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In biology, an organism is any contiguous living system, such as an animal, plant or bacterium. More than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described.
All known types of organisms are capable of some degree of response to stimuli, reproduction, growth and development and homeostasis. An organism consists of one or more cells; when it has one cell is a unicellular organism; and when it has more than one it is known as a multicellular organism. Most unicellular organisms are of microscopic size and are thus classified as microorganisms. Humans are multicellular organisms composed of many trillions of cells grouped into specialized tissues and organs.
An organism may be either a prokaryote or a eukaryote. Prokaryotes are represented by two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles (such as mitochondria in animals and plants and plastids in plants and algae, all generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes.
In 2002, Thomas Cavalier-Smith, author of Branching order of bacterial phyla, proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved from Bacteria, more specifically from Actinobacteria.
- 1 Etymology
- 2 Semantics
- 3 Organizational terminology
- 4 Chemistry
- 5 Structure
- 6 Life span
- 7 Evolution
- 8 See also
- 9 References
- 10 External links
The term "organism" (from Greek ὀργανισμός, organismos, from ὄργανον, organon, i.e. "instrument, implement, tool, organ of sense or apprehension") first appeared in the English language in 1703 and took on its current definition by 1834 (Oxford English Dictionary). It is directly related to the term "organization". There is a long tradition of defining organisms as self-organizing beings.
There has been controversy about the best way to define the organism and indeed about whether or not such a definition is necessary. Several contributions are responses to the suggestion that the category of "organism" may well not be adequate in biology.
The word organism may broadly be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. However, many sources propose definitions that exclude viruses and theoretically possible man-made non-organic life forms. Viruses are dependent on the biochemical machinery of a host cell for reproduction.
In multicellular terms, "organism" usually describes the whole hierarchical assemblage of systems (for example circulatory, digestive, or reproductive) themselves collections of organs; these are, in turn, collections of tissues, which are themselves made of cells. In some plants and the nematode Caenorhabditis elegans, individual cells are totipotent.
Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, growth or metabolism. This controversy is problematic because some cellular organisms are also incapable of independent survival (but not of independent metabolism and procreation) and live as obligatory intracellular parasites. Although viruses have a few enzymes and molecules characteristic of living organisms, they have no metabolism of their own and cannot synthesize and organize the organic compounds that form them. Naturally, this rules out autonomous reproduction and they can only be passively replicated by the machinery of the host cell. In this sense they are similar to inanimate matter. While viruses sustain no independent metabolism, and thus are usually not accounted organisms, they do have their own genes and they do evolve by similar mechanisms by which organisms evolve.
The most common argument in support of viruses as living organisms is their ability to undergo evolution and replicate through self-assembly. Some scientists argue that viruses neither evolve, nor self- reproduce. In fact, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible. This is not true for cells. If viruses did not exist, the direction of evolution could be different; however, the ability to evolve would not be affected. As for the reproduction, viruses totally rely on hosts' machinery to replicate themselves. The discovery of viral megagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong on the tree of life. The presence of these genes suggested that viruses could metabolize in the past. It was found later that the genes coding for energy and protein metabolism have cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts.
To give an example, Homo sapiens is the Latin binomial equating to modern humans. All members of the species sapiens are, at least in theory, genetically able to interbreed. Several species may belong to a genus, but the members of different species within a genus are usually unable to interbreed to produce fertile offspring. (The red wolf Canis lupus rufus, however, may be a result of interbreeding between the grey wolf Canis lupus and the coyote Canis latrans.) Homo only has one surviving species (sapiens), Homo erectus, Homo neanderthalensis, etc. having become extinct thousands of years ago; some scientists argue for interbreeding between H. sapiens and H. neanderthalensis with fertile progeny. Several genera belong to the same family and so on up the hierarchy. Eventually, the relevant kingdom (Animalia, in the case of humans) is placed into one of the three domains depending upon certain genetic and structural characteristics.
All living organisms known to science are given classification by this system such that the species within a particular family are more closely related and genetically similar than the species within a particular phylum.
