wanweipedia

Life

Life
Temporal range: 4280–0 Ma Hadean - Present
Ruwenpflanzen.jpg
Plants in the Rwenzori Mountains, Uganda
Scientific classification
Domains and Supergroups

Life on Earth:

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. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. Biology is the science concerned with the study of life.

There is currently no consensus regarding the definition of life. 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. Other definitions sometimes include non-cellular life forms such as viruses and viroids.

Abiogenesis is the natural process of life arising from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity. 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.[1][2][3][4] The earliest known life forms are microfossils of bacteria.[5][6] Researchers generally think that current life on Earth descends from an RNA world,[7] although RNA-based life may not have been the first life to have existed.[8][9] The classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. 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.[10][11][12][13]

Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale, but it has also adapted to survive in most ecosystems and conditions. Some microorganisms, called extremophiles, thrive in physically or geochemically extreme environments that are detrimental to most other life on Earth. The cell is considered the structural and functional unit of life.[14][15] 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. Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells.

In the past, there have been many attempts to define what is meant by "life" through obsolete concepts such as odic force, hylomorphism, spontaneous generation and vitalism, that have now been disproved by biological discoveries. Aristotle is considered to be the first person to classify organisms. Later, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Eventually new groups and categories of life were discovered, such as cells and microorganisms, forcing dramatic revisions of the structure of relationships between living organisms. Though currently only known on Earth, life need not be restricted to it, and many scientists speculate in the existence of extraterrestrial life. 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.

Death is the permanent termination of all biological processes which sustain an organism, and as such, is the end of its life. Extinction is the term describing the dying out of a group or taxon, usually a species. Fossils are the preserved remains or traces of organisms.

Definitions

The definition of life has long been a challenge for scientists and philosophers, with many varied definitions put forward.[16][17][18] This is partially because life is a process, not a substance.[19][20][21] This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside of Earth.[22][23] Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living.[24] Legal definitions of life have also been described and debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.[25] As many as 123 definitions of life have been compiled.[26] One definition seems to be favored by NASA: “a self-sustaining chemical system capable of Darwinian evolution.”[27][28][29][30] More simply, life is, "matter that can reproduce itself and evolve as survival dictates".[31][32][33]

Biology

The characteristics of life

Since there is no unequivocal 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 characteristic exhibits all or most of the following traits:[18][34][35][36][37][38][39]

  1. Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
  2. Organization: being structurally composed of one or more cells – 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 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.
  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 chemotaxis.
  7. 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

From a physics perspective, living beings are thermodynamic systems with an organized molecular structure that can reproduce itself and evolve as survival dictates.[40][41] Thermodynamically, life has been described as an open system which makes use of gradients in its surroundings to create imperfect copies of itself.[42] Another way of putting this is to define life as "a self-sustained chemical system capable of undergoing Darwinian evolution", a definition adopted by a NASA committee attempting to define life for the purposes of exobiology, based on a suggestion by Carl Sagan.[43][44][45] A major strength of this definition is that it distinguishes life by the evolutionary process rather than its chemical composition.[46]

Others take a systemic viewpoint that does not necessarily depend on molecular chemistry. One systemic definition of life is that living things are self-organizing and autopoietic (self-producing). Variations of this definition include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.[47] This definition is extended by the apparition of novel functions over time.[48]

Viruses

Adenovirus as seen under an electron microscope

Whether or not viruses should be considered as alive is controversial. They are most often considered as just gene coding replicators rather than forms of life.[49] They have been described as "organisms at the edge of life"[50] because they possess genes, evolve by natural selection,[51][52] and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolize and they 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.[53][54][55]

Biophysics

To reflect the minimum phenomena required, other biological definitions of life have been proposed,[56] with many of these being based upon chemical systems. Biophysicists have commented that living things function on negative entropy.[57][58] In other words, living processes can be viewed as a delay of the spontaneous diffusion or dispersion of the internal energy of biological molecules towards more potential microstates.[16] 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.[59][60]

Living systems theories

Living systems are open self-organizing living things that interact with their environment. These systems are maintained by flows of information, energy, and matter.

Definition of cellular life according to Budisa, Kubyshkin and Schmidt.

Budisa, Kubyshkin and Schmidt defined cellular life as an organizational unit resting on four pillars/cornerstones: (i) energy, (ii) metabolism, (iii) information and (iv) form. This system is able to regulate and control metabolism and energy supply and contains at least one subsystem that functions as an information carrier (genetic information). Cells as self-sustaining units are parts of different populations that are involved in the unidirectional and irreversible open-ended process known as evolution.[61]

Some scientists have proposed in the last few decades that a general living systems theory is required to explain the nature of life.[62] Such a general theory would arise out of the ecological and biological sciences and attempt 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.[63]

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.[64]:10 The Gaia hypothesis, proposed in the 1960s by scientist James Lovelock,[65][66] suggests that life on Earth functions as a single organism that defines and maintains environmental conditions necessary for its survival.[64] This hypothesis served as one of the foundations of the modern Earth system science.

