Life is a characteristic that distinguishes objects that have self-sustaining biological processes which are "alive," or "living"), from those which do not; either because such functions have ceased, or else because they lack such functions and are classified as "inanimate".
In biology, the science that studies living organisms, "life" is the condition which distinguishes active organisms from inorganic matter, including the capacity for growth, functional activity and the continual change preceding death.
A diverse array of living organisms or life forms can be found in the biosphere on Earth, and properties common to these organisms such as plants, animals, fungi, protists, archaea, and bacteria, are a carbon and water-based cellular form with complex organization and heritable genetic information.
A living organism is a life form.
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.
Humans are living organisms.
Life is the opposite of death.
Death is the opposite of life.
In philosophy and religion, the conception and nature of life varies, and offer interpretations in the frameworks of existence and consciousness, and touch on many other related issues, such as, ontology, value, life stance, purpose, conceptions of God, the soul and the afterlife.
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 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.
Democritus, the disciple of Leucippus, thought that the essential characteristic of life is having a soul or psychê.
Hylomorphism is the theory that all things are a combination of matter and form.
Hylomorphism is the theory that originates with Aristotle.
Aristotle was one of the first ancient writers to approach the subject of life in a scientific way.
Biology was one of Aristotle's main interests, and there is extensive biological material in his extant writings.
According to Aristotle, all things in the material universe have both matter and form.
There are three kinds of souls: the 'vegetative soul' of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the 'animal soul' which causes animals to move and feel; and the rational soul which is the source of consciousness and reasoning which is found only in man.
Each higher soul has all the attributes of the lower one.
Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.
A teleological explanation accounts for phenomena in terms of their purpose or goal-directedness.
Vitalism is the belief that the life-principle is essentially immaterial.
Vitalism appealed to philosophers such as Henri Bergson, Nietzsche, Wilhelm Dilthey, anatomists like Bichat, and chemists like Liebig.
Vitalism underpinned the idea of a fundamental separation of 'organic' and inorganic material, and the belief that organic material can only be derived from living things.
The idea of a fundamental separation of 'organic' and inorganic material, and the belief that organic material can only be derived from living things was disproved in 1828.
It is still a challenge for scientists and philosophers to define life in unequivocal terms.
Any definition of life 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.
Therefore, life is a characteristic of organisms that exhibit all or most of the following phenomena: Homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction.
Reproduction is a characteristic of life.
Adaptation is a characteristic of life.
homeostasis is a characteristic of life.
Growth is a characteristic of life.
Metabolism is a characteristic of life.
Organization is a characteristic of life.
There are several characteristics of life.
Homeostasis is the regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
The characteristic of organization means to be structurally composed of one or more cells, which are the basic units of life.
Metabolism is the transformation of energy by converting chemicals and energy into cellular components and decomposing organic matter.
Living things require energy to maintain internal organization and to produce the other phenomena associated with life.
Living things require energy to maintain homeostasis and to produce the other phenomena associated with life.
The characteristic of growth in life is the 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.
Adaptation is the ability to change over a period of time in response to the environment.
Adaptation 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.
You must learn to adapt in order to survive.
I must learn to adapt in order to survive.
A response to stimuli 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 to stimuli is often expressed by motion, for example, the leaves of a plant turning toward the sun (and by chemotaxis.
Reproduction is ability to produce new individual organisms either asexually, from a single parent organism, or sexually, from at least two parent organisms.
Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.
If you respond to changes in your environment, are you living?
if you promote your own continuation, are you not living?
A systemic definition of life is that living things are self-organizing and autopoietic or self-producing.
Viruses are most often considered replicators rather than forms of life.
Viruses have been described as "organisms at the edge of life", since they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly.
Viruses do not metabolise and require a host cell to make new products.
Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.
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.
Some scientists have recently proposed that a general Living systems theory is required.
Living systems theory, arising out of the biological sciences, attempts to map general principles for how all living systems work.
A systems view of life treat 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.
Evidence suggests that life on Earth has existed for about three point seven billion years.
All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates.
There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre-cellular life to protocells and metabolism. 
There is no scientific consensus as to how life originated and all proposed theories are highly speculative.
