which a homeostatic metabolizing organism, which actively helps reproduce and/or protect the replicator(s), has evolved
- Stuart Kauffman’s definition of life as an autonomous agent or a multi-agent system capable of reproducing itself or itself, and of completing at least one thermodynamic work cycle
- Robert Pirsig’s definition of life, found in his book, as that which maximizes its range of possible futures, in other words, that which makes decisions that result in the most future choices, or that which strives to keep its options open.
Descent with modification: a “useful” characteristic
A useful characteristic upon which to base a definition of life is that of descent with modification: the ability of a life form to produce offspring that are like its parent or parents, but with the possibility of some variation due to chance. Descent with modification is sufficient by itself to allow evolution, assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called genetics. In all known life forms (assuming prions are not counted as such), the genetic material is primarily DNA or the related molecule, RNA. Another exception might be the software code of certain forms of viruses and programs created through genetic programming, but whether computer programs can be alive even by this definition is still a matter of some contention.
Exceptions to the common definition
Note that many individual organisms are incapable of reproduction and yet are still generally considered to be “alive”; see mules and ants for examples. However, these exceptions can be accounted for by applying the definition of life on the level of entire species or of individual genes. (For example, see kin selection for information about one way by which non-reproducing individuals can still enhance the spread of their genes and the survival of their species.)
Viruses reproduce, flames grow, some software programs mutate and evolve, future software programs will probably evince (even high-order) behavior, machines move, and proto-life, consisting of metabolizing cells without reproduction apparatus, can have exist. Still, some would not call these entities alive. Generally, all five characteristics are required for a population to be considered alive.
Origin of life
There is no truly “standard” model of the origin of life, however, most currently accepted models build in one way or another upon the following discoveries, which are listed in rough order of postulated emergence:
- Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Urey-Miller experiment.
- Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
- Procedures for producing random RNA molecules can produce “ribozymes”, which are able to produce more of themselves under very specific conditions.
There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many of the possibilities have tended to fall into either “genes-first” or “metabolism-first”; a recent trend is the emergence of hybrid models that combine aspects of both.
The possibility of extraterrestrial life
As of 2004, Earth is the only planet in the universe known by humans to support life. The question of whether life exists elsewhere in the universe remains open, but analyses such as the Drake equation have been used to estimate the probability of such life existing. There have been a number of false alarms of life elsewhere in the universe, but none of these apparent discoveries have so far survived scientific scrutiny.
Today, the closest that scientists have come to finding extraterrestrial life is fossil evidence of possible bacterial life on Mars (via the ALH84001 meteorite). Searches for extraterrestrial life are currently focusing on planets and moons believed to possess liquid water, at present or in the past. Recent evidence from the NASA rovers Spirit and Opportunity supports the theory that Mars once had surface water.
Jupiter’s moons are also considered good candidates for extraterrestrial life, especially Europa, which seems to possess oceans of liquid water.
Bibliography of the Biology of Life
Below is a carefully curated list of books, organized by topic, focusing on the biology of life. These works span molecular biology, evolution, ecology, and more, providing a comprehensive perspective on life sciences.
Foundational Texts in Biology
- “Molecular Biology of the Cell” by Bruce Alberts et al. (6th edition, 2014)
- Why Read This? A definitive guide to cellular processes, from the structure of the cell to the regulation of gene expression. It’s a cornerstone for understanding life’s molecular machinery.
- “Biology” by Neil A. Campbell and Jane B. Reece (12th edition, 2021)
- Why Read This? This comprehensive textbook is ideal for students and professionals, covering all aspects of biology, from basic principles to advanced topics.
- “Principles of Biochemistry” by David L. Nelson and Michael M. Cox (7th edition, 2017)
- Why Read This? An essential guide to the molecular basis of life, exploring metabolism, bioenergetics, and the role of macromolecules in living systems.
Biochemistry and Molecular Biology
- “Biochemical Pathways: An Atlas of Biochemistry and Molecular Biology” by Gerhard Michal and Dietmar Schomburg (2nd edition, 2012)
- Why Read This? A visual guide to the complex biochemical pathways that sustain life, ideal for researchers and students.
- “Lehninger Principles of Biochemistry” by David L. Nelson and Michael M. Cox (8th edition, 2021)
- Why Read This? A deep dive into biochemistry, focusing on the chemical processes that underpin biological systems.
- “The Machinery of Life” by David S. Goodsell (2nd edition, 2009)
- Why Read This? A beautifully illustrated book that brings molecular biology to life, showing how cellular components work together.
Evolution and the Origin of Life
- “The Origin of Species” by Charles Darwin (1859)
- Why Read This? A groundbreaking text that introduced the theory of natural selection, foundational for understanding evolutionary biology.
- “The Selfish Gene” by Richard Dawkins (1976, revised 2016)
- Why Read This? This influential book popularizes the gene-centric view of evolution, explaining how genes drive the behavior and evolution of organisms.
- “The Vital Question: Energy, Evolution, and the Origins of Complex Life” by Nick Lane (2015)
- Why Read This? Lane explores the connection between bioenergetics and the evolution of complex life forms.
- “The RNA World” edited by Raymond F. Gesteland, Thomas R. Cech, and John F. Atkins (4th edition, 2006)
- Why Read This? Essential for understanding the hypothesis that life originated from self-replicating RNA molecules.
Cell Biology and Genetics
- “Essential Cell Biology” by Alberts et al. (5th edition, 2019)
- Why Read This? A user-friendly introduction to cell biology, perfect for students new to the field.
- “Genomes 4” by Terry A. Brown (4th edition, 2017)
- Why Read This? A thorough exploration of genome biology, from sequencing to function, essential for understanding modern genetics.
- “Genome: The Autobiography of a Species in 23 Chapters” by Matt Ridley (1999)
- Why Read This? A compelling narrative of human genetics, exploring the stories encoded in our DNA.
Physiology and Systems Biology
- “Guyton and Hall Textbook of Medical Physiology” by John E. Hall (14th edition, 2020)
- Why Read This? A comprehensive reference for understanding the physiological processes of the human body.
- “Biophysics: Searching for Principles” by William Bialek (2012)
- Why Read This? Explores the physics underlying biological systems, making it ideal for readers interested in the intersection of biology and physics.
Ecology and Environmental Biology
- “Ecology: The Economy of Nature” by Robert E. Ricklefs and Rick Relyea (8th edition, 2020)
- Why Read This? A foundational text that introduces ecological principles and the interdependence of life.
- “Silent Spring” by Rachel Carson (1962)
- Why Read This? A classic that brought environmental awareness to the forefront, emphasizing the impact of human activity on ecosystems.
Popular Science and Interdisciplinary Perspectives
- “Life on the Edge: The Coming of Age of Quantum Biology” by Jim Al-Khalili and Johnjoe McFadden (2014)
- Why Read This? An engaging look at the role of quantum mechanics in biological processes, such as photosynthesis and enzyme action.
- “Power, Sex, Suicide: Mitochondria and the Meaning of Life” by Nick Lane (2005)
- Why Read This? Explains the central role of mitochondria in energy production and evolutionary biology.
- “Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body” by Neil Shubin (2008)
- Why Read This? A fascinating exploration of evolutionary biology, linking the anatomy of humans to their ancient ancestors.
Why This Bibliography?
This collection covers the breadth of biology, from foundational theories to cutting-edge research. Whether you’re a student, researcher, or general reader, these books provide essential knowledge about life’s complexity, origins, and interconnectedness. Each book offers a unique perspective, making it a valuable addition to any reader’s exploration of life sciences. It is an invitation to read and discover more on the subject.
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