life

For other uses, see Life (disambiguation). Life (Biota) Life on a rocky peak in the Waitakere Ranges Scientific classification Domains and Kingdoms

Life on Earth:

Non-cellular life (viruses) ** Cellular life Bacteria Archaea Eukarya Protista Fungi Plantae Animalia

Life (cf. biota) is a characteristic that distinguishes objects that have self-sustaining biological processes from those that do not12–either because such functions have ceased (death), or else because they lack such functions and are classified as "inanimate."3

In biology, the science of 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.45 A diverse array of living organisms (life forms) can be found in the biosphere on Earth, and properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. 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.16

In philosophy and religion, the conception of life and its nature varies. Both offer interpretations as to how life relates to existence and consciousness, and both touch on many related issues, including life stance, purpose, conception of a god or gods, a soul or an afterlife.

Contents 1 Early theories about life 1.1 Materialism 1.2 Hylomorphism 1.3 Vitalism 2 Definitions 2.1 Biology 2.2 Biophysics 2.3 Living systems theories 3 Origin of life 4 Conditions for life 4.1 Range of tolerance 4.2 Extremophiles 5 Classification of life 6 Extraterrestrial life 7 Death 7.1 Extinction 7.2 Fossils 8 See also 9 References 10 Further reading 11 External links // Early theories about life Materialism Plant life Herds of zebra and impala gathering on the Masai 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 all life is merely a complex form or arrangement of matter. Empedocles (430 B.C.) 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. 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. For example, growth in plants is explained by the natural downward movement of earth and the natural upward movement of fire.7

Democritus (460 B.C.), the disciple of Leucippus, thought that the essential characteristic of life is having a soul (psychê). In common with other ancient writers, he used the term to mean the principle of living things that causes them to function as a living thing. He thought the soul was composed of fire atoms, because of the apparent connection between life and heat, and because fire moves.8 He also suggested that humans originally lived like animals, gradually developing communities to help one another, originating language, and developing crafts and agriculture.9

In the scientific revolution of the seventeenth century, mechanistic ideas were revived by philosophers like Descartes.

Hylomorphism

Hylomorphism is the theory (originating with Aristotle (322 BC)) that all things are a combination of matter and form. Aristotle was one of the first ancient writers to approach the subject of life in a scientific way. Biology was one of his main interests, and there is extensive biological material in his extant writings. According to him, all things in the material universe have both matter and form. 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.10 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.11

Consistent with this account is a teleological explanation of life. A teleological explanation accounts for phenomena in terms of their purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained by its purpose of camouflage. The direction of causality is the other way round from materialistic science, which explains the consequence in terms of a prior cause. Modern biologists now reject this functional view in terms of a material and causal one: biological features are to be explained not by looking forward to future optimal results, but by looking backwards to the past evolutionary history of a species, which led to the natural selection of the features in question.

Vitalism

Vitalism is the belief that the life-principle is essentially immaterial. This originated with Stahl, and held sway until the middle of the nineteenth century. It 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. This was disproved in 1828 when Wöhler prepared urea from inorganic materials. This so-called Wöhler synthesis is considered the starting point of modern organic chemistry. It is of great historical significance because for the first time an organic compound was produced from inorganic reactants.

Later, Helmholtz, anticipated by Mayer, demonstrated that no energy is lost in muscle movement, suggesting that there were no vital forces necessary to move a muscle. These empirical results led to the abandonment of scientific interest in vitalistic theories, although the belief lingered on in non-scientific theories such as homeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.

Definitions

It is still a challenge for scientists and philosophers to define life in unequivocal terms.121314 Any definition 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.15 Defining life is difficult —in part— because life is a process, not a pure substance.16

Biology

Since there is no unequivocal definition of life, the current understanding is descriptive, where life is a 'characteristic' of organisms that exhibit all or most of the following phenomena:161718

Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature. Organization: Being structurally composed of one or more cells, which are 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 a period of time in response to the environment. This ability 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. 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 by chemotaxis. Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms. Proposed

To reflect the minimum phenomena required, some have proposed other biological definitions of life:

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.18 A network of inferior negative feedbacks (regulatory mechanisms) subordinated to a superior positive feedback (potential of expansion, reproduction).19 A 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.20 Life is a self-sustained chemical system capable of undergoing Darwinian evolution.21 Things with the capacity for metabolism and motion.16 Viruses