Since viruses are not living organisms, their classification is a challenging task. At first, viruses were classified according to their hosts: plant viruses, animal viruses, bacteriophages. Later, they were classified by the disease that they cause. For example, respiratory viruses, enterics. Now, viruses are classified based on the nucleic acid content, capsid symmetry and the presence or absence of the envelope.
Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival. The same laws that govern non-living chemistry govern the chemical processes of life. It is generally the phenomena of entire organisms that determine their fitness to an environment and therefore the survivability of their DNA-based genes.
Organisms clearly owe their origin, metabolism, and many other internal functions to chemical phenomena, especially the chemistry of large organic molecules. Organisms are complex systems of chemical compounds that, through interaction and environment, play a wide variety of roles.
Organisms are semi-closed chemical systems. Although they are individual units of life (as the definition requires), they are not closed to the environment around them. To operate they constantly take in and release energy. Autotrophs produce usable energy (in the form of organic compounds) using light from the sun or inorganic compounds while heterotrophs take in organic compounds from the environment.
The primary chemical element in these compounds is carbon. The chemical properties of this element such as its great affinity for bonding with other small atoms, including other carbon atoms, and its small size making it capable of forming multiple bonds, make it ideal as the basis of organic life. It is able to form small three-atom compounds (such as carbon dioxide), as well as large chains of many thousands of atoms that can store data (nucleic acids), hold cells together, and transmit information (protein).
Compounds that make up organisms may be divided into macromolecules and other, smaller molecules. The four groups of macromolecule are nucleic acids, proteins, carbohydrates and lipids. Nucleic acids (specifically deoxyribonucleic acid, or DNA) store genetic data as a sequence of nucleotides. The particular sequence of the four different types of nucleotides (adenine, cytosine, guanine, and thymine) dictate many characteristics that constitute the organism. The sequence is divided up into codons, each of which is a particular sequence of three nucleotides and corresponds to a particular amino acid. Thus a sequence of DNA codes for a particular protein that, due to the chemical properties of the amino acids it is made from, folds in a particular manner and so performs a particular function.
These protein functions have been recognized:
- Enzymes, which catalyze all of the reactions of metabolism
- Structural proteins, such as tubulin, or collagen
- Regulatory proteins, such as transcription factors or cyclins that regulate the cell cycle
- Signaling molecules or their receptors such as some hormones and their receptors
- Defensive proteins, which can include everything from antibodies of the immune system, to toxins (e.g., dendrotoxins of snakes), to proteins that include unusual amino acids like canavanine
A bilayer of phospholipids makes up the membrane of cells that constitutes a barrier, containing everything within the cell and preventing compounds from freely passing into, and out of, the cell. Due to the selective permeability of the phospholipid membraine only specific compounds can pass through it. In some multicellular organisms they serve as a storage of energy and mediate communication between cells. Carbohydrates are more easily broken down than lipids and yield more energy to compare to lipids and proteins.In fact, carbohydrates are the number one source of energy for all living organisms.
All organisms consist of monomeric units called cells; some contain a single cell (unicellular) and others contain many units (multicellular). Multicellular organisms are able to specialize cells to perform specific functions. A group of such cells is a tissue, and in animals these occur as four basic types, namely epithelium, nervous tissue, muscle tissue, and connective tissue. Several types of tissue work together in the form of an organ to produce a particular function (such as the pumping of the blood by the heart, or as a barrier to the environment as the skin). This pattern continues to a higher level with several organs functioning as an organ system to allow for reproduction, digestion, etc. Many multicellular organisms consist of several organ systems, which coordinate to allow for life.
The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells, and cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
There are two types of cells, eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multicellular organisms. Prokaryotic cells lack a nuclear membrane so DNA is unbound within the cell; eukaryotic cells have nuclear membranes.
All cells, whether prokaryotic or eukaryotic, have a membrane, which envelops the cell, separates its interior from its environment, regulates what moves in and out, and maintains the electric potential of the cell. Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells.
All cells share several similar characteristics of:
- Reproduction by cell division (binary fission, mitosis or meiosis).
