User:MartinZ/sandbox/Life
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This article is one of a series on: |
Life in the universe |
---|
Outline |
Planetary habitability in the Solar System |
Life outside the Solar System |
Habitability of... |
Life is a characteristic distinguishing
The definition of life is controversial. The current definition is that
Since appearing, life on Earth has changed its environment on a
Though only confirmed on
Definitions
It is a challenge for scientists and philosophers to define life.[1][2][3][4][5] This is partially because life is a process, not a substance.[6][7][8] Any definition must be general enough to both encompass all known life and any unknown life that may be different from life on Earth.[9][10][11]
Biology
Since there is no unequivocal definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits:[10][12][13][14][15][16][17]
- Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
- Organization: being structurally composed of one or more cells— the basic units of life
- 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.
- 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.
- Adaptation: the ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
- Response to multicellular organisms. A response is often expressed by motion; for example, 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.
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.
Alternative definitions
At a higher level, living beings are
Others take a
Viruses
Whether or not
Biophysics
To reflect the minimum phenomena required, other biological definitions of life have been proposed,
Living systems theories
Living systems are open
Some scientists have proposed in the last few decades that a general living systems theory is required to explain the nature of life.[38] Such a general theory, arising out of the ecological and biological sciences, attempts to map general principles for how all living systems work. Instead of examining phenomena by attempting to break things down into components, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment.[39]
Gaia hypothesis
The idea that the Earth is alive is found in philosophy and religion, but the first scientific discussion of it was by the Scottish scientist James Hutton. In 1785, he stated that the Earth was a superorganism and that its proper study should be physiology. Hutton is considered the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th century.[40] The Gaia hypothesis, proposed in the 1960s by scientist James Lovelock,[41][42] suggests that life on Earth functions as a single organism that defines and maintains environmental conditions necessary for its survival.[43]
Nonfractionability
The first attempt at a general
Life as a property of ecosystems
A systems view of life treats environmental
Complex systems biology
Complex systems biology (CSB) is a field of science that studies the emergence of complexity in functional organisms from the viewpoint of
Darwinian dynamic
It has also been argued that the evolution of order in living systems and certain physical systems obey a common fundamental principle termed the Darwinian dynamic.[53][54] The Darwinian dynamic was formulated by first considering how macroscopic order is generated in a simple non-biological system far from thermodynamic equilibrium, and then extending consideration to short, replicating RNA molecules. The underlying order generating process for both types of system was concluded to be basically similar.[53]
Operator theory
Another systemic definition, called the Operator theory, proposes that 'life is a general term for the presence of the typical closures found in organisms; the typical closures are a membrane and an autocatalytic set in the cell',[55] and also proposes that an organism is 'any system with an organisation that complies with an operator type that is at least as complex as the cell.[56][57][58][59] Life can also be modeled as a network of inferior negative feedbacks of regulatory mechanisms subordinated to a superior positive feedback formed by the potential of expansion and reproduction.[60]
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.[61]
Democritus (460 BC) thought that the essential characteristic of life is having a soul (psyche). Like other ancient writers, he was attempting to explain what makes something a living thing. His explanation was that fiery atoms make a soul in exactly the same way atoms and void account for any other thing. He elaborates on fire because of the apparent connection between life and heat, and because fire moves.[62]
Plato's world of eternal and unchanging
Stoics adopted a divine teleology ... The choice seems simple: either show how a structured, regular world could arise out of undirected processes, or inject intelligence into the system.[63]— R. J. Hankinson, Cause and Explanation in Ancient Greek Thought
The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes, who held that animals and humans were assemblages of parts that together functioned as a machine. In the 19th century, the advances in cell theory in biological science encouraged this view. The evolutionary theory of Charles Darwin (1859) is a mechanistic explanation for the origin of species by means of natural selection.[64]
Hylomorphism
Hylomorphism is a theory first expressed by Greek philosopher
This account is consistent with
Spontaneous generation
Spontaneous generation was the belief on the ordinary formation of living organisms 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.[68]
The theory of spontaneous generation was proposed by Aristotle,[69] who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it held sway 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.[70][71] Disproof of the traditional ideas of spontaneous generation is no longer controversial among biologists.[72][73][74]
Vitalism
Vitalism is the belief that the life-principle is non-material. This originated with
During the 1850s,
Origin
million years ago) |
The
There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the Miller–Urey experiment, and the work of Sidney Fox, which shows that conditions on the primitive Earth favored chemical reactions that synthesize amino acids and other organic compounds from inorganic precursors,[99] and phospholipids spontaneously forming lipid bilayers, the basic structure of a cell membrane.
