In biology Biology is the science that studies living organisms. Prior to the nineteenth century, biology came under the general study of all natural objects called natural history. The term biology was first coined by Gottfried Reinhold Treviranus.[citation needed] It is now a standard subject of instruction at schools and universities around the world, and, an organism is any living Life is a characteristic that distinguishes objects that have self-sustaining biological processes ("alive," "living"), from those which do not —either because such functions have ceased (death), or else because they lack such functions and are classified as "inanimate." thing (such as animal Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. Most animals are also, plant Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. About 350,000 species of plants, defined as seed plants, bryophytes, ferns and fern allies, are estimated to exist currently. As of 2004, some 287,655 species had been, fungus A fungus is a eukaryotic organism that is a member of the kingdom Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/). The fungi are a monophyletic group, also called the Eumycota (true fungi or Eumycetes), that is phylogenetically distinct from the morphologically similar slime molds (myxomycetes) and water molds (oomycetes). The fungi are, or micro-organism A microorganism or microbe is an organism that is microscopic (usually too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design). In at least some form, all organisms are capable of response to stimuli In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it elicits or influences a reflex via stimulus transduction. A stimulus is often the first component of a homeostatic, reproduction Reproduction is the biological process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. The known methods of reproduction are broadly grouped into two main types: sexual and asexual, growth Growth refers to an increase in some quantity over time. The quantity can be physical or abstract (e.g., a system becoming more complex, an organism becoming more mature). It can also refer to the mode of growth, i.e. numeric models for describing how much a particular quantity grows over time and development Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis," which is the process that gives rise to tissues, organs and anatomy. Developmental biology is that branch of life science, which deals with, and maintenance of homeostasis Homeostasis is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition. Typically used to refer to a living organism, the concept came from that of milieu interieur that was created by Claude Bernard and published in 1865. Multiple dynamic equilibrium adjustment as a stable whole. An organism may either be unicellular A microorganism or microbe is an organism that is microscopic (usually too small to be seen by the naked human eye). The study of microorganisms is called microbiology, a subject that began with Anton van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design (single-celled) or be composed of, as in humans, many billions of cells The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014 grouped into specialized tissues Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues and organs In biology and anatomy, an organ is a tissue that performs a specific function or group of functions within an organism. The term multicellular Multicellular organisms are organisms consisting of more than one cell, and having differentiated cells that perform specialized functions in the organism. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdoms Plantae and Animalia (many-celled) describes any organism made up of more than one cell The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014.

The terms "organism" (Greek Greek , an Indo-European language native to the southern Balkan peninsula, is the language of the Greeks. It forms an independent branch within Indo-European. It has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical Ancient Greek literature ὀργανισμός - organismos, from Ancient Greek Ancient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical (c. 5th–4th centuries BC), and Hellenistic (c. 3rd century BC–6th century AD) periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek. Its Hellenistic phase is known as Koine ὄργανον - organon "organ, instrument, tool") first appeared in the English language in 1701 and took on its current definition by 1834 (Oxford English Dictionary According to the publishers, it would take a single person 120 years to type the 59 million words of the OED second edition, 60 years to proofread it, and 540 megabytes to store it electronically. As of 30 November 2005, the Oxford English Dictionary contained approximately 301,100 main entries. Supplementing the entry headwords, there are 157,000).