Viruses are most often considered replicators rather than forms of life. They have been described as "organisms at the edge of life",22 since they possess genes, evolve by natural selection,23 and replicate by creating multiple copies of themselves through self-assembly. However, 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.2425

Biophysics

Biophysicists have also commented on the nature and qualities of life forms—notably that they function on negative entropy.2627 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 (see: entropy and life).282930

Living systems theories

In order to answer the question ‘What is life?’, some scientists have recently proposed that a general Living systems theory is required.31 Such 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 component parts, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment.32

Gaia hypothesis

The idea that the Earth is alive is probably as old as humankind, but the first public expression of it as a fact of science was by a Scottish scientist, James Hutton. In 1785 he stated that the Earth was a superorganism and that its proper study should be physiology. Hutton is rightly remembered as the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the nineteenth century.33 The Gaia hypothesis, originally proposed in the 1960s by scientist James Lovelock,3435 explores the idea that the life on Earth functions as a single organism which actually defines and maintains environmental conditions necessary for its survival.36

Nonfractionability

Robert Rosen (1991) built on the assumption that the explanatory powers of the mechanistic worldview cannot help understand the realm of living systems. One of several important clarifications he made was to define a system component as "a unit of organization; a part with a function, i.e., a definite relation between part and whole." From this and other starting concepts, he developed a "relational theory of systems" that attempts to explain the special properties of life. Specifically, he identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and 'biological machines.'37

Life as a property of ecosystems

A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity of influence",38 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.39 He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. Robert Ulanowicz (2009) also highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems.40

Origin of life Main article: Origin of life For religious beliefs about the creation of life, see creation myth.

Evidence suggests that life on Earth has existed for about 3.7 billion years.41 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. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that combine both categories.42

There is no scientific consensus as to how life originated and all proposed theories are highly speculative. However, most currently accepted scientific models build in one way or another on the following hypotheses:

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 amino acids and other organic compounds from inorganic precursors. Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.

Life as we know it today synthesizes proteins, which are polymers of amino acids using instructions encoded by cellular genes—which are polymers of deoxyribonucleic acid (DNA). Protein synthesis also entails intermediary ribonucleic acid (RNA) polymers. One possibility is that genes came first43 and then proteins. Another possibility is that proteins came first44 and then genes. However, because genes are required to make proteins, and proteins are required to make genes, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because DNA and proteins function together so intimately, it's unlikely that they arose independently.45 Therefore, many scientists consider the possibility, apparently first suggested by Francis Crick,46 that the first life was based on the DNA-protein intermediary: RNA.45 In fact, RNA has the DNA-like properties of information storage and replication and the catalytic properties of some proteins. Crick and others actually favored the RNA-first hypothesis47 even before the catalytic properties of RNA had been demonstrated by Thomas Cech.48

A significant issue with the RNA-first hypothesis is that experiments designed to synthesize RNA from simple precursors have not been nearly as successful as the Miller-Urey experiments that synthesized other organic molecules from inorganic precursors. One reason for the failure to create RNA in the laboratory is that RNA precursors are very stable and don't react with each other under ambient conditions. However, the successful synthesis of certain RNA molecules under conditions hypothesized to exist prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction.49 This study makes the RNA-first hypothesis more plausible to many scientists.50

Recent experiments have demonstrated true Darwinian evolution of unique RNA enzymes (ribozymes) made up of two separate catalytic components that replicate each other in vitro.51 In describing this work from his laboratory, Gerald Joyce stated: "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system."52 Such experiments make the possibility of a primordial RNA World even more attractive to many scientists.

Conditions for life

The diversity of life on Earth today is a result of the dynamic interplay between genetic opportunity, metabolic capability and environmental challenges,53 as well as symbiosis.545556 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.53 For example, the release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced fundamental, global changes in the Earth's environment. The altered environment, in turn, posed novel evolutionary challenges to the organisms present, which ultimately resulted in the formation of our planet's major animal and plant species. Therefore this 'co-evolution' between organisms and their environment is apparently an inherent feature of living systems.53

