Life is a characteristic that distinguishes physical entities that do have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate.

Biological processes are the processes vital for a living organism to live.

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Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria.

A protist is any eukaryotic organism that is not an animal, plant or fungus.

Archaea constitute a domain of single-celled microorganisms.

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The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential artificial life as "living".

Viroids are the smallest infectious pathogens known.


Biology is the primary science concerned with the study of life, although many other sciences are involved.

Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development and evolution.

Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, evolution, distribution, identification and taxonomy.


The definition of life is controversial.


The current definition is that organisms maintain homeostasis, are composed of cells, undergo metabolism, can grow, adapt to their environment, respond to stimuli, and reproduce.

Homeostasis is the property of a system within the body of an organism in which a variable, such as the concentration of a substance in solution, is actively regulated to remain very nearly constant.


However, many other biological definitions have been proposed, and there are some borderline cases of life, such as viruses.


Throughout history, there have been many attempts to define what is meant by "life" and many theories on the properties and emergence of living things, such as materialism, the belief that everything is made out of matter and that life is merely a complex form of it; hylomorphism, the belief that all things are a combination of matter and form, and the form of a living thing is its soul; spontaneous generation, the belief that life repeatedly emerges from non-life; and vitalism, a now largely discredited hypothesis that living organisms possess a "life force" or "vital spark".

Vitalism is the belief that "living organisms are fundamentally different from non-living entities because they contain some non-physical element or are governed by different principles than are inanimate things".a Where vitalism explicitly invokes a vital principle, that element is often referred to as the "vital spark", "energy" or "élan vital", which some equate with the soul.

Materialism is a form of philosophical monism which holds that matter is the fundamental substance in nature, and that all things, including mental states and consciousness, are results of material interactions.

Spontaneous generation refers to an obsolete body of thought on the ordinary formation of living organisms without descent from similar organisms.


Modern definitions are more complex, with input from a diversity of scientific disciplines.


Biophysicists have proposed many definitions based on chemical systems; there are also some living systems theories, such as the Gaia hypothesis, the idea that the Earth itself is alive.

The Gaia hypothesis, also known as the Gaia theory or the Gaia principle, proposes that living organisms interact with their inorganic surroundings on Earth to form a synergistic and self-regulating, complex system that helps to maintain and perpetuate the conditions for life on the planet.

Earth is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to harbor life.

Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena.


Another theory is that life is the property of ecological systems, and yet another is elaborated in complex systems biology, a branch or subfield of mathematical biology.

Complex systems biology is a branch or subfield of mathematical and theoretical biology concerned with complexity of both structure and function in biological organisms, as well as the emergence and evolution of organisms and species, with emphasis being placed on the complex interactions of, and within, bionetworks, and on the fundamental relations and relational patterns that are essential to life.

An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system.

Systems biology is the computational and mathematical analysis and modeling of complex biological systems.


Abiogenesis describes the natural process of life arising from non-living matter, such as simple organic compounds.


Properties common to all organisms include the need for certain core chemical elements to sustain biochemical functions.

Biochemistry, sometimes called biological chemistry, is the study of chemical processes within and relating to living organisms.

A chemical element or element is a species of atoms having the same number of protons in their atomic nuclei.


Life on Earth first appeared as early as 4.28 billion years ago, soon after ocean formation 4.41 billion years ago, and not long after the formation of the Earth 4.54 billion years ago.


Earth's current life may have descended from an RNA world, although RNA-based life may not have been the first.

The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins.

Ribonucleic acid is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes.


The mechanism by which life began on Earth is unknown, though many hypotheses have been formulated and are often based on the Miller–Urey experiment.

The Miller–Urey experiment was a chemical experiment that simulated the conditions thought at the time to be present on the early Earth, and tested the chemical origin of life under those conditions.


The earliest known life forms are microfossils of bacteria.

The earliest known life forms on Earth are putative fossilized microorganisms found in hydrothermal vent precipitates.

A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age.


In July 2016, scientists reported identifying a set of 355 genes thought to be present in the last universal common ancestor of all living organisms.

The last universal common ancestor, also called the last universal ancestor, or concestor, is the most recent population of organisms from which all organisms now living on Earth have a common descent, the most recent common ancestor of all current life on Earth.


Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale.

The geologic time scale is a system of chronological dating that relates geological strata to time.


To survive in most ecosystems, life must often adapt to a wide range of conditions.

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