Scientific classification Biological classification or scientific classification in biology, is a method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy, but should be distinguished from folk taxonomy, which lacks scientific basis. Modern biological classification in biology considers organisms synonymous with life on Earth. Based on cell type, organisms may be divided into the prokaryotic The prokaryotes are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but a few prokaryotes such as myxobacteria have multicellular stages in their life cycles. The word prokaryote comes from the Greek πρό- (pro-) & and eukaryotic A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Most living organisms, including all animals, plants, fungi, and protists, are eukaryotes. The defining membrane-bound structure that differentiates eukaryotic cells from prokaryotic cells is the nucleus. The presence of a nucleus gives these organisms groups. The prokaryotes represent two separate domains Each of the three cell types tends to fit into recurring specialties or roles. Bacteria tend to be the most prolific reproducers, at least in moderate environments. Archaeans tend to adapt quickly to extreme environments, such as high temperatures, high acids, high sulfur, etc. This includes adapting to use a wide variety of food sources, the Bacteria The bacteria [bækˈtɪərɪə] (singular: bacterium)[α] are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, water, and deep and Archaea The Archaea [ɑrˈkiə] are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon (sometimes spelled "archeon"). They have no cell nucleus or any other organelles within their cells. In the past they were viewed as an unusual group of bacteria and named archaebacteria but since the. Eukaryotic organisms, with a membrane-bounded cell nucleus In cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes, also contain organelles In cell biology, an organelle (pronunciation: /ɔːgəˡnɛl/) is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane, namely mitochondria In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers (μm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical and (in plants) plastids Plastids are major organelles found in plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Plastids often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's colour, generally considered to be derived from endosymbiotic The endosymbiotic theory concerns the origins of mitochondria and plastids , which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms which were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) bacteria.[1] Fungi A fungus is a eukaryotic organism that is a member of the kingdom Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/). The fungi are a monophyletic group, also called the Eumycota (true fungi or Eumycetes), that is phylogenetically distinct from the morphologically similar slime molds (myxomycetes) and water molds (oomycetes). The fungi are, animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. Most animals are also and plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. About 350,000 species of plants, defined as seed plants, bryophytes, ferns and fern allies, are estimated to exist currently. As of 2004, some 287,655 species had been are examples of species that are eukaryotes.

More recently a clade Ever since Darwin showed that all organisms share common ancestry, taxonomy has consistently attempted to represent and reflect the evolutionary history of organisms. The DNA and RNA analysis used in modern molecular biology has greatly helped in illuminating this history, by providing large amounts of new phylogenetic information which was, Neomura Neomura is a speculative clade composed of the two domains of life of Archaea and Eukaryota. The group was first proposed by Thomas Cavalier-Smith and its name means "new walls"; so called because it is thought to have evolved from Bacteria, and one of the major changes was the replacement of peptidoglycan cell walls with other, has been proposed, which groups together the Archaea The Archaea [ɑrˈkiə] are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon (sometimes spelled "archeon"). They have no cell nucleus or any other organelles within their cells. In the past they were viewed as an unusual group of bacteria and named archaebacteria but since the and Eukarya A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Most living organisms, including all animals, plants, fungi, and protists, are eukaryotes. The defining membrane-bound structure that differentiates eukaryotic cells from prokaryotic cells is the nucleus. The presence of a nucleus gives these organisms. Neomura is thought to have evolved from Bacteria The bacteria [bækˈtɪərɪə] (singular: bacterium)[α] are a large group of unicellular microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria are ubiquitous in every habitat on Earth, growing in soil, acidic hot springs, radioactive waste, water, and deep, more specifically from Actinobacteria They include some of the most common soil life, freshwater and marine life, playing an important role in decomposition of organic materials, such as cellulose and chitin and thereby playing a vital part in organic matter turnover and carbon cycle. This replenishes the supply of nutrients in the soil and is an important part of humus formation.[2]

Contents

Semantics

The word "organism" may broadly be defined as an assembly of molecules In chemistry, a molecule is defined as a sufficiently stable, electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense. In organic chemistry and biochemistry, the term molecule is used less strictly and also is that function as a more or less stable whole and has the properties of life. However, many sources propose definitions that exclude viruses A virus is a microscopic infectious agent that can only reproduce inside a host cell. Viruses infect all types of organisms: from animals and plants, to bacteria and archaea. Since the initial discovery of tobacco mosaic virus by Martinus Beijerinck in 1898, more than 5,000 types of virus have been described in detail, although most types of virus and theoretically-possible man-made non-organic life The hypothetical types of biochemistry are the different types of speculative biochemistries of alien life forms that differ radically from those known on Earth. It includes biochemistries that use elements other than carbon to construct primary cellular structures and/or use solvents besides water forms.[3] Viruses A virus is a microscopic infectious agent that can only reproduce inside a host cell. Viruses infect all types of organisms: from animals and plants, to bacteria and archaea. Since the initial discovery of tobacco mosaic virus by Martinus Beijerinck in 1898, more than 5,000 types of virus have been described in detail, although most types of virus are dependent on the biochemical machinery of a host cell for reproduction.