Range of tolerance

The inert components of an ecosystem are the physical and chemical factors necessary for life – energy (sunlight or chemical energy), water, temperature, atmosphere, gravity, nutrients, and ultraviolet solar radiation protection.57 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'.58 Outside of that are the 'zones of physiological stress', where the survival and reproduction are possible but not optimal. Outside of these zones are the 'zones of intolerance', where life for that organism is implausible. It has been determined that organisms that have a wide range of tolerance are more widely distributed than organisms with a narrow range of tolerance.58

Extremophiles Deinococcus radiodurans can resist radiation exposure. Main article: Extremophile

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. Furthermore, some microorganisms can survive exposure to such conditions for weeks, months, years, or even centuries.53 Extremophiles are microbial life forms that thrive outside the ranges life is commonly found in. They also 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. Characterization of the structure and metabolic diversity of microbial communities in such extreme environments is ongoing. 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.53

Classification of life Main article: Biological classification

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The hierarchy of biological classification's eight major taxonomic ranks. Life is divided into domains, which are subdivided into further groups. Intermediate minor rankings are not shown.

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 (384-322 BC). He classified all living organisms known at that time as either a plant or an animal. Aristotle distinguished animals with blood from animals without blood (or at least without red blood), which can be compared with the concepts of vertebrates and invertebrates respectively. He divided the blooded animals into five groups: viviparous quadrupeds (mammals), birds, oviparous quadrupeds (reptiles and amphibians), fishes and whales. The bloodless animals were also divided into five groups: cephalopods, crustaceans, insects (which also included the spiders, scorpions, and centipedes, in addition to what we now define as insects), shelled animals (such as most molluscs and echinoderms) and "zoophytes". 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.59

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.60 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. This convention for naming species is referred to as binomial nomenclature.

The fungi were originally treated as plants. For a short period Linnaeus had placed them in the taxon Vermes in Animalia. He later placed them back in Plantae. Copeland classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledged their special status.61 The problem was eventually solved by Whittaker, when he gave them their own kingdom in his five-kingdom system. As it turned out, the fungi are more closely related to animals than to plants.62

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. These new organisms were originally described separately in protozoa as animals and protophyta/thallophyta as plants, but were united by Haeckel in his kingdom protista, later the group of prokaryotes were split off in the kingdom Monera, eventually this kingdom would be divided in two separate groups, the Bacteria and the Archaea, leading to the six-kingdom system and eventually to the current three-domain system.63 The classification of eukaryotes is still controversial, with protist taxonomy especially problematic.64

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.65

Since the 1960s a trend called cladistics has emerged, arranging taxa in an evolutionary or phylogenetic tree. It is unclear, should this be implemented, how the different codes will coexist.66

Linnaeus 173567 2 kingdoms Haeckel 186668 3 kingdoms Chatton 19256970 2 empires Copeland 19386171 4 kingdoms Whittaker 196972 5 kingdoms Woese et al. 19777374 6 kingdoms Woese et al. 199063 3 domains (not treated) Protista Prokaryota Monera Monera Eubacteria Bacteria Archaebacteria Archaea Eukaryota Protista Protista Protista Eukarya Vegetabilia Plantae Fungi Fungi Plantae Plantae Plantae Animalia Animalia Animalia Animalia Animalia

Extraterrestrial life Main articles: Extraterrestrial life and astrobiology Panspermia hypothesis

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. Depending on those values, the equation may either suggest that life arises frequently or infrequently.

Panspermia, also called exogenesis, is a hypothesis 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. However, this hypothesis does not help explain the ultimate origin of life.

Death Main article: Death

Death is the permanent termination of all vital functions or life processes in an organism or cell.7576 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.77 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 some kind of afterlife, reincarnation, or resurrection.

Extinction Main article: Extinction

Extinction is the gradual process by which a group of taxa or species dies out, reducing biodiversity.78 The moment of extinction is generally considered to be the death of the last individual of that species. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively after a period of apparent absence. 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.79

Fossils Main article: Fossil

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 (strata) is known as the fossil record. Such a preserved specimen is called a "fossil" if it is older than the arbitrary date of 10,000 years ago.80 Hence, 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.