- Use of enzymes and other proteins coded by DNA genes and made via messenger RNA intermediates and ribosomes.
- Metabolism, including taking in raw materials, building cell components, converting energy, molecules and releasing by-products. The functioning of a cell depends upon its ability to extract and use chemical energy stored in organic molecules. This energy is derived from metabolic pathways.
- Response to external and internal stimuli such as changes in temperature, pH or nutrient levels.
- Cell contents are contained within a cell surface membrane that contains proteins and a lipid bilayer.
One of the basic parameters of an organism is its life span. Some organisms live as short as one day, while some plants and fungi can live thousands of years.
Last universal ancestor
The last universal ancestor is the most recent organism from which all organisms now living on Earth descend. Thus it is the most recent common ancestor (MRCA) of all current life on Earth. The LUA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era). The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia. Although more than 99 percent of all species that ever lived on the planet are estimated to be extinct, there are currently 10–14 million species of life on Earth.
Information about the early development of life includes input from many different fields, including geology and planetary science. These sciences provide information about the history of the Earth and the changes produced by life. However, a great deal of information about the early Earth has been destroyed by geological processes over the course of time.
All organisms on Earth are descended from a common ancestor or ancestral gene pool. Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds have wings, even those that do not fly.
Today, there is strong evidence from genetics that all organisms have a common ancestor. For example, every living cell makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. The universality of these traits strongly suggests common ancestry, because the selection of many of these traits seems arbitrary.
In 2000, W. Ford Doolittle discussed variability in the genetic code (see: genetic code variation) and suggested that horizontal gene transfer might make it difficult to study the last universal ancestor. In 2010, a formal mathematical test calculated from the genetic record (and in particular the universal use of the same genetic code, same nucleotides, and same amino acids), that the factor in favor of a common ancestor's existence is 10^2489.
Sexual reproduction (sex) is widespread among current eukaryotes. Dacks and Rogers proposed, on the basis of a phylogenetic analysis, that sex was present in the common ancestor of all eukaryotes. The finding of a core set of genes necessary for meiosis in the descendants of lineages that diverged early form the eukaryotic evolutionary tree also led Ramesh et al. and Malik et al. to suggest that sex was present in a common ancestor of all eukaryotes. Furthermore, this view is supported by evidence that eukaryotes previously regarded as "ancient asexuals", such as amoeba, were likely sexual in the past, and that most present day asexual amoeboid lineages likely arose recently and independently.
In prokaryotes, natural bacterial transformation involves the transfer of DNA from one bacterium to another and integration of the donor DNA into the recipient chromosome by recombination. Natural bacterial transformation is considered to be a primitive sexual process and occurs in both bacteria and archaea, although it has been studied mainly in bacteria. Transformation is clearly a bacterial adaptation and not an accidental occurrence, because it depends on numerous gene products that specifically interact with each other to enter a state of natural competence to perform this complex process. Transformation is a common mode of DNA transfer, and over 60 prokaryotic species are known to be naturally competent for transformation.
History of life
The chemical evolution from self-catalytic chemical reactions to life (see Origin of life) is not a part of biological evolution, but it is unclear at which point such increasingly complex sets of reactions became what we would consider, today, to be living organisms.
Little is known about the earliest developments in life. However, all existing organisms share certain traits, including cellular structure and genetic code. Most scientists interpret this to mean all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, but there is no scientific consensus on the relationship of the three domains of life (Archaea, Bacteria, Eukaryota) or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of macromolecules, particularly RNA, and the behavior of complex systems.
The emergence of oxygenic photosynthesis (around 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, believed to have emerged around 2 billion years ago.
In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals, the Cambrian explosion (a period of unrivaled and remarkable, but brief, organismal diversity documented in the fossils found at the Burgess Shale) saw the creation of all the major body plans, or phyla, of modern animals. This event is now believed to have been triggered by the development of the Hox genes. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of today's land ecosystems.