Chambers Online Reference The eleventh edition of The Chambers Dictionary of the English language was published in August 2008 by Chambers Harrap Publishers provides a broad definition: "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction".[4]

In multicellular life the word "organism" usually describes the whole hierarchical assemblage of systems (for example circulatory The circulatory system is an organ system that transports nutrients , gases, hormones, blood cells, nitrogen waste products, etc. to and from cells in the body to help fight diseases and help stabilize body temperature and pH to maintain homeostasis. This system may be seen strictly as a blood distribution network, but some consider the, digestive The digestive tract is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The major function of the GI tract are ingestion, digestion, absorption, and defecation. The GI tract differs substantially from animal to animal. Some animals have multi-chambered, or reproductive The reproductive system is a system of organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes of differentiated species often have significant differences. These) themselves collections of organs In biology and anatomy, an organ is a tissue that performs a specific function or group of functions within an organism; these are, in turn, collections of tissues, which are themselves made of cells The cell is the structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. Some organisms, such as most bacteria, are unicellular . Other organisms, such as humans, are multicellular. (Humans have an estimated 100 trillion or 1014. In some plants and the nematode Caenorhabditis elegans, individual cells are totipotent.

A polypore mushroom has parasitic relationship with its host

A superorganism is an organism consisting of many individuals working together as a single functional or social unit.

An ericoid mycorrhizal fungus Herpes simplex virus

Viruses

Viruses are not typically considered to be organisms because they are incapable of "independent" reproduction or metabolism. This controversy is problematic, though, since some parasites and endosymbionts are also incapable of independent life. Although viruses have a few enzymes and molecules characteristic of living organisms, they are incapable of reproducing outside a host cell and most of their metabolic processes require a host and its 'genetic machinery' such as organelles in eukaryotic hosts and the assemblage of ready-made enzymes (which the virus cannot make by itself) in prokaryotic hosts. While viruses sustain no independent metabolism, and thus are usually not accounted organisms, they do have their own genes and they do evolve by the same mechanisms by which organisms evolve.

Organizational terminology

Species Genus Family Order Class Phylum Kingdom Domain Life
The hierarchy of biological classification's eight major taxonomic ranks. Intermediate minor rankings are not shown.

All organisms are classified by the science of alpha taxonomy into either taxa or clades.

Taxa are ranked groups of organisms which run from the general (domain) to the specific (species). A broad scheme of ranks in hierarchical order is:

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 unable to interbreed to produce fertile offspring. Homo, however, only has one surviving species (sapiens); Homo erectus, Homo neanderthalensis, &c. having become extinct thousands of years ago. 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.

A crab is an example of an organism.

Chemistry

Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival. The molecular phenomena of chemistry are fundamental in understanding organisms, but it is a philosophical error (reductionism) to reduce organismal biology to mere chemistry. 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 which, through interaction with each other and the 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 physical properties of this element such as its great affinity for bonding with other small atoms, including other carbon atoms, and its small size makes it capable of forming multiple bonds, make it ideal as the basis of organic life. It is able to form small compounds containing three atoms (such as carbon dioxide) as well as large chains of many thousands of atoms which are able to store data (nucleic acids), hold cells together and transmit information (protein).

Macromolecules

The compounds which 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 the many characteristics which 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 which, due to the chemical properties of the amino acids of which it is made, folds in a particular manner and so performs a particular function.

The following functions of protein have been recognized:

  1. Enzymes, which catalyze all of the reactions of metabolism;
  2. Structural proteins, such as tubulin, or collagen;
  3. Regulatory proteins, such as transcription factors or cyclins that regulate the cell cycle;
  4. Signaling molecules or their receptors such as some hormones and their receptors;
  5. 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.

Lipids make up the membrane of cells which constitutes a barrier, containing everything within the cell and preventing compounds from freely passing into, and out of, the cell. In some multicellular organisms they serve to store energy and mediate communication between cells. Carbohydrates also store and transport energy in some organisms, but are more easily broken down than lipids.

Structure

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 tissue the four basic types of which are 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, &c. Many multicelled organisms comprise of several organ systems which coordinate to allow for life.

The cell

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 multi-cellular 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 abilities:[5]

Life span

One of the basic parameters of organism is its life span. Some organisms live as short as one day, while some plants can live thousands of years. Aging is important when determining life span of most organisms, bacterium, a virus or even a prion.[citation needed]

Evolution

See also: Common descent and Origin of life

In biology, the theory of universal common descent proposes that 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 which do not fly.

Today, there is debate over whether or not all organisms descended from a common ancestor, or a "last universal ancestor" (LUA), also called the "last universal common ancestor" (LUCA). The universality of genetic coding suggests common ancestry. 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, although exceptions to the basic twenty amino acids have been found. However, throughout history groupings based on appearance or function of species have sometimes been polyphyletic due to convergent evolution.