See also Alpha taxonomy Artificial life Biology - the study of life Carbon-based life Cellular life Conway's Game of Life Death Entropy and life Extraterrestrial life Extremophile Gaia hypothesis Genetics Genetic engineering Hierarchy of life Hypothetical types of biochemistry Kingdom (biology) Life on Mars Meaning of life Nature Non-cellular life Organic life Organism Origin of life Personal life Phylogenetics Prehistoric life Quality of life Synthetic life References ^ a b Koshland Jr, Daniel E. (March 22, 2002). "The Seven Pillars of Life". Science 295. (5563): 2215–2216. doi:10.1126/science.1068489. PMID 11910092. http://www.sciencemag.org/cgi/content/full/295/5563/2215. Retrieved 2009-05-25.  ^ The American Heritage Dictionary of the English Language, 4th edition, published by Houghton Mifflin Company, via Answers.com: "The property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli or adaptation to the environment originating from within the organism." "The characteristic state or condition of a living organism." ^ Definition of inanimate. WordNet Search by Princeton University. ^ The Concise Oxford Dictionary. English Edition 1991 ^ "Merriam-Webster Dictionary". Merriam-Webster Dictionary. http://www.merriam-webster.com/dictionary/life. Retrieved 2009-06-21.  ^ "organism". Chambers 21st Century Dictionary (online ed.). 1999.  ^ SEP ^ SEP ^ Ibidem ^ Aristotle, De Anima, Book II ^ Introduction to Ancient Philosophy, Don Marietta, p.104. ^ Defining Life : Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond ^ Defining Life, Explaining Emergence ^ "Can We Define Life". Colorado Arts & Sciences. 2009. http://artsandsciences.colorado.edu/magazine/2009/03/can-we-define-life/. Retrieved 2009-06-22.  ^ Nealson KH, Conrad PG (December 1999). "Life: past, present and future". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 354 (1392): 1923–39. doi:10.1098/rstb.1999.0532. PMID 10670014.& PMC 1692713. http://journals.royalsociety.org/content/7r10hqn3rp1g1vag/fulltext.pdf.  ^ a b c McKay, Chris P. (September 14, 2004). "What Is Life—and How Do We Search for It in Other Worlds?". PLoS Biol. 2 (2(9)): 302. PMID PMC516796 doi:10.1371/journal.pbio.0020302. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC516796/?tool=pubmed. Retrieved 2010-02-02.  ^ Davison, Paul G.. "How to Define Life". The University of North Alabama. http://www2.una.edu/pdavis/BI%20101/Overview%20Fall%202004.htm. Retrieved 2008-10-17.  ^ a b Witzany, G. (2007). The Logos of the Bios 2. Bio-Communication. Helsinki, Umweb. ^ Korzeniewski, Bernard (2001). "Cybernetic formulation of the definition of life". Journal of Theoretical Biology. 2001 April 7. 209 (3) pp. 275–86. ^ 2004, "Autonomous Agents", in John D. Barrow, P.C.W. Davies, and C.L. Harper Jr., eds., Science and Ultimate Reality: Quantum Theory, Cosmology, and Complexity, Cambridge University Press. ^ Gerald Francis Joyce, "The RNA World: Life Before DNA and Protein". ^ Rybicki EP (1990) "The classification of organisms at the edge of life, or problems with virus systematics." S Aft J Sci 86:182–186 ^ Holmes EC (October 2007). "Viral evolution in the genomic age". PLoS Biol. 5 (10): e278. doi:10.1371/journal.pbio.0050278. PMID 17914905.& PMC 1994994. http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0050278. Retrieved 2008-09-13.  ^ Koonin EV, Senkevich TG, Dolja VV (2006). "The ancient Virus World and evolution of cells". Biol. Direct 1: 29. doi:10.1186/1745-6150-1-29. PMID 16984643.& PMC 1594570. http://www.biology-direct.com/content/1//29. Retrieved 2008-09-14.  ^ Rybicki, Ed (November 1997). "Origins of Viruses". http://www.mcb.uct.ac.za/tutorial/virorig.html#Virus%20Origins. Retrieved 2009-04-12.  ^ Schrödinger, Erwin (1944). What is Life?. Cambridge University Press. ISBN 0-521-42708-8.  ^ Margulis, Lynn; Sagan, Dorion (1995). What is Life?. University of California Press. ISBN 0-520-22021-8.  ^ Lovelock, James (2000). Gaia – a New Look at Life on Earth. Oxford University Press. ISBN 0-19-286218-9.  ^ Avery, John (2003). Information Theory and Evolution. World Scientific. ISBN 9812383999.  ^ "BIOPHYSICS: DEFINITION OF LIFE AND BRIEF EXPLANATION OF EACH TERM". Biology Cabinet. September 29, 2006. http://biocab.org/Exobiology.html#anchor_41. Retrieved 2009-07-22.  ^ Woodruff, T. Sullivan; John Baross (Oct 8, 2007). Planets and Life: The Emerging Science of Astrobiology. Cambridge University Press.  Cleland and Chyba wrote a chapter in Planets and Life: "In the absence of such a theory, we are in a position analogous to that of a 16th-century investigator trying to define 'water' in the absence of molecular theory." [...] "Without access to living things having a different historical origin, it is difficult and perhaps ultimately impossible to formulate an adequately general theory of the nature of living systems". ^ "Patterns, Flows, and Interrelationship". 2002. http://www.mollyyoungbrown.com/systems_article.htm. Retrieved 2009-06-27.  ^ GAIA - A new look at life on Earth. James Lovelock 1979. pp. 10. Oxford University Press. ISBN 0-19-286030-5. ^ Lovelock, J.E. (1965). "A physical basis for life detection experiments". Nature 207 (7): 568–570. doi:10.1038/207568a0.  ^ Geophysiology ^ GAIA - A new look at life on Earth. James Lovelock. 1979. Oxford University Press. ISBN 0-19-286030-5. ^ Life Itself: A Comprehensive Inquiry into the Nature, Origin, and Fabrication of Life. Rosen, Robert. November, 1991. ISBN 978-0-231-07565-7 ^ "The Ecosystemic Life Hypothesis". Bulletin of the Ecological Society of America. April 2002. http://www.calresco.org/fiscus/esl.htm. Retrieved 2009-08-28.  ^ Morowitz, Harold J. (1992) "Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis". Yale University Press. ISBN 0-300-05483-1 ^ A Third Window: Natural Life Beyond Newton and Darwin, Templeton Foundation Press (2009) ISBN 159947154X ^ "History of life through time". University of California Museum of Paleontology. ^ Coveney, Peter V.; Philip W. Fowler. "Modelling biological complexity: a physical scientist's perspective". Journal of the Royal Society Interface. 2005. 2 (4) pp. 267–280. doi:10.1098/rsif.2005.0045 ^ Senapathy, Periannan, Independent Birth of Organisms, Madison, WI. Genome Press, 1994. ^ Eigen, Manfred, Steps Towards Life: A Perspective on Evolution (German edition, 1987), Oxford University Press, 1992. p 31. ^ a b Barazesh, Solmaz, How RNA Got Started: Scientists Look for the Origins of Life, Science News, May 13, 2009. ^ Watson, James D., Prologue: Early Speculations and Facts about RNA Templates, p xv-xxiii, The RNA World, R.F. Gesteland and J.F. Atkins, Eds., Cold Spring Harbor Laboratory Press, 1993. ^ Gilbert, Walter, The RNA world, p 618 v 319, Nature, 1986. ^ Cech, Thomas R., A model for the RNA-catalyzed replication of RNA, p 4360-4363 v 83, Proc. Nat. Acad. Sci., USA, 1986. ^ Powner, Matthew W., Béatrice Gerland and John D. Sutherland, Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions, Nature 459, 239-242 (14 May 2009). ^ Szostak, Jack W., Origins of life: Systems chemistry on early Earth, Nature 459, 171-172 (14 May 2009). ^ Lincoln, Tracey A. and Gerald F. Joyce, Self-Sustained Replication of an RNA Enzyme, Science 27 February 2009:Vol. 323, No. 5918, pp. 1229-1232, DOI: 10.1126/science.1167856. ^ Joyce, Gerald F., Evolution in an RNA World, Cold Spring Harb Symp Quant Biol sqb.2009.74.004; Published in Advance August 10, 2009, doi:10.1101/sqb.2009.74.004. ^ a b c d e Rothschild, Lynn (September, 2003). "Understand the evolutionary mechanisms and environmental limits of life". NASA. http://astrobiology.arc.nasa.gov/roadmap/g5.html. Retrieved 2009-07-13.  ^ "Symbiosis and the origin of life". Origins of Life and Evolution of Biospheres 8 (1): 39-53. April, 1977. doi:10.1007/BF00930938. http://www.springerlink.com/content/n10p775113175l67/. Retrieved 2010-02-22.  ^ Margulis, Lynn (2001). The Symbiotic Planet: A New Look at Evolution. London, England: Orion Books Ltd.. ISBN 0-75380-785-8.  ^ Douglas J. Futuyma; Janis Antonovics (1992, Volume 8). Oxford surveys in evolutionary biology: Symbiosis in evolution. London, England: Oxford University Press. pp. 347-374. ISBN 0-19-507623-0.  ^ "Essential requirements for life". CMEX-NASA. http://cmapsnasacmex.ihmc.us/servlet/SBReadResourceServlet?rid=1025200161109_2045745605_1714&partName=htmltext. Retrieved 2009-07-14.  ^ a b Chiras, Daniel C. (2009). Environmental Science – Creating a Sustainable Future.  ^ "Aristotle -biography". University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/history/aristotle.html. Retrieved 2008-10-20.  ^ Knapp S, Lamas G, Lughadha EN, Novarino G (April 2004). "Stability or stasis in the names of organisms: the evolving codes of nomenclature". Philosophical transactions of the Royal Society of London. Series B, Biological sciences 359 (1444): 611–22. doi:10.1098/rstb.2003.1445. PMID 15253348.& PMC 1693349. http://journals.royalsociety.org/openurl.asp?genre=article&issn=0962-8436&volume=359&issue=1444&spage=611.  ^ a b Copeland, H. F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology 13 (4): 383. doi:10.1086/394568. http://links.jstor.org/sici?sici=0033-5770(193812)13%3A4%3C383%3ATKOO%3E2.0.CO%3B2-K.  ^ Whittaker RH (January 1969). "New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms". Science 163 (863): 150–60. doi:10.1126/science.163.3863.150. PMID 5762760.  ^ a b Woese C, Kandler O, Wheelis M (1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya.". Proc Natl Acad Sci U S A 87 (12): 4576–9. doi:10.1073/pnas.87.12.4576. PMID 2112744.& PMC 54159. http://www.pnas.org/cgi/reprint/87/12/4576.  ^ Adl SM, Simpson AG, Farmer MA, et al. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". J. Eukaryot. Microbiol. 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873.  ^ Van Regenmortel MH (January 2007). "Virus species and virus identification: past and current controversies". Infection, genetics and evolution : journal of molecular epidemiology and evolutionary genetics in infectious diseases 7 (1): 133–44. doi:10.1016/j.meegid.2006.04.002. PMID 16713373.  ^ Pennisi E (March 2001). "Taxonomy. Linnaeus's last stand?". Science (New York, N.Y.) 291 (5512): 2304–7. doi:10.1126/science.291.5512.2304. PMID 11269295. http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=11269295.  ^ C. Linnaeus (1735). Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species.  ^ E. Haeckel (1866). Generelle Morphologie der Organismen. Reimer, Berlin.  ^ É. Chatton (1925). "Pansporella perplexa. Réflexions sur la biologie et la phylogénie des protozoaires". Ann. Sci. Nat. Zool 10-VII: 1–84.  ^ É. Chatton (1937). Titres et Travaux Scientifiques (1906–1937). Sette, Sottano, Italy.  ^ H. F. Copeland (1956). The Classification of Lower Organisms. Palo Alto: Pacific Books.  ^ Whittaker RH (January 1969). "New concepts of kingdoms of organisms". Science 163 (863): 150–60. doi:10.1126/science.163.3863.150. PMID 5762760.  ^ C. R. Woese, W. E. Balch, L. J. Magrum, G. E. Fox and R. S. Wolfe (August 1977). "An ancient divergence among the bacteria". Journal of Molecular Evolution 9 (4): 305–311. doi:10.1007/BF01796092. PMID 408502.  ^ Woese CR, Fox GE (November 1977). "Phylogenetic structure of the prokaryotic domain: the primary kingdoms". Proc. Natl. Acad. Sci. U.S.A. 74 (11): 5088–90. PMID 270744.  ^ "Definition of death.". Archived from the original on 2009-10-31. http://www.webcitation.org/5kwsdvU8f.  ^ Defining of death. ^ Encyclopedia of Death and Dying ^ "Extinction - definition.". Archived from the original on 2009-10-31. http://www.webcitation.org/5kwseRB80.  ^ What is an extinction? ^ FAQs - San Diego Natural History Museum Further reading Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman Nealson KH, Conrad PG (December 1999). "Life: past, present and future". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 354 (1392): 1923–39. doi:10.1098/rstb.1999.0532. PMID 10670014.& PMC 1692713. http://journals.royalsociety.org/content/7r10hqn3rp1g1vag/.  