Living organisms synthesize
However, since genes and proteins are both required to produce the other, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because of this, it is unlikely that genes and proteins arose independently.[102]
Therefore, a possibility, first suggested by
One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic precursors is more difficult than for other organic molecules. One reason for this is that RNA precursors are very stable and react with each other very slowly under ambient conditions, and it has also been proposed that living organisms consisted of other molecules before RNA.[106] However, the successful synthesis of certain RNA molecules under the conditions that existed prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction.[107] This study makes the RNA world hypothesis more plausible.[108]
Geological findings in 2013 showed that reactive
In 2009, experiments demonstrated
Prebiotic compounds may have extraterrestrial origin.
In March 2015, NASA scientists reported that, for the first time, complex
According to the
Environmental conditions
The diversity of life on Earth is a result of the dynamic interplay between
Biosphere
The biosphere is the global sum of all ecosystems. It can also be termed as the zone of life on Earth, a closed system (apart from solar and cosmic radiation and heat from the interior
of the Earth), and largely self-regulating.
In a general sense, biospheres are any closed, self-regulating systems containing ecosystems. This includes artificial biospheres such as Biosphere 2 and BIOS-3, and potentially ones on other planets or moons.[125]
Range of tolerance
The inert components of an ecosystem are the physical and chemical factors necessary for life — energy (sunlight or
Extremophiles
To survive, selected microorganisms can assume forms that enable them to withstand
Investigation 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, is important for the search for
Chemical elements
All life forms require certain core chemical elements needed for biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—the elemental macronutrients for all organisms[134]—often represented by the acronym CHNOPS. 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 biologically abundant of these elements is carbon, which has the desirable attribute of forming multiple, stable covalent bonds. This allows carbon-based (organic) molecules to form an immense variety of chemical arrangements.[135] 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.[136][137]
DNA
Deoxyribonucleic acid is a
DNA stores biological information. The DNA backbone is resistant to cleavage, and both strands of the double-stranded structure store the same biological information. Biological information is replicated as the two strands are separated. A significant portion of DNA (more than 98% for humans) is non-coding, meaning that these sections do not serve as patterns for protein sequences.
The two strands of DNA run in opposite directions to each other and are therefore
Within cells, DNA is organized into long structures called
DNA was first isolated by
RNA
Ribonucleic acid (RNA) is a
Some RNA molecules play an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals. One of these active processes is protein synthesis, a universal function wherein mRNA molecules direct the assembly of proteins on ribosomes. This process uses transfer RNA (tRNA) molecules to deliver amino acids to the ribosome, where ribosomal RNA (rRNA) then links amino acids together to form proteins.
Proteins
Proteins are large
A linear chain of amino acid residues is called a
Once formed, proteins only exist for a certain period of time and are then degraded and recycled by the cell's machinery through the process of protein turnover. A protein's lifespan is measured in terms of its half-life and covers a wide range. They can exist for minutes or years with an average lifespan of 1–2 days in mammalian cells. Abnormal and or misfolded proteins are degraded more rapidly either due to being targeted for destruction or due to being unstable.
