Life (cf. biota) is a characteristic that distinguishes objects that have signaling and self-sustaining processes (biology) from those that do not, either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate.
In biology, the science of living
Organisms,
Life is the condition which distinguishes active organisms from inorganic matter. Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means. A diverse array of living organisms (
Life forms) can be found in the biosphere on Earth, and the properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information.
In philosophy and religion, the conception of
Life and its nature varies. Both offer interpretations as to how
Life relates to existence and consciousness, and both touch on many related issues, including
Life stance, purpose, conception of a god or gods, a soul or an after
Life.
Early theories about life
Some of the earliest theories of
Life were materialist, holding that all that exists is matter, and that all
Life is merely a complex form or arrangement of matter. Empedocles (430 BC) argued that every thing in the universe is made up of a combination of four eternal "elements" or "roots of all": earth, water, air, and fire. All change is explained by the arrangement and rearrangement of these four elements. The various forms of
Life are caused by an appropriate mixture of elements. For example, growth in plants is explained by the natural downward movement of earth and the natural upward movement of fire.
Democritus (460 BC), the disciple of Leucippus, thought that the essential characteristic of
Life is having a soul (psyche). In common with other ancient writers, he used the term to mean the principle of living things that causes them to function as a living thing. He thought the soul was composed of fire atoms, because of the apparent connection between
Life and heat, and because fire moves. He also suggested that humans originally lived like animals, gradually developing communities to help one another, originating language, and developing crafts and agriculture.
In the scientific revolution of the 17th century, mechanistic ideas were revived by philosophers like Descartes.
Definitions
It is still a challenge for scientists and philosophers to define
Life in unequivocal terms. Defining
Life is difficult—in part—because
Life is a process, not a pure substance. Any definition must be sufficiently broad to encompass all
Life with which we are familiar, and it should be sufficiently general that, with it, scientists would not miss
Life that may be fundamentally different from earthly
Life.
Biology
Since there is no unequivocal definition of
Life, the current understanding is descriptive, where
Life is a characteristic of organisms that exhibit all or most of the following phenomena:
1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
2. Organization: Being structurally composed of one or more cells, which are the basic units of
Life.
3. Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with
Life.
4. Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and by chemotaxis.
7. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms.
Biophysics
Biophysicists have also commented on the nature and qualities of
Life forms—notably that they function on negative entropy. In more detail, according to physicists such as John Bernal, Erwin Schrödinger, Eugene Wigner, and John Avery,
Life is a member of the class of phenomena which are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form (see: entropy and
Life).
Gaia hypothesis
The idea that the Earth is alive is probably as old as humankind, but the first public expression of it as a fact of science was by a Scottish scientist, James Hutton. In 1785 he stated that the Earth was a superorganism and that its proper study should be physiology. Hutton is rightly remembered as the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th century. The Gaia hypothesis, originally proposed in the 1960s by scientist James Lovelock, explores the idea that the
Life on Earth functions as a single organism which actually defines and maintains environmental conditions necessary for its survival.
Life as a property of ecosystems
A systems view of
Life treats environmental fluxes and biological fluxes together as a "reciprocity of influence", and a reciprocal relation with environment is arguably as important for understanding
Life as it is for understanding ecosystems. As Harold J. Morowitz (1992) explains it,
Life is a property of an ecological system rather than a single organism or species. He argues that an ecosystemic definition of
Life is preferable to a strictly biochemical or physical one. Robert Ulanowicz (2009) also highlights mutualism as the key to understand the systemic, order-generating behavior of
Life and ecosystems.
Extinction
Extinction is the gradual process by which a group of taxa or species dies out, reducing biodiversity. The moment of extinction is generally considered to be the death of the last individual of that species. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively after a period of apparent absence. Species become extinct when they are no longer able to survive in changing habitat or against superior competition. Over the history of the Earth, over 99% of all the species that have ever lived have gone extinct, however, mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.
See :
Life,
Domain,
Kingdom,
Phylum,
Class,
Order,
Family,
Genus, and
Species
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