Walker, Martin G. LIFE! Why We Exist...And What We Must Do to Survive Dog Ear Publishing, 2006, ISBN 1598582437 External links Wikimedia Commons has media related to: Tree of life Wikiquote has a collection of quotations related to: Life Look up life or living in Wiktionary, the free dictionary. Wikispecies has information related to: The Taxonomy of Life Wikispecies - a free directory of life "The Adjacent Possible: A Talk with Stuart Kauffman" Stanford Encyclopedia of Philosophy entry Life under extreme conditions An in depth look at how life can form under the most extreme conditions and circumstaces. v • d • e Elements of Nature Universe Space · Time · Matter · Energy Earth Earth science · Geology · History of the Earth · Geological history of Earth · Future of the Earth · Structure of the Earth · Plate tectonics Weather Earth's atmosphere · Climate Environment Ecology · Ecosystem · Wilderness Life Hierarchy of life · Origin of life · Life on Earth · Eukaryota (Plants/Flora, Animals/Fauna, Fungi, Protista) · Prokaryote (Archaea, Bacteria) · Virus · Evolutionary history of life · Biology Category · Portal v • d • e Hierarchy of life Biosphere >  Ecosystem > Community (Biocoenosis) > Population >  Organism > Organ system > Organ > Tissue > Cell > Organelle > Molecule (Macromolecule · Biomolecule) > Atom v • d • e    Basic topics in evolutionary biology Evidence of common descent Processes of evolution Adaptation · Macroevolution · Microevolution · Speciation Population genetic mechanisms Genetic drift · Gene flow · Mutation · Natural selection Evolutionary developmental biology (Evo-devo) concepts Canalisation · Modularity · Phenotypic plasticity The evolution of... Aging · Birds · Dinosaurs · DNA · Dolphins and whales · The Ear · The Eye · Flight · Fungi · Horses · Humans · Human intelligence · Insects · Life · Molluscs · Plants · Sex · Sirenians (sea cows) · Spiders Modes of speciation Anagenesis · Catagenesis · Cladogenesis History of evolutionary thought Charles Darwin · On the Origin of Species · Modern evolutionary synthesis · Gene-centered view of evolution · Life (classification trees) Other subfields Ecological genetics · Molecular evolution · Phylogenetics · Systematics List of evolutionary biology topics · Timeline of evolution v • d • e Taxonomic ranks Magnorder Domain/Superkingdom Superphylum/Superdivision Superclass Superorder Superfamily Superspecies Kingdom Phylum/Division Class Legion Order Family Tribe Genus Species Subkingdom Subphylum Subclass Cohort Suborder Subfamily Subtribe Subgenus Subspecies Infrakingdom/Branch Infraphylum Infraclass Infraorder Alliance Infraspecies Microphylum Parvclass Parvorder

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Men more interested, have better sex life
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Accident claims second life in two weeks in Ringwood
For the second time in just about a week, an accident has claimed a life in the borough, and winter weather had nothing to do with it.

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Standard Life has broken ranks with two of its largest domestic competitors by insisting the UK will remain its major source of future growth.

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Air NZ mercy mission saves man's life
A Gisborne man who was left without essential medication when his luggage went missing on a Pacific Blue flight to Rarotonga says he owes his life to rival airline Air New Zealand.Liver transplant patient Robert DeLong packed...

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A Gisborne man who was left without essential medication when his luggage went missing on a Pacific Blue flight to Rarotonga says he owes his life to rival airline Air New Zealand.

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Sex Life Ends at 70 as Health Declines, Study Says (Update1)
The average person’s sex life ends by the age of 70, according to a report published today in the British Medical Journal.

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Sex Life Ends at 70 as Health Declines, Study Says (Update1)
Men age 30 have an average of 35 years of sexually active life remaining, compared with 31 years for women, researchers at the University of Chicago ’s department of obstetrics and gynecology estimated after reviewing a survey of 3,000 people.

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