Nonfractionability

Robert Rosen devoted a large part of his career, from 1958[67] onwards, to developing a comprehensive theory of life as a self-organizing complex system, "closed to efficient causation"[68] He defined a system component as "a unit of organization; a part with a function, i.e., a definite relation between part and whole." He identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and "biological machines." He summarized his views in his book Life Itself.[69] Similar ideas may be found in the book Living Systems[70] by James Grier Miller.

Life as a property of ecosystems

A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity of influence,"[71] 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.[72] He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. Robert Ulanowicz (2009) highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems.[73]

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 dynamic systems theory.[74] The latter is also often called systems biology and aims to understand the most fundamental aspects of life. A closely related approach to CSB and systems biology called relational biology is concerned mainly with understanding life processes in terms of the most important relations, and categories of such relations among the essential functional components of organisms; for multicellular organisms, this has been defined as "categorical biology", or a model representation of organisms as a category theory of biological relations, as well as an algebraic topology of the functional organization of living organisms in terms of their dynamic, complex networks of metabolic, genetic, and epigenetic processes and signaling pathways.[75][76] Alternative but closely related approaches focus on the interdependance of constraints, where constraints can be either molecular, such as enzymes, or macroscopic, such as the geometry of a bone or of the vascular system.[77]

Darwinian dynamic

It has also been argued that the evolution of order in living systems and certain physical systems obeys a common fundamental principle termed the Darwinian dynamic.[78][79] 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 was concluded to be basically similar for both types of systems.[78]

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"[80] and that an organism is any system with an organisation that complies with an operator type that is at least as complex as the cell.[81][82][83] [84] 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.[85]

History of study

Materialism

Plant growth in the Hoh Rainforest
Herds of zebra and impala gathering on the Maasai Mara plain
An aerial photo of microbial mats around the Grand Prismatic Spring of Yellowstone National Park

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.[86]

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.[87]

Plato's world of eternal and unchanging Forms, imperfectly represented in matter by a divine Artisan, contrasts sharply with the various mechanistic Weltanschauungen, of which atomism was, by the fourth century at least, the most prominent ... This debate persisted throughout the ancient world. Atomistic mechanism got a shot in the arm from Epicurus ... while the 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.[88]

— 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 (1596–1650), who held that animals and humans were assemblages of parts that together functioned as a machine. This idea was developed further by Julien Offray de La Mettrie (1709–1750) in his book L'Homme Machine.[89]

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.[90]

At the beginning of the 20th century Stéphane Leduc (1853–1939) promoted the idea that biological processes could be understood in terms of physics and chemistry, and that their growth resembled that of inorganic crystals immersed in solutions of sodium silicate. His ideas, set out in his book La biologie synthétique[91] was widely dismissed during his lifetime, but has incurred a resurgence of interest in the work of Russell, Barge and colleagues.[92]

Hylomorphism

The structure of the souls of plants, animals, and humans, according to Aristotle

Hylomorphism is a theory first expressed by the Greek philosopher Aristotle (322 BC). The application of hylomorphism to biology was important to Aristotle, and biology is extensively covered in his extant writings. In this view, everything in the material universe has both matter and form, and the form of a living thing is its soul (Greek psyche, Latin anima). 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 (Aristotle believed) is found only in man.[93] Each higher soul has all of the attributes of the lower ones. Aristotle believed that while matter can exist without form, form cannot exist without matter, and that therefore the soul cannot exist without the body.[94]

This account is consistent with teleological explanations of life, which account for phenomena in terms of purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained by its purpose of camouflage. The direction of causality (from the future to the past) is in contradiction with the scientific evidence for natural selection, which explains the consequence in terms of a prior cause. Biological features are explained not by looking at future optimal results, but by looking at the past evolutionary history of a species, which led to the natural selection of the features in question.[95]

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.[96]

The theory of spontaneous generation was proposed by Aristotle,[97] 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.[98][99] Disproof of the traditional ideas of spontaneous generation is no longer controversial among biologists.[100][101][102]

Vitalism

Vitalism is the belief that the life-principle is non-material. This originated with Georg Ernst Stahl (17th century), and remained popular until the middle of the 19th century. It appealed to philosophers such as Henri Bergson, Friedrich Nietzsche, and Wilhelm Dilthey,[103] anatomists like Xavier Bichat, and chemists like Justus von Liebig.[104] Vitalism included the idea that there was a fundamental difference between organic and inorganic material, and the belief that organic material can only be derived from living things. This was disproved in 1828, when Friedrich Wöhler prepared urea from inorganic materials.[105] This Wöhler synthesis is considered the starting point of modern organic chemistry. It is of historical significance because for the first time an organic compound was produced in inorganic reactions.[104]

During the 1850s, Hermann von Helmholtz, anticipated by Julius Robert von Mayer, demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital forces" necessary to move a muscle.[106] These results led to the abandonment of scientific interest in vitalistic theories, especially after Buchner's demonstration that alcoholic fermentation could occur in cell-free extracts of yeast. [107] Nonetheless, the belief still exists in pseudoscientific theories such as homeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.[108]

Origin


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