Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate. Various forms of life exist, such as plants, fungi, protists and bacteria; the criteria can at times be ambiguous and may or may not define viruses, viroids, or potential synthetic life as "living". Biology is the science concerned with the study of life. There is no consensus regarding the definition of life. One popular definition is that organisms are open systems that maintain homeostasis, are composed of cells, have a life cycle, undergo metabolism, can grow, adapt to their environment, respond to stimuli and evolve. However, several other definitions have been proposed, there are some borderline cases of life, such as viruses or viroids. Abiogenesis attempts to describe the natural process of life arising from non-living matter, such as simple organic compounds.
The prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity. Life on Earth first appeared as early as 4.28 billion years ago, soon after ocean formation 4.41 billion years ago, not long after the formation of the Earth 4.54 billion years ago. The earliest known life forms are microfossils of bacteria. Earth's current life may have descended from an RNA world, although RNA-based life may not have been the first; the mechanism by which life began on Earth is unknown, though many hypotheses have been formulated and are based on the Miller–Urey experiment. Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale, but it has adapted to survive in most ecosystems and conditions; some microorganisms, called extremophiles, thrive in physically or geochemically extreme environments that are detrimental to most other life on Earth. The cell is considered the functional unit of life.
There are two kinds of cells and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many biomolecules such as proteins and nucleic acids. Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells. In the past, there have been many attempts to define what is meant by "life" through obsolete concepts such as odic force, spontaneous generation and vitalism, that have now been disproved by biological discoveries. Aristotle was the first person to classify organisms. Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. New groups and categories of life were discovered, such as cells and microorganisms, forcing dramatic revisions of the structure of relationships between living organisms. Though only known on Earth, life need not be restricted to it, many scientists speculate in the existence of extraterrestrial life. Artificial life is a computer simulation or man-made reconstruction of any aspect of life, used to examine systems related to natural life.
Death is the permanent termination of all biological functions which sustain an organism, as such, is the end of its life. Extinction is the term describing the dying out of a group or taxon a species. Fossils are the preserved traces of organisms; the definition of life has long been a challenge for scientists and philosophers, with many varied definitions put forward. This is because life is a process, not a substance; this is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside of Earth. Philosophical definitions of life have been put forward, with similar difficulties on how to distinguish living things from the non-living. Legal definitions of life have been described and debated, though these focus on the decision to declare a human dead, the legal ramifications of this decision. Since there is no unequivocal definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment.
This characteristic exhibits all or most of the following traits: Homeostasis: regulation of the internal environment to maintain a constant state. Living things require energy to maintain internal organization 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 accumulating matter. Adaptation: the ability to change over time in response to the environment; this ability is fundamental to the process of evolution and is determined by the organism's heredity and external factors. 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 expressed by motion. Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms; these complex processes, called physiological functions, have under
Gram-positive bacteria are bacteria that give a positive result in the Gram stain test, traditionally used to classify bacteria into two broad categories according to their cell wall. Gram-positive bacteria take up the crystal violet stain used in the test, appear to be purple-coloured when seen through a microscope; this is because the thick peptidoglycan layer in the bacterial cell wall retains the stain after it is washed away from the rest of the sample, in the decolorization stage of the test. Gram-negative bacteria cannot retain the violet stain after the decolorization step, their peptidoglycan layer is much thinner and sandwiched between an inner cell membrane and a bacterial outer membrane, causing them to take up the counterstain and appear red or pink. Despite their thicker peptidoglycan layer, gram-positive bacteria are more receptive to certain cell wall targeting antibiotics than gram-negative bacteria, due to the absence of the outer membrane. In general, the following characteristics are present in gram-positive bacteria: Cytoplasmic lipid membrane Thick peptidoglycan layer Teichoic acids and lipoids are present, forming lipoteichoic acids, which serve as chelating agents, for certain types of adherence.