The evolutionary process may be exceedingly slow. Fossil evidence indicates that the diversity and complexity of modern life has developed over much of the history of the earth. Geological evidence indicates that the Earth is approximately 4.6 billion years old. Studies on guppies by David Reznick at the University of California, Riverside, however, have shown that the rate of evolution through natural selection can proceed 10 thousand to 10 million times faster than what is indicated in the fossil record. Such comparative studies however are invariably biased by disparities in the time scales over which evolutionary change is measured in the laboratory, field experiments, and the fossil record.
Horizontal gene transfer
The ancestry of living organisms has traditionally been reconstructed from morphology, but is increasingly supplemented with phylogenetics—the reconstruction of phylogenies by the comparison of genetic (DNA) sequence.
Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic "domains". Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes.
Biologist Gogarten suggests "the original metaphor of a tree no longer fits the data from recent genome research", therefore "biologists (should) use the metaphor of a mosaic to describe the different histories combined in individual genomes and use (the) metaphor of a net to visualize the rich exchange and cooperative effects of HGT among microbes."
Future of life (cloning and synthetic organisms)
In modern terms, the category of organism cloning refers to the procedure of creating a new multicellular organism, genetically identical to another. However, the techniques used in cloning have the potential of creating entirely new species of organisms. Organism cloning is the subject of much ethical debate. (see Bioethics, Ethics of cloning, and Designer baby articles)
In 2008 the J. Craig Venter Institute assembled a synthetic bacterial genome, Mycoplasma genitalium, by using recombination in yeast of 25 overlapping DNA fragments in a single step. The use of yeast recombination greatly simplifies the assembly of large DNA molecules from both synthetic and natural fragments. Other companies, such as Synthetic Genomics, have already been formed to take advantage of the many commercial uses of custom designed genomes.
Guillin Barr syndrome
- Kunin, W.E.; Gaston, Kevin, eds. (31 December 1996). The Biology of Rarity: Causes and consequences of rare—common differences. ISBN 978-0412633805. Retrieved 26 May 2015.
- Stearns, Beverly Peterson; Stearns, S. C.; Stearns, Stephen C. (2000). Watching, from the Edge of Extinction. Yale University Press. p. 1921. ISBN 978-0-300-08469-6. Retrieved 2014-12-27.
- Novacek, Michael J. (8 November 2014). "Prehistory’s Brilliant Future". New York Times. Retrieved 2014-12-25.
- G. Miller; Scott Spoolman (2012). Environmental Science - Biodiversity Is a Crucial Part of the Earth's Natural Capital. Cengage Learning. p. 62. ISBN 1-133-70787-4. Retrieved 2014-12-27.
- Mora, C.; Tittensor, D.P.; Adl, S.; Simpson, A.G.; Worm, B. (23 August 2011). "How many species are there on Earth and in the ocean?". PLOS Biology 9: e1001127. doi:10.1371/journal.pbio.1001127. PMC 3160336. PMID 21886479.
- Cavalier-Smith T. (1987). "The origin of eukaryotic and archaebacterial cells". Annals of New York Academy of Sciences 503: 17–54. doi:10.1111/j.1749-6632.1987.tb40596.x. PMID 3113314.
- "The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification" (PDF). Int. J. Syst. Evol. Microbiol. 52 (Pt 1): 7–76. January 2002. PMID 11837318.
- ὄργανον. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
- "organism". Online Etymology Dictionary.
- Kant I., Critique of Judgment: §64.
- Dupré, J. (2010). "The polygenomic organism". The Sociological Review 58: 19–99. doi:10.1111/j.1467-954X.2010.01909.x.
- Folse Hj, 3.; Roughgarden, J. (2010). "What is an individual organism? A multilevel selection perspective". The Quarterly review of biology 85 (4): 447–472. doi:10.1086/656905. PMID 21243964.
- Pradeu, T. (2010). "What is an organism? An immunological answer". History and philosophy of the life sciences 32 (2–3): 247–267. PMID 21162370.
- Gardner, A.; Grafen, A. (2009). "Capturing the superorganism: A formal theory of group adaptation". Journal of Evolutionary Biology 22 (4): 659–671. doi:10.1111/j.1420-9101.2008.01681.x. PMID 19210588.