Bacteria Archaea Eucaryota Aquifex Thermotoga Planctomyces Cyanobacteria Proteobacteria Spirochetes Gram-positive bacteria Green filantous bacteria Pyrodicticum Thermoproteus Thermococcus celer Methanococcus Methanobacterium Methanosarcina Halophiles Entamoebae Slime mold Animal Fungus Plant Ciliate Flagellate Trichomonad Microsporidia Diplomonad A hypothetical phylogenetic tree of all extant organisms, based on 16S rRNA gene sequence data, showing the evolutionary history of the three domains of life, bacteria, archaea, and eukaryotes. Originally proposed by Carl Woese.

The "last universal ancestor" (LUA), or "last universal common ancestor" (LUCA), is the name given to the hypothetical single cellular organism or single cell that gave rise to all life on Earth 3.5 to 3.8 billion years ago[6]; however, this hypothesis has since been refuted on many grounds. For example, it was once thought that the genetic code was universal (see: universal genetic code), but many variations have been discovered[7] including various alternative mitochondrial codes.[8] Back in the early 1970s, evolutionary biologists thought that a given piece of DNA specified the same protein subunit in every living thing, and that the genetic code was thus universal. This was interpreted as evidence that every organism had inherited its genetic code from a single common ancestor, aka, an LUCA. In 1979, however, exceptions to the code were found in mitochondria, the tiny energy factories inside cells. Researchers studying human mitochondrial genes discovered that they used an alternative code, and many slight variants have been discovered since,[9] including various alternative mitochondrial codes,[10] as well as small variants such as Mycoplasma translating the codon UGA as tryptophan. Biologists subsequently found exceptions in bacteria and in the nuclei of algae and single-celled animals. For example, certain proteins may use alternative initiation (start) codons not normally used by that species.[11] In certain proteins, non-standard amino acids are substituted for standard stop codons, depending upon associated signal sequences in the messenger RNA: UGA can code for selenocysteine and UAG can code for pyrrolysine. Selenocysteine is now viewed as the 21st amino acid, and pyrrolysine is viewed as the 22nd. A detailed description of variations in the genetic code can be found at the NCBI web site.

It is now clear that the genetic code is not the same in all living things and this provides credence that all living things did not evolve on a firmly-rooted tree of life from a single LUCA. Further support that there is no LUCA has been provided over the years by horizontal/lateral gene transfer in both prokaryote and eukaryote single cell organisms. This is why phylogenetic trees cannot be rooted; why almost all phylogenetic trees have different branching structures, particularly near the base of the tree; and why many organisms have been found with codons and sections of their DNA sequence that are sometimes unrelated to other species.[12]

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.

History of life

Main article: Timeline of evolution

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.

Precambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, William Schopf of UCLA published a controversial paper in the journal Nature arguing that formations such as this possess 3.5 billion year old fossilized algae microbes. If true, they would be the earliest known life on earth.

Not much 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 land ecosystems with which we are familiar.

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.[13]. 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, and the history of life

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."[14]

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."[15]

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, cloning also has 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).

The J. Craig Venter Institute has recently assembled a synthetic yeast genome, Mycoplasma genitalium, by recombination 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."[16] Other companies, such as Synthetic Genomics, have already been formed to take advantage of the many commercial uses of custom designed genomes.