Peptidoglycan chains are cross-linked to form rigid cell walls by a bacterial enzyme DD-transpeptidase. A much smaller volume of periplasm than that in gram-negative bacteria. Only some species have a capsule consisting of polysaccharides. Only some species are flagellates, when they do have flagella, have only two basal body rings to support them, whereas gram-negative have four. Both gram-positive and gram-negative bacteria have a surface layer called an S-layer. In gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Gram-negative bacteria's S-layer is attached directly to the outer membrane. Specific to gram-positive bacteria is the presence of teichoic acids in the cell wall; some of these are lipoteichoic acids, which have a lipid component in the cell membrane that can assist in anchoring the peptidoglycan. Along with cell shape, Gram staining is a rapid method used to differentiate bacterial species; such staining, together with growth requirement and antibiotic susceptibility testing, other macroscopic and physiologic tests, forms the full basis for classification and subdivision of the bacteria.
The kingdom Monera was divided into four divisions based on Gram staining: Firmicutes, Gracilicutes and Mendocutes. Based on 16S ribosomal RNA phylogenetic studies of the late microbiologist Carl Woese and collaborators and colleagues at the University of Illinois, the monophyly of the gram-positive bacteria was challenged, with major implications for the therapeutic and general study of these organisms. Based on molecular studies of the 16S sequences, Woese recognised twelve bacterial phyla. Two of these were both gram-positive and were divided on the proportion of the guanine and cytosine content in their DNA; the high G + C phylum was made up of the Actinobacteria and the low G + C phylum contained the Firmicutes. The Actinobacteria include the Corynebacterium, Mycobacterium and Streptomyces genera; the Firmicutes, have a 45 -- 60 % GC content. Although bacteria are traditionally divided into two main groups, gram-positive and gram-negative, based on their Gram stain retention property, this classification system is ambiguous as it refers to three distinct aspects, which do not coalesce for some bacterial species.
The gram-positive and gram-negative staining response is not a reliable characteristic as these two kinds of bacteria do not form phylogenetic coherent groups. However, although Gram staining response is an empirical criterion, its basis lies in the marked differences in the ultrastructure and chemical composition of the bacterial cell wall, marked by the absence or presence of an outer lipid membrane. All gram-positive bacteria are bounded by a single-unit lipid membrane, and, in general, they contain a thick layer of peptidoglycan responsible for retaining the Gram stain. A number of other bacteria—that are bounded by a single membrane, but stain gram-negative due to either lack of the peptidoglycan layer, as in the Mycoplasmas, or their inability to retain the Gram stain because of their cell wall composition—also show close relationship to the Gram-positive bacteria. For the bacterial cells bounded by a single cell membrane, the term "monoderm bacteria" or "monoderm prokaryotes" has been proposed.
In contrast to gram-positive bacteria, all archetypical gram-negative bacteria are bounded by a cytoplasmic membrane and an outer cell membrane. The presence of inner and outer cell membranes defines a new compartment in these cells: the periplasmic space or the periplasmic compartment; these bacteria have been designated as "diderm bacteria." The distinction between the monoderm and diderm bacteria is supported by conserved signature indels in a number of important proteins. Of these two structurally distinct groups of bacteria, monoderms are indicated to be ancestral. Based upon a number of observations including that the gram-positive bacteria are the major producers of antibiotics and that, in general, gram-negative bacteria are resistant to them, it h
A spirochaete or spirochete is a member of the phylum Spirochaetes, which contains distinctive diderm bacteria, most of which have long, helically coiled cells. Spirochaetes are chemoheterotrophic in nature, with lengths between 3 and 500 µm and diameters around 0.09 to at least 3 µm. Spirochaetes are distinguished from other bacterial phyla by the location of their flagella, sometimes called axial filaments, which run lengthwise between the bacterial inner membrane and outer membrane in periplasmic space; these cause a twisting motion. When reproducing, a spirochaete will undergo asexual transverse binary fission. Most spirochaetes are free-living and anaerobic. Spirochaetes bacteria are diverse in their pathogenic capacity and the ecological niches that they inhabit, as well as molecular characteristics including guanine-cytosine content and genome size. Many organisms within the Spirochaetes phylum cause prevalent diseases. Pathogenic members of this phylum include the following: Leptospira species, which causes leptospirosis Borrelia burgdorferi, B. garinii, B. afzelii, which cause Lyme disease Borrelia recurrentis, which causes relapsing fever Treponema pallidum subspecies which cause treponematoses such as syphilis and yaws.