- Michod, R E (1999). Darwinian dynamics: evolutionary transitions in fitness and individuality. Princeton University Press. ISBN 978-0-691-05011-9.
- Queller, D. C; J. E Strassmann (2009). "Beyond society: the evolution of organismality". Philosophical Transactions of the Royal Society B: Biological Sciences 364 (1533): 3143–3155. doi:10.1098/rstb.2009.0095.
- Santelices B. (1999). "How many kinds of individual are there?". Trends in ecology & evolution 14 (4): 152–155. doi:10.1016/s0169-5347(98)01519-5. PMID 10322523.
- Wilson, R (2007). "The biological notion of individual". Stanford Encyclopedia of Philosophy.
- Pepper, J. W; M. D Herron (2008). "Does biology need an organism concept?". Biological Reviews 83 (4): 621–627. doi:10.1111/j.1469-185X.2008.00057.x. PMID 18947335.
- Wilson, J (2000). "Ontological butchery: organism concepts and biological generalizations". Philosophy of Science 67: 301–311. JSTOR 188676.
- Bateson, P. (2005). "The return of the whole organism". Journal of biosciences 30 (1): 31–39. doi:10.1007/BF02705148. PMID 15824439.
- Dawkins, Richard (1982). The Extended Phenotype. Oxford University Press. ISBN 0192860887.
- "organism". Oxford English Dictionary (online ed.). 2004.
- "organism". Chambers 21st Century Dictionary (online ed.). 1999.
- Moreira, D.; López-García, P. N. (2009). "Ten reasons to exclude viruses from the tree of life". Nature Reviews Microbiology. doi:10.1038/nrmicro2108.
- The Universal Features of Cells on Earth in Chapter 1 of Molecular Biology of the Cell fourth edition, edited by Bruce Alberts (2002) published by Garland Science.
- Theobald, D. L.I (2010), "A formal test of the theory of universal common ancestry", Nature 465 (7295): 219–22, Bibcode:2010Natur.465..219T, doi:10.1038/nature09014, PMID 20463738
- Doolittle, W. F. (2000), "Uprooting the tree of life" (PDF), Scientific American 282 (6): 90–95, doi:10.1038/scientificamerican0200-90, PMID 10710791.
- Glansdorff, N.; Xu, Y; Labedan, B. (2008), "The Last Universal Common Ancestor: Emergence, constitution and genetic legacy of an elusive forerunner", Biology Direct 3: 29, doi:10.1186/1745-6150-3-29, PMC 2478661, PMID 18613974.
- Yoko Ohtomo; Takeshi Kakegawa; Akizumi Ishida; Toshiro Nagase; Minik T. Rosing (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. doi:10.1038/ngeo2025. Retrieved 9 Dec 2013.
- Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News. Retrieved 15 November 2013.
- Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (8 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology (journal) 13 (12): 1103–24. doi:10.1089/ast.2013.1030. PMC 3870916. PMID 24205812. Retrieved 15 November 2013.
- Doolittle, W. Ford (2000). "Uprooting the tree of life" (PDF). Scientific American 282 (6): 90–95.
- Theobald, Douglas L. (13 May 2010), "A formal test of the theory of universal common ancestry", Nature (London: Macmillan Publishers Limited) 465 (7295): 219–22, Bibcode:2010Natur.465..219T, doi:10.1038/nature09014, ISSN 0028-0836, PMID 20463738.
- Dacks J; Roger AJ (June 1999). "The first sexual lineage and the relevance of facultative sex". J. Mol. Evol. 48 (6): 779–83. doi:10.1007/PL00013156. PMID 10229582.
- Ramesh MA; Malik SB; Logsdon JM (January 2005). "A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis". Curr. Biol. 15 (2): 185–91. doi:10.1016/j.cub.2005.01.003. PMID 15668177.
- Malik SB; Pightling AW; Stefaniak LM; Schurko AM; Logsdon JM (2008). "An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis". PLoS ONE 3 (8): e2879. doi:10.1371/journal.pone.0002879. PMC 2488364. PMID 18663385.