Notes

  1. ^ T.Cavalier-Smith (1987) The origin of eukaryote and archaebacterial cells, Annals of the New York Academy of Sciences 503, 17–54
  2. ^ T. Cavalier-Smith (2002) The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification. International Journal of Systematic and Evolutionary Microbiology 52, 7–76
  3. ^ "organism". Oxford English Dictionary (online ed.). 2004.
  4. ^ "organism". Chambers 21st Century Dictionary (online ed.). 1999.
  5. ^ 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.
  6. ^ Doolittle, W. Ford (February, 2000). Uprooting the tree of life. Scientific American 282 (6): 90–95.
  7. ^ NCBI: "The Genetic Codes", Compiled by Andrzej (Anjay) Elzanowski and Jim Ostell
  8. ^ Jukes TH, Osawa S, The genetic code in mitochondria and chloroplasts., Experientia. 1990 Dec 1;46(11-12):1117-26.
  9. ^ NCBI: "The Genetic Codes", Compiled by Andrzej (Anjay) Elzanowski and Jim Ostell
  10. ^ Jukes TH, Osawa S, The genetic code in mitochondria and chloroplasts., Experientia. 1990 Dec 1;46(11-12):1117-26.
  11. ^ Genetic Code page in the NCBI Taxonomy section (Downloaded 27 April 2007.)
  12. ^ Syoso Osawa (1995). Evolution of the Genetic Code. Oxford University Press. pp. 232. ISBN 978-0198547815.
  13. ^ Evaluation of the Rate of Evolution in Natural Populations of Guppies (Poecilia reticulata) "[1]"
  14. ^ Oklahoma State - Horizontal Gene Transfer
  15. ^ esalenctr.org
  16. ^ Gibsona, Daniel G., Gwynedd A. Benders, Kevin C. Axelroda, Jayshree Zaveria, Mikkel A. Algirea, Monzia Moodiea, Michael G. Montaguea, J. Craig Ventera, Hamilton O. Smith, and Clyde A. Hutchison III (2008). "One-step assembly in yeast of 25 overlapping DNA fragments to form a complete synthetic Mycoplasma genitalium genome". PNAS 105 (51): 20404–20409. doi:10.1073/pnas.0811011106. http://www.pnas.org/content/105/51/20404.full.pdf.

External links

Wikipedia:Books has a book on: Organism
Elements of Nature
Universe Outer space · Matter · Energy · Time
Earth Earth science · Geology · History of the Earth · Geological history of Earth · Structure of the Earth · Plate tectonics
Weather Earth's atmosphere · Climate
Environment Ecology · Ecosystem · Wilderness
Life Biosphere · Origin of life · Life on Earth · Plants · Animals · Microbe · Virus · Protista · Flora · Fungi · Fauna · Evolutionary history of life · Biology
Category · Portal
Stucture of vital system
Biosphere > Ecosystem > Biocoenosis > Population > Organism > Organ system > Organ > Tissue > Cell > Organelle > Molecule > Atom > Subatomic particle (Composite particle · Elementary particle)

Categories: Life | Organisms | Greek loanwords

 

The above information uses material from Wikipedia and is licensed under the GNU Free Documentation License.
Some facts may not have been fully verified for accuracy. [Disclaimers]
This page was last archived by our server on Tue Jul 7 17:03:06 2009. [ refresh local cache ]
Displaying this page or its contents does not use any Wikimedia Foundation's resources.
The owners of this site proudly support the Wikimedia Foundation.

[Hide]▼
'Organisms' News
Prehistoric Paintings Threatened by Bacteria 'Cocktail' The ... - Softpedia
news.softpedia.com
Prehistoric Paintings Threatened by Bacteria 'Cocktail' The ...

Softpedia, Romania

By Tudor Vieru, Science Editor More than 16000 years ago, a group of humans took refuge in the Lascaux caves, in what is now southwestern France, probably living a nomadic existence and seeking refuge from predators. In the course of their stay, ...
Google News Search: Organisms,
Sun May 31 02:11:13 2009
'Organisms' Blogs
Genetically modified organisms (GMOs): The significance of gene ...
eea.europa.eu
Genetically modified organisms (GMOs): The significance of gene ...

unknown

2002-03-20 23:00:00

This report, written for the EEA by experts from the European Science Foundation, considers the significance of the transfer by pollen of genes from six major genetically modified (GM) crop types that are close to commercial release in ...

Google Blogs Search: Organisms,
Fri May 29 02:53:13 2009
'Organisms' Answers
How do organisms obtain food from hydrothermal vents?
Q. This is all i know: Organisms deep in the sea cannot use photosynthesis so they use the rich nutrients that come out of hydrothermal vents. Many bacteria (chemoautotrophic bacteria) can grow/live using the nutrients, then organisms such as copepods and amphipods, and then larger organisms devour the organisms that feed on bacteria for food, and so on. (Is this right?!)
Asked by Woof - Thu Mar 19 22:11:45 2009 - - 2 Answers - 0 Comments

A. You're right. The "nutrients" from the hydrothermal vents are hydrogen sulfides. The chemoautrophic bacteria oxidize them for energy.
Answered by Dynamic Fetch - Fri Mar 20 00:14:51 2009

Yahoo Answers Search: Organisms,
Mon Jun 1 05:02:18 2009
[Hide]▲