Brachyspira pilosicoli and Brachyspira aalborgi, which cause intestinal spirochaetosisSpirochaetes may cause dementia and may be involved in the pathogenesis of Alzheimer's disease. Salvarsan, the first organic synthetic antimicrobial drug in medical history, was effective against spirochaetes only and was used to cure syphilis; the class consists of 14 validly named genera across 4 orders and 5 families. The orders Brachyspirales and Leptospirales each contain a single family, Brachyspiraceae and Leptospiraceae, respectively; the Spirochaetales order harbours two families and Borreliaceae. Molecular markers in the form of conserved signature indels and CSPs have been found specific for each of the orders, with the exception of Brevinimetales, that provide a reliable means to demarcate these clades from one another within the diverse phylum. Additional CSIs have been found shared by each family within the Spirochaetales; these molecular markers are in agreement with the observed phylogenetic tree branching of two monophyletic clades within the Spirochaetales order.
CSIs have been found that further differentiate taxonomic groups within the Borreliaceae family that further delineate evolutionary relationships that are in accordance with physical characteristics such as pathogenicity. A CSI has been found shared by all Spirochaetes species; this CSI is a 3 amino acid insert in the flagellar basal body rod protein FlgC, an important part of the unique endoflagellar structure shared by Spirochaetes species. Given that the CSI is shared by members within this phylum, it has been postulated that it may be related to the characteristic flagellar properties observed among Spirochaetes species. All families belonging to the Spirochaetes phylum were assigned to a single order, the Spirochaetales. However, the current taxonomic view is more connotative of accurate evolutionary relationships; the distribution of a CSI is indicative of shared ancestry within the clade for which it is specific. It thus functions as a synapomorphic characteristic, so that the distributions of different CSIs provide the means to identify different orders and families within the phylum and so justify the phylogenetic divisions.
The phylogeny is based on 16S rRNA-based LTP release 132 by The All-Species Living Tree Project. The accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature and National Center for Biotechnology Information. Phylum Spirochaetes Garrity & Holt 2001 Class Spirochaetae Cavalier-Smith 2002 Order Leptospirales Gupta et al. 2014 Family Leptospiraceae Hovind-Hougen 1979 emend. Levett et al. 2005 Genus Leptonema Hovind-Hougen 1983 Genus Leptospira Noguchi 1917 emend. Faine and Stallman 1982 Genus Turneriella Levett et al. 2005 Order Brachyspirales corrig. Gupta et al. 2014 Family Brachyspiraceae Paster 2012 Genus Brachyspira Hovind-Hougen et al. 1982 Order Brevinematales Gupta et al. 2014 Family Brevinemataceae Paster 2012 Genus Brevinema Defosse et al. 1995 Order Spirochaetales Buchanan 1917 emend. Gupta et al. 2013 Genus Exilispira Imachi et al. 2008 Genus Alkalispirochaeta Sravanthi et al. 2016 Genus Oceanispirochaeta Subhash & Lee 2017b Genus Pleomorphochaeta Arroua et al. 2016 Genus Sediminispirochaeta Shivani et al. 2016 Genus Sphaerochaeta Ritalahti et al. 2012 emend.
