Life
Life | |
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Diverse forms of life on a coral reef | |
Scientific classification ![]() | |
Domains and Supergroups | |
Life on Earth:
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Life is a quality that distinguishes
Life has been studied since ancient times, with theories such as
Living things are composed of biochemical molecules, formed mainly from a few core chemical elements. All living things contain two types of large molecule, proteins and nucleic acids, the latter usually both DNA and RNA: these carry the information needed by each species, including the instructions to make each type of protein. The proteins, in turn, serve as the machinery which carries out the many chemical processes of life. The cell is the structural and functional unit of life. Smaller organisms, including prokaryotes (bacteria and archaea), consist of small single cells. Larger organisms, mainly eukaryotes, can consist of single cells or may be multicellular with more complex structure. Life is confirmed only on Earth but extraterrestrial life is thought probable. Artificial life is being simulated and explored by scientists and engineers.
Definitions
Challenge
The definition of life has long been a challenge for scientists and philosophers.[2][3][4] This is partially because life is a process, not a substance.[5][6][7] This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside Earth.[8][9] Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living.[10] Legal definitions of life have been debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.[11] At least 123 definitions of life have been compiled.[12]
Descriptive
Since there is no consensus for a definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment. This implies all or most of the following traits:[4][13][14][15][16][17]
- Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
- Organisation: being structurally composed of one or more cells– the basic units of life
- Metabolism: transformation of energy, used to convert chemicals into cellular components (anabolism) and to decompose organic matter (catabolism). Living things require energy for homeostasis and other activities.
- Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size and structure.
- Adaptation: the evolutionary process whereby an organism becomes better able to live in its habitat.[18][19][20]
- Response to multicellular organisms, or the motion of the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
- Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms.
Physics
From a
Living systems
Others take a
Death
Death is the termination of all vital functions or life processes in an organism or cell.[30][31] One of the challenges in defining death is in distinguishing it from life. Death would seem to refer to either the moment life ends, or when the state that follows life begins.[31] However, determining when death has occurred is difficult, as cessation of life functions is often not simultaneous across organ systems.[32] Such determination, therefore, requires drawing conceptual lines between life and death. This is problematic because there is little consensus over how to define life. The nature of death has for millennia been a central concern of the world's religious traditions and of philosophical inquiry. Many religions maintain faith in either a kind of afterlife or reincarnation for the soul, or resurrection of the body at a later date.[33]
"At the edge of life": viruses
Whether or not viruses should be considered as alive is controversial.
History of study
Materialism
Some of the earliest theories of life were materialist, holding that all that exists is matter, and that life is merely a complex form or arrangement of matter. Empedocles (430 BC) argued that everything 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.[42] Democritus (460 BC) was an atomist; he thought that the essential characteristic of life was having a soul (psyche), and that the soul, like everything else, was composed of fiery atoms. He elaborated on fire because of the apparent connection between life and heat, and because fire moves.[43]
Hylomorphism

Hylomorphism is a theory first expressed by the Greek philosopher
This account is consistent with
Spontaneous generation
Spontaneous generation was the belief that living organisms can form without descent from similar organisms. Typically, the idea was that certain forms such as fleas could arise from inanimate matter such as dust or the supposed seasonal generation of mice and insects from mud or garbage.[52]
The theory of spontaneous generation was proposed by Aristotle,[53] who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it was considered the best explanation for two millennia. It was decisively dispelled by the experiments of Louis Pasteur in 1859, who expanded upon the investigations of predecessors such as Francesco Redi.[54][55] Disproof of the traditional ideas of spontaneous generation is no longer controversial among biologists.[56][57][58]
Vitalism
Vitalism is the belief that there is a non-material life-principle. This originated with
During the 1850s
Development
Origin of life
The
The biosphere is postulated to have developed, from the origin of life onwards, at least some 3.5 billion years ago.