- Lahr DJ; Parfrey LW; Mitchell EA; Katz LA; Lara E (July 2011). "The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms". Proc. Biol. Sci. 278 (1715): 2081–90. doi:10.1098/rspb.2011.0289. PMC 3107637. PMID 21429931.
- Chen I; Dubnau D (March 2004). "DNA uptake during bacterial transformation". Nat. Rev. Microbiol. 2 (3): 241–9. doi:10.1038/nrmicro844. PMID 15083159.
- Johnsborg O; Eldholm V; Håvarstein LS (December 2007). "Natural genetic transformation: prevalence, mechanisms and function". Res. Microbiol. 158 (10): 767–78. doi:10.1016/j.resmic.2007.09.004. PMID 17997281.
- Reznick, D. N.; Shaw, F. H.; Rodd, F. H.; Shaw, R. G. (1997). "Evaluation of the Rate of Evolution in Natural Populations of Guppies (Poecilia reticulata)". Science 275 (5308): 1934–1937. doi:10.1126/science.275.5308.1934. PMID 9072971.
- Oklahoma State – Horizontal Gene Transfer
- Peter Gogarten. "Horizontal Gene Transfer - A New Paradigm for Biology". esalenctr.org. Retrieved 2011-08-20.
- Gibsona, Daniel G.; Benders, Gwynedd A.; Axelroda, Kevin C.; et al. (2008). "One-step assembly in yeast of 25 overlapping DNA fragments to form a complete synthetic Mycoplasma genitalium genome" (PDF). PNAS 105 (51): 20404–20409. doi:10.1073/pnas.0811011106. PMC 2600582. PMID 19073939.
- BBCNews: 27 September 2000, When slime is not so thick Citat: "It means that some of the lowliest creatures in the plant and animal kingdoms, such as slime and amoeba, may not be as primitive as once thought"
- SpaceRef.com, July 29, 1997: Scientists Discover Methane Ice Worms On Gulf Of Mexico Sea Floor
- The Eberly College of Science: Methane Ice Worms discovered on Gulf of Mexico Sea Floor download Publication quality photos
- Artikel, 2000: Methane Ice Worms: Hesiocaeca methanicola. Colonizing Fossil Fuel Reserves
- SpaceRef.com, May 04, 2001: Redefining "Life as We Know it" Hesiocaeca methanicola In 1997, Charles Fisher, professor of biology at Penn State, discovered this remarkable creature living on mounds of methane ice under half a mile of ocean on the floor of the Gulf of Mexico.
- SpaceRef.com, July 29, 1997: Scientists Discover Methane Ice Worms On Gulf Of Mexico Sea Floor
- BBCNews, 18 December 2002, 'Space bugs' grown in lab Citat: "Bacillus simplex and Staphylococcus pasteuri...Engyodontium album The strains cultured by Dr Wainwright seemed to be resistant to the effects of UV - one quality required for survival in space"
- BBCNews, 19 June 2003, Ancient organism challenges cell evolution Citat: "It appears that this organelle has been conserved in evolution from prokaryotes to eukaryotes, since it is present in both"
- Interactive Syllabus for General Biology - BI 04, Saint Anselm College, Summer 2003
- Jacob Feldman: Stramenopila
- NCBI Taxonomy entry: root (rich)
- Saint Anselm College: Survey of representatives of the major Kingdoms Citat: "Number of kingdoms has not been resolved...Bacteria present a problem with their diversity...Protista present a problem with their diversity...",
- Species 2000 Indexing the world's known species. Species 2000 has the objective of enumerating all known species of plants, animals, fungi and microbes on Earth as the baseline dataset for studies of global biodiversity. It will also provide a simple access point enabling users to link from here to other data systems for all groups of organisms, using direct species-links.
- The largest organism in the world may be a fungus carpeting nearly 10 square kilometers of an Oregon forest, and may be as old as 10500 years.
- The Tree of Life.
- Frequent questions from kids about life and their answers