Miyazaki et al. 2014 Family Borreliaceae Gupta et al. 2014 Genus Borreliella Adeolu & Gupta 2015 Genus Borrelia Swellengrebel 1907 emend. Adeolu & Gupta 2014 Genus Cristispira pectinis Gross 1910 Family Spirochaetaceae Swellengrebel 1907 Genus? Clevelandina reticulitermitidis ♦ Bermudes et al. 1988 Genus? Diplocalyx calotermitidis ♦ Bermudes et al. 1988 Genus? Hollandina pterotermitidis ♦ Bermudes et al. 1988 Genus? Pillotina calotermitidis ♦ Bermudes et al. 1988 Genus Marispirochaeta Shivani et al. 2017 Genus Spirochaeta Ehrenberg 1835 emend. Pikuta et al. 2009 ["Ehre
Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, unlike prokaryotes, which have no membrane-bound organelles. Eukaryotes belong to Eukarya, their name comes from the Greek εὖ and κάρυον. Eukaryotic cells contain other membrane-bound organelles such as mitochondria and the Golgi apparatus, in addition, some cells of plants and algae contain chloroplasts. Unlike unicellular archaea and bacteria, eukaryotes may be multicellular and include organisms consisting of many cell types forming different kinds of tissue. Animals and plants are the most familiar eukaryotes. Eukaryotes can reproduce both asexually through mitosis and sexually through meiosis and gamete fusion. In mitosis, one cell divides to produce two genetically identical cells. In meiosis, DNA replication is followed by two rounds of cell division to produce four haploid daughter cells; these act as sex cells. Each gamete has just one set of chromosomes, each a unique mix of the corresponding pair of parental chromosomes resulting from genetic recombination during meiosis.
The domain Eukaryota appears to be monophyletic, makes up one of the domains of life in the three-domain system. The two other domains and Archaea, are prokaryotes and have none of the above features. Eukaryotes represent a tiny minority of all living things. However, due to their much larger size, their collective worldwide biomass is estimated to be about equal to that of prokaryotes. Eukaryotes evolved 1.6–2.1 billion years ago, during the Proterozoic eon. The concept of the eukaryote has been attributed to the French biologist Edouard Chatton; the terms prokaryote and eukaryote were more definitively reintroduced by the Canadian microbiologist Roger Stanier and the Dutch-American microbiologist C. B. van Niel in 1962. In his 1937 work Titres et Travaux Scientifiques, Chatton had proposed the two terms, calling the bacteria prokaryotes and organisms with nuclei in their cells eukaryotes; however he mentioned this in only one paragraph, the idea was ignored until Chatton's statement was rediscovered by Stanier and van Niel.
In 1905 and 1910, the Russian biologist Konstantin Mereschkowski argued that plastids were reduced cyanobacteria in a symbiosis with a non-photosynthetic host, itself formed by symbiosis between an amoeba-like host and a bacterium-like cell that formed the nucleus. Plants had thus inherited photosynthesis from cyanobacteria. In 1967, Lynn Margulis provided microbiological evidence for endosymbiosis as the origin of chloroplasts and mitochondria in eukaryotic cells in her paper, On the origin of mitosing cells. In the 1970s, Carl Woese explored microbial phylogenetics, studying variations in 16S ribosomal RNA; this helped to uncover the origin of the eukaryotes and the symbiogenesis of two important eukaryote organelles and chloroplasts. In 1977, Woese and George Fox introduced a "third form of life", which they called the Archaebacteria. In 1979, G. W. Gould and G. J. Dring suggested that the eukaryotic cell's nucleus came from the ability of Gram-positive bacteria to form endospores. In 1987 and papers, Thomas Cavalier-Smith proposed instead that the membranes of the nucleus and endoplasmic reticulum first formed by infolding a prokaryote's plasma membrane.
In the 1990s, several other biologists proposed endosymbiotic origins for the nucleus reviving Mereschkowski's theory. Eukaryotic cells are much larger than those of prokaryotes having a volume of around 10,000 times greater than the prokaryotic cell, they have a variety of internal membrane-bound structures, called organelles, a cytoskeleton composed of microtubules and intermediate filaments, which play an important role in defining the cell's organization and shape. Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubular spindle during nuclear division. Eukaryote cells include a variety of membrane-bound structures, collectively referred to as the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and pinches off to form a vesicle, it is probable that most other membrane-bound organelles are derived from such vesicles.
Alternatively some products produced by the cell can leave in a vesicle through exocytosis. The nucleus is surrounded with pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, involved in protein transport and maturation, it includes the rough endoplasmic reticulum where ribosomes are attached to synthesize proteins, which enter the interior space or lumen. Subsequently, they enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and further modified in stacks of flattened vesicles, the Golgi apparatus. Vesicles may be specialized for various purposes. For instance, lysosomes contain digestive enzymes that break down most biomolecules in the cytoplasm. Peroxisomes are used to break down peroxide, otherwise toxic. Many protozoans have contractile vacuoles, which collect and expel excess water, extrusomes, which expel material used to deflect predators or capture prey.