The Miller-Urey experiment, and the work of Sidney Fox, suggest that that conditions on the primitive Earth may have favored chemical reactions that synthesized some organic compounds from inorganic precursors.
The diversity of life on Earth today is a result of the dynamic interplay between genetic opportunity, metabolic capability and environmental challenges.
For most of its existence, Earth's habitable environment has been dominated by microorganisms and subjected to their metabolism and evolution.
As a consequence of such microbial activities on a geologic time scale, the physical-chemical environment on Earth has been changing, thereby determining the path of evolution of subsequent life.
The release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced fundamental, global changes in the Earth's environment.
The inert components of an ecosystem are the physical and chemical factors necessary for life.
The physical and chemical factors necessary for life are energy, water, temperature, atmosphere, gravity, nutrients, and ultraviolet solar radiation protection.
In most ecosystems the conditions vary during the day and often shift from one season to the next.
To live in most ecosystems, then, organisms must be able to survive a range of conditions, called range of tolerance.
Life has evolved strategies that allow it to survive even beyond the physical and chemical limits to which it has adapted to grow.
To survive, some microorganisms can assume forms that enable them to withstand freezing, complete desiccation, starvation, high-levels of radiation exposure, and other physical or chemical challenges.
Some microorganisms can survive exposure to such conditions for weeks, months, years, or even centuries.
Extremophiles are microbial life forms that thrive outside the ranges life is commonly found in.
Extremophiles excel at exploiting uncommon sources of energy.
While all organisms are composed of nearly identical molecules, evolution has enabled such microbes to cope with this wide range of physical and chemical conditions.
An understanding of the tenacity and versatility of life on Earth, as well as an understanding of the molecular systems that some organisms utilize to survive such extremes, will provide a critical foundation for the search for life beyond Earth.
Traditionally, people have divided organisms into the classes of plants and animals, based mainly on their ability of movement.
The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle.
Aristotle classified all living organisms known at that time as either a plant or an animal.
Aristotle distinguished animals with blood from animals without blood, which can be compared with the concepts of vertebrates and invertebrates respectively.
Aristotle divided the blooded animals into five groups: viviparous quadrupeds (mammals), birds, oviparous quadrupeds (reptiles and amphibians), fishes and whales.
Aristotle divided the blooded animals into five groups: mammals, birds, reptiles and amphibians, fishes and whales.
Though Aristotle's work in zoology was not without errors, it was the grandest biological synthesis of the time and remained the ultimate authority for many centuries after his death.
The exploration of the American continent revealed large numbers of new plants and animals that needed descriptions and classification.
In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced and was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification.
In the late 1740s, Carolus Linnaeus introduced his method, still used, to formulate the scientific name of every species.
Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and defining their meaning with an unprecedented precision.
By consistently using his system, Linnaeus separated nomenclature from taxonomy.
As new discoveries enabled us to study cells and microorganisms, new groups of life were revealed, and the fields of cell biology and microbiology were created.
As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and viroids.
Sometimes these entities are considered to be alive but others argue that viruses are not living organisms since they lack characteristics such as cell membrane, metabolism and do not grow or respond to their environments.
Viruses can however be classed into "species" based on their biology and genetics but many aspects of such a classification remain controversial.
Earth is the only planet in the universe known to harbour life.
The Drake equation, which relates the number of extraterrestrial civilizations in our galaxy with which we might come in contact, has been used to discuss the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation.
Panspermia and exogenesis are hypotheses proposing that life originated elsewhere in the universe and was subsequently transferred to Earth in the form of spores perhaps via meteorites, comets or cosmic dust.
Death is the permanent termination of all vital functions or life processes in an organism or cell.
After death, the remains of an organism become part of the biogeochemical cycle.
Organisms may be consumed by a predator or a scavenger and leftover organic material may then be further decomposed by detritivores, organisms which recycle detritus, returning it to the environment for reuse in the food chain.
One of the challenges in defining death is in distinguishing it from life.
Death would seem to refer to either the moment at which life ends, or when the state that follows life begins.
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.
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.
Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past.
The totality of fossils, both discovered and undiscovered, and their placement in fossil-containing rock formations and sedimentary layers is known as the fossil record.
Fossils range in age from the youngest at the start of the Holocene Epoch to the oldest from the Archaean Eon, a few billion years old.