Like other biological
Proteins may be
Classification
Life is usually classified by eight levels of taxa—domains, kingdoms, phyla, class, order, family, genus, and species. In May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with only one-thousandth of one percent described.[144]
The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle (384–322 BC), who classified all living organisms known at that time as either a
The exploration of the
The
As new discoveries enabled detailed study of
As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and viroids. 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 can still be classed into "species" based on their biology and genetics, but many aspects of such a classification remain controversial.[151]
In the 1960s a trend called
Linnaeus 1735[153] |
Haeckel 1866[154] |
Chatton 1925[155] |
Copeland 1938[147] |
Whittaker 1969[156] |
Woese et al. 1990[149] |
Cavalier-Smith 1998,[157] 2015[158] |
---|---|---|---|---|---|---|
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 |
Cells
Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division. Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted.[159] 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.[160]
There are two primary types of cells.
The molecular mechanisms of cell biology are based on proteins. Most of these are synthesized by the ribosomes through an enzyme-catalyzed process called protein biosynthesis. A sequence of amino acids is assembled and joined together based upon gene expression of the cell's nucleic acid.[163] In eukaryotic cells, these proteins may then be transported and processed through the Golgi apparatus in preparation for dispatch to their destination.[164]
Cells reproduce through a process of
Multicellular organisms may have first evolved through the formation of colonies like cells. These cells can form group organisms through cell adhesion. The individual members of a colony are capable of surviving on their own, whereas the members of a true multi-cellular organism have developed specializations, making them dependent on the remainder of the organism for survival. Such organisms are formed clonally or from a single germ cell that is capable of forming the various specialized cells that form the adult organism. This specialization allows multicellular organisms to exploit resources more efficiently than single cells.[166] In January 2016, scientists reported that, about 800 million years ago, a minor genetic change in a single molecule, called GK-PID, may have allowed organisms to go from a single cell organism to one of many cells.[167]
Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability. Cell signaling coordinates cellular activities, and hence governs the basic functions of multicellular organisms. Signaling between cells can occur through direct cell contact using juxtacrine signalling, or indirectly through the exchange of agents as in the endocrine system. In more complex organisms, coordination of activities can occur through a dedicated nervous system.[168]
Extraterrestrial
Though life is confirmed only on the Earth, many think that
Beyond the Solar System, the region around another main-sequence star that could support Earth-like life on an Earth-like planet is known as the habitable zone. The inner and outer radii of this zone vary with the luminosity of the star, as does the time interval during which the zone survives. Stars more massive than the Sun have a larger habitable zone, but remain on the main sequence for a shorter time interval. Small red dwarfs have the opposite problem, with a smaller habitable zone that is subject to higher levels of magnetic activity and the effects of tidal locking from close orbits. Hence, stars in the intermediate mass range such as the Sun may have a greater likelihood for Earth-like life to develop.[174] The location of the star within a galaxy may also affect the likelihood of life forming. Stars in regions with a greater abundance of heavier elements that can form planets, in combination with a low rate of potentially habitat-damaging supernova events, are predicted to have a higher probability of hosting planets with complex life.[175] The variables of the Drake equation are used to discuss the conditions in planetary systems where civilization is most likely to exist.[176] This suggests that life could also form on other planets.[84]Artificial
Artificial life is a field of study that examines
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.[178]
Death
Death is the permanent termination of all vital functions or life processes in an organism or cell.
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.[180] However, determining when death has occurred requires drawing precise conceptual boundaries between life and death. This is problematic, however, 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.
Extinction
Extinction is the process by which a group of
Fossils
Fossils are the preserved remains or
See also
Notes
References
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Further reading
- Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman
- Seeding the Universe With Life Legacy Books, Washington D. C., 2000, ISBN 0-476-00330-X
- Walker, Martin G. LIFE! Why We Exist ... And What We Must Do to Survive Dog Ear Publishing, 2006, ISBN 1-59858-243-7
External links
- Wikispecies – a free directory of life
- Resources for life in the Solar System and in galaxy, and the potential scope of life in the cosmological future
- "The Adjacent Possible: A Talk with Stuart Kauffman"
- Stanford Encyclopedia of Philosophy entry
- The Kingdoms of Life