In higher plants, most of a cell's volume is taken up by a central vacuole, whi
Carl Richard Woese was an American microbiologist and biophysicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese which revolutionized the discipline of microbiology, he was the originator of the RNA world hypothesis in 1967, although not by that name. He held the Stanley O. Ikenberry Chair and was professor of microbiology at the University of Illinois at Urbana–Champaign. Carl Woese was born in Syracuse, New York on July 15, 1928. Woese attended Deerfield Academy in Massachusetts, he received a bachelor's degree in mathematics and physics from Amherst College in 1950. During his time at Amherst, Woese took only one biology course and had "no scientific interest in plants and animals" until advised by William M. Fairbank an assistant professor of physics at Amherst, to pursue biophysics at Yale. In 1953, he completed a Ph. D. in biophysics at Yale University, where his doctoral research focused on the inactivation of viruses by heat and ionizing radiation.
He studied medicine at the University of Rochester for two years, quitting two days into a pediatrics rotation. He became a postdoctoral researcher in biophysics at Yale University investigating bacterial spores. From 1960–63, he worked as a biophysicist at the General Electric Research Laboratory in Schenectady, New York. In 1964, Woese joined the microbiology faculty of the University of Illinois at Urbana–Champaign, where he focused on Archaea and molecular evolution as his areas of expertise, he became a professor at the University of Illinois at Urbana–Champaign's Carl R. Woese Institute for Genomic Biology, renamed in his honor in 2015, after his death. Woese died on December 30, 2012, following complications from pancreatic cancer. Woese turned his attention to the genetic code while setting up his lab at General Electric's Knolls Laboratory in the fall of 1960. Interest among physicists and molecular biologists had begun to coalesce around deciphering the correspondence between the twenty amino acids and the four letter alphabet of nucleic acid bases in the decade following James D. Watson, Francis Crick, Rosalind Franklin's discovery of the structure of DNA in 1953.
Woese published a series of papers on the topic. In one, he deduced a correspondence table between what was known as "soluble RNA" and DNA based upon their respective base pair ratios, he re-evaluated experimental data associated with the hypothesis that viruses used one base, rather than a triplet, to encode each amino acid, suggested 18 codons predicting one for proline. Other work established the mechanistic basis of protein translation, but in Woese's view overlooked the genetic code's evolutionary origins as an afterthought. In 1962 Woese spent several months as a visiting researcher at the Pasteur Institute in Paris, a locus of intense activity on the molecular biology of gene expression and gene regulation. While in Paris, he met Sol Spiegelman, who invited Woese to visit the University of Illinois after hearing his research goals. With the freedom to patiently pursue more speculative threads of inquiry outside the mainstream of biological research, Woese began to consider the genetic code in evolutionary terms, asking how the codon assignments and their translation into an amino acid sequence might have evolved.
For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on their biochemistry and metabolism. In a influential 1962 paper, Roger Stanier and C. B. van Niel first established the division of cellular organization into prokaryotes and eukaryotes, defining prokaryotes as those organisms lacking a cell nucleus. Adapted from Édouard Chatton's generalization and Van Niel's concept was accepted as the most important distinction among organisms. However, it became assumed that all life shared a common prokaryotic ancestor. In 1977, Carl Woese and George E. Fox experimentally disproved this universally held hypothesis about the basic structure of the tree of life. Woese and Fox discovered a kind of microbial life which they called the “archaebacteria”, they reported that the archaebacteria comprised "a third kingdom" of life as distinct from bacteria as plants and animals. Having defined Archaea as a new "urkingdom" which were neither bacteria nor eukaryotes, Woese redrew the taxonomic tree.