Evolution
Fossils
Fossils are the preserved remains or
Extinction
Extinction is the process by which a
Environmental conditions
The diversity of life on Earth is a result of the dynamic interplay between
Biosphere
The
Range of tolerance
The inert components of an ecosystem are the physical and chemical factors necessary for life—energy (sunlight or
Extremophiles

To survive, some microorganisms have evolved to withstand freezing, complete desiccation, starvation, high levels of radiation exposure, and other physical or chemical challenges. These microorganisms may survive exposure to such conditions for long periods.[91][112] They excel at exploiting uncommon sources of energy. Characterization of the structure and metabolic diversity of microbial communities in such extreme environments is ongoing.[113]
Classification
Antiquity
The first classification of organisms was made by the Greek philosopher Aristotle (384–322 BC), who grouped living things as either plants or animals, based mainly on their ability to move. He distinguished animals with blood from animals without blood, which can be compared with the concepts of
Linnaean
In the late 1740s, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Linnaeus attempted to improve the composition and reduce the length of the previously used many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and precisely defining their meaning.[115]
The fungi were originally treated as plants. For a short period Linnaeus had classified them in the taxon
As advances in
As microbiology developed, viruses, which are non-cellular, were discovered. Whether these are considered alive has been a matter of debate; viruses lack characteristics of life such as cell membranes, metabolism and the ability to grow or respond to their environments. Viruses have been classed into "species" based on their genetics, but many aspects of such a classification remain controversial.[120]
The original Linnaean system has been modified many times, for example as follows:
Linnaeus 1735[121] |
Haeckel 1866[122] |
Chatton 1925[123] |
Copeland 1938[124] |
Whittaker 1969[125] |
Woese et al. 1990[118] |
Cavalier-Smith 1998,[126] 2015[127] |
---|---|---|---|---|---|---|
2 kingdoms | 3 kingdoms | 2 empires | 4 kingdoms | 5 kingdoms | 3 domains | 2 empires, 6/7 kingdoms |
(not treated) | Protista | Prokaryota
|
Monera | Monera | Bacteria | Bacteria |
Archaea | Archaea (2015) | |||||
Eukaryota
|
Protoctista
|
Protista | Eucarya | Protozoa | ||
Chromista | ||||||
Vegetabilia
|
Plantae | Plantae | Plantae | Plantae | ||
Fungi | Fungi | |||||
Animalia | Animalia | Animalia | Animalia | Animalia |
The attempt to organise the Eukaryotes into a small number of kingdoms has been challenged. The Protozoa do not form a clade or natural grouping,[128] and nor do the Chromista (Chromalveolata).[129]
Composition
Chemical elements
All life forms require certain core chemical elements for their biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—the elemental macronutrients for all organisms.[130] Together these make up nucleic acids, proteins and lipids, the bulk of living matter. Five of these six elements comprise the chemical components of DNA, the exception being sulfur. The latter is a component of the amino acids cysteine and methionine. The most abundant of these elements in organisms is carbon, which has the desirable attribute of forming multiple, stable covalent bonds. This allows carbon-based (organic) molecules to form the immense variety of chemical arrangements described in organic chemistry.[131] Alternative hypothetical types of biochemistry have been proposed that eliminate one or more of these elements, swap out an element for one not on the list, or change required chiralities or other chemical properties.[132][133]
DNA
Deoxyribonucleic acid or
Cells
Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division.[137][138] Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted.[139] The activity of an organism depends on the total activity of its cells, with energy flow occurring within and between them. Cells contain hereditary information that is carried forward as a genetic code during cell division.[140]
There are two primary types of cells, reflecting their evolutionary origins.
The molecular mechanisms of cell biology are based on proteins. Most of these are synthesised by the ribosomes through an enzyme-catalyzed process called protein biosynthesis. A sequence of amino acids is assembled and joined based upon gene expression of the cell's nucleic acid.[143] In eukaryotic cells, these proteins may then be transported and processed through the Golgi apparatus in preparation for dispatch to their destination.[144]
Cells reproduce through a process of
Multicellular structure
Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability.
Extraterrestrial
Though life is confirmed only on Earth, many think that
Investigation of the tenacity and versatility of life on Earth,
Beyond the Solar System, the region around another
Artificial
Artificial life is the simulation of any aspect of life, as through computers, robotics, or biochemistry.[161] Synthetic biology is a new area of biotechnology that combines science and biological engineering. The common goal is the design and construction of new biological functions and systems not found in nature. Synthetic biology includes the broad redefinition and expansion of biotechnology, with the ultimate goals of being able to design and build engineered biological systems that process information, manipulate chemicals, fabricate materials and structures, produce energy, provide food, and maintain and enhance human health and the environment.[162]
See also
- Biology, the study of life
- Biosignature
- History of life
- Lists of organisms by population
- Viable system theory
- Central dogma of molecular biology
- Carbon-based life
Notes
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External links
- Wikispecies – A free directory of life
- Vitae (BioLib)
- Biota (Taxonomicon)
- Entry on the Stanford Encyclopedia of Philosophy