Life

livinglife on Earthbiotaorganicaliveliving organismsliving thingsbioticlivesbiological life
For other uses, see Life (disambiguation).wikipedia
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Organism

organismsflora and faunaliving organisms
One popular definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve.
In biology, an organism (from Greek: ὀργανισμός, organismos) is any individual entity that embodies the properties of life.

Metabolism

metabolicmetabolizedmetabolic pathways
One popular definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve.
Metabolism (, from μεταβολή metabolē, "change") is the set of life-sustaining chemical reactions in organisms.

Reproduction

reproductiveprocreationreproduce
One popular definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve.
Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction.

Archaea

archaeonarcheaarchaebacteria
Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria.
Archaea are a major part of Earth's life, and may play roles in the carbon cycle and the nitrogen cycle.

Evolution

evolvedtheory of evolutionevolutionary
One popular definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli, reproduce and evolve.
All life on Earth shares a last universal common ancestor (LUCA) that lived approximately 3.5–3.8 billion years ago.

Biology

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

Nucleic acid

nucleic acidsNAmolecular basis
There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many biomolecules such as proteins and nucleic acids.
Nucleic acids are the biopolymers, or small biomolecules, essential to all known forms of life.

Death

mortalitydeceaseddead
Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased (they have died), or because they never had such functions and are classified as inanimate.
One of the challenges in defining death is in distinguishing it from life.

Synthetic biology

synthetic lifesyntheticsynthetic biologist
The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential synthetic life as "living".
May 2019: researchers, in a milestone effort, reported the creation of a new synthetic (possibly artificial) form of viable life, a variant of the bacteria Escherichia coli, by reducing the natural number of 64 codons in the bacterial genome to 59 codons instead, in order to encode 20 amino acids.

Earth

Earth's surfaceterrestrialworld
Though currently only known on Earth, life need not be restricted to it, and many scientists speculate in the existence of extraterrestrial life.
Earth is the third planet from the Sun and the only astronomical object known to harbor life.

Extraterrestrial life

alienextraterrestrialaliens
Though currently only known on Earth, life need not be restricted to it, and many scientists speculate in the existence of extraterrestrial life.
Extraterrestrial life is hypothetical life which may occur outside of Earth and which did not originate on Earth.

Artificial life

artificialA-lifeALife
Artificial life is a computer simulation or human-made reconstruction of any aspect of life, which is often used to examine systems related to natural life.
Artificial life (often abbreviated ALife or A-Life) is a field of study wherein researchers examine systems related to natural life, its processes, and its evolution, through the use of simulations with computer models, robotics, and biochemistry.

Physiology

physiologistphysiologicalphysiologically
These complex processes, called physiological functions, have underlying physical and chemical bases, as well as signaling and control mechanisms that are essential to maintaining life.
Physiology is the scientific study of the functions and mechanisms which work within a living system.

Extinction

extinctspecies extinction
Extinction is the term describing the dying out of a group or taxon, usually a species.
More than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to have died out.

Abiogenesis

origin of lifeorigins of lifeformation
Abiogenesis is the natural process of life arising from non-living matter, such as simple organic compounds.
Abiogenesis, or informally the origin of life, is the natural process by which life has arisen from non-living matter, such as simple organic compounds.

Biological organisation

biological organizationhierarchy of lifeOrganization
Biological organization is the hierarchy of complex biological structures and systems that define life using a reductionistic approach.

DNA replication

replicationreplication forklagging strand
They are most often considered as just replicators rather than forms of life.
DNA replication occurs in all living organisms acting as the basis for biological inheritance.

List of interstellar and circumstellar molecules

List of molecules in interstellar spacedetected in deep spaceinterstellar molecules
Complex organic molecules occur in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth.
In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites.

Gene

genesnumber of genesgene sequence
They have been described as "organisms at the edge of life" because they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly.
Some genetic traits are instantly visible, such as eye color or number of limbs, and some are not, such as blood type, risk for specific diseases, or the thousands of basic biochemical processes that constitute life.

Natural environment

environmentenvironmentalpaleoenvironment
The Gaia hypothesis, proposed in the 1960s by scientist James Lovelock, suggests that life on Earth functions as a single organism that defines and maintains environmental conditions necessary for its survival.
Earth science generally recognizes four spheres, the lithosphere, the hydrosphere, the atmosphere, and the biosphere as correspondent to rocks, water, air, and life respectively.

Biophysical environment

environmentenvironmentalthe environment
Living systems are open self-organizing living things that interact with their environment.
A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution.

Earliest known life forms

earliest known lifeearliest evidenceearly life on Earth
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.
Currently, Earth remains the only place in the universe known to harbor life.

Entropy and life

biologically available sources of energycapacity for entropyentropy
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 that 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.
Research concerning the relationship between the thermodynamic quantity entropy and the evolution of life began around the turn of the 20th century.

Bioenergetics

energy metabolismenergybioenergetic
Life is dependent on energy transformations; living organisms survive because of exchange of energy between living tissues/ cells and the outside environment.

Living systems

Living systems theoryliving systemJ.G. Millers theory of open and self organizing systems
Others take a systemic viewpoint that does not necessarily depend on molecular chemistry.
Some scientists have proposed in the last few decades that a general living systems theory is required to explain the nature of life.