His three-domain system, based on phylogenetic relationships rather than obvious morphological similarities, divided life into 23 main divisions, incorporated within three domains: Bacteria and Eucarya. Acceptance of the validity of Woese's phylogenetically valid classification was a slow process. Prominent biologists including Salvador Luria and Ernst Mayr objected to his division of the prokaryotes. Not all criticism of him was restricted to the scientific level. A decade of labor-intensive oligonucleotide cataloging left him with a reputation as "a crank," and Woese would go on to be dubbed as "Microbiology's Scarred Revolutionary" by a news article printed in the journal Science; the growing amount of supporting data led the scientific community to accept the Archaea by the mid-1980s. Today, few scientists cling to the idea of a unified Prokarya. Woese's work on Archaea is significant in its implications for the search for life on other planets. Before the discovery by Woese and Fox, scientists thoug
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are uni- and multicellular, like slime molds and social amoebae such as the genus Dictyostelium. Multicellular organisms arise in various ways, for example by cell division or by aggregation of many single cells. Colonial organisms are the result of many identical individuals joining together to form a colony. However, it can be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct. Multicellularity has evolved independently at least 46 times in eukaryotes, in some prokaryotes, like cyanobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina. However, complex multicellular organisms evolved only in six eukaryotic groups: animals, brown algae, red algae, green algae, land plants, it evolved for Chloroplastida, once or twice for animals, once for brown algae, three times in the fungi and several times for slime molds and red algae.
The first evidence of multicellularity is from cyanobacteria-like organisms that lived 3–3.5 billion years ago. To reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells, an issue, studied in evolutionary developmental biology. Animals have evolved a considerable diversity of cell types in a multicellular body, compared with 10–20 in plants and fungi. Loss of multicellularity occurred in some groups. Fungi are predominantly multicellular, though early diverging lineages are unicellular and there have been numerous reversions to unicellularity across fungi, it may have occurred in some red algae, but it is possible that they are primitively unicellular. Loss of multicellularity is considered probable in some green algae. In other groups parasites, a reduction of multicellularity occurred, in number or types of cells. Multicellular organisms long-living animals, face the challenge of cancer, which occurs when cells fail to regulate their growth within the normal program of development.
Changes in tissue morphology can be observed during this process. Cancer in animals has been described as a loss of multicellularity. There is a discussion about the possibility of existence of cancer in other multicellular organisms or in protozoa. For example, plant galls have been characterized as tumors, but some authors argue that plants do not develop cancer. In some multicellular groups, which are called Weismannists, a separation between a sterile somatic cell line and a germ cell line evolved. However, Weismannist development is rare, as a great part of species have the capacity for somatic embryogenesis. One hypothesis for the origin of multicellularity is that a group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit; this is what slime molds do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a coenocyte. A membrane would form around each nucleus, thereby resulting in a group of connected cells in one organism.
A third hypothesis is that as a unicellular organism divided, the daughter cells failed to separate, resulting in a conglomeration of identical cells in one organism, which could develop specialized tissues. This is what animal embryos do as well as colonial choanoflagellates; because the first multicellular organisms were simple, soft organisms lacking bone, shell or other hard body parts, they are not well preserved in the fossil record. One exception may be the demosponge; the earliest fossils of multicellular organisms include the contested Grypania spiralis and the fossils of the black shales of the Palaeoproterozoic Francevillian Group Fossil B Formation in Gabon. The Doushantuo Formation has yielded 600 million year old microfossils with evidence of multicellular traits; until phylogenetic reconstruction has been through anatomical similarities. This is inexact, as living multicellular organisms such as animals and plants are more than 500 million years removed from their single-cell ancestors.
Such a passage of time allows both divergent and convergent evolution time to mimic similarities and accumulate differences between groups of modern and extinct ancestral species. Modern phylogenetics uses sophisticated techniques such as alloenzymes, satellite DNA and other molecular markers to describe traits that are shared between distantly related lineages; the evolution of multicellularity could have occurred in a number of different ways, some of which are described below: This theory suggests that the first multicellular organisms occurred from symbiosis of different species of single-cell organisms, each with different roles. Over time these organisms would become so dependent on each other they would not be able to survive