multicellular organism
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A multicellular organism is an
organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multice ...

organism
that consists of more than one
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
, in contrast to a
unicellular organism A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Org ...
. All species of
animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...

animal
s,
land plants The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophyta is a clade within the Phragmoplastophyta, a larger clade that also includes several groups of green algae including the ...

land plants
and most
fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

fungi
are multicellular, as are many
algae Algae (; singular alga ) is an informal term for a large and diverse group of s. It is a grouping that includes species from multiple distinct s. Included organisms range from , such as '','' and the s, to forms, such as the , a large whi ...

algae
, whereas a few organisms are partially uni- and partially multicellular, like
slime mold Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...

slime mold
s and social amoebae such as the genus ''
Dictyostelium ''Dictyostelium'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also ...

Dictyostelium
''. Multicellular organisms arise in various ways, for example by
cell division Cell division is the process by which a parent cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious ...

cell division
or by aggregation of many single cells.
Colonial organism In biology, a colony is composed of two or more conspecific individuals living in close association with, or connected to, one another. This association is usually for mutual benefit such as stronger defense or the ability to attack bigger prey. It ...
s are the result of many identical individuals joining together to form a
colony In , a colony is a subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the ' (or "mother country"). This administrative co ...
. However, it can often be hard to separate colonial protists from true multicellular organisms, because the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular". There are also
multinucleate Multinucleate cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a roo ...
though technically unicellular organisms that are macroscopic, such as the
xenophyophorea Xenophyophorea is a clade of foraminiferans. Members of this class are multinucleate Unicellular organism, unicellular organisms found on the ocean floor throughout the world's oceans, at depths of . They are a kind of foraminiferan that extracts ...
that can reach 20 cm.


Evolutionary history


Occurrence

Multicellularity has evolved independently at least 25 times in
eukaryotes Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline ...

eukaryotes
, and also in some
prokaryote A prokaryote () is a Unicellular organism, single-celled organism that lacks a cell nucleus, nucleus, and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek language, Greek wikt:πρό#Ancient Greek, πρό (, 'before') a ...
s, like
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", t ...

cyanobacteria
,
myxobacteria The myxobacteria ("slime bacteria") are a group of bacteria that predominantly live in the soil and feed on insoluble organic substances. The myxobacteria have very large genomes relative to other bacteria, e.g. 9–10 million nucleotides except ...
,
actinomycetes The Actinomycetales are an order of Actinobacteria. A member of the order is often called an actinomycete. The actinomycetes are diverse and contain a variety of subdivisions, as well as yet-unclassified isolates, mainly because some genera ar ...

actinomycetes
, '' Magnetoglobus multicellularis'' or '' Methanosarcina''. However, complex multicellular organisms evolved only in six
eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryotic
groups:
animals Animals (also called Metazoa) are multicellular Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the L ...

animals
, ,
brown algae The brown algae (singular: alga), comprising the class (biology), class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Most brown algae live in marine enviro ...
,
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
,
green algae The green algae (singular: green alga) are a large, informal grouping of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to ...

green algae
, and
land plant The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that w ...
s. It evolved repeatedly for
Chloroplastida Viridiplantae (literally "green plants") are a of organisms that comprise approximately 450,000–500,000 species and play important roles in both terrestrial and aquatic ecosystems. They are made up of the , which are primarily aquatic, and t ...
(green algae and land plants), once for animals, once for brown algae, three times in the
fungi A fungus (plural: fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as ...

fungi
(
chytrid Chytridiomycota are a division of zoosporic organisms in the kingdom (biology), kingdom Fungi, informally known as chytrids. The name is derived from the Ancient Greek ('), meaning "little pot", describing the structure containing unreleased zoos ...
s,
ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...
s and
basidiomycete Basidiomycota () is one of two large divisions Division or divider may refer to: Mathematics *Division (mathematics) Division is one of the four basic operations of arithmetic, the ways that numbers are combined to make new numbers. The other ...
s) and perhaps several times for
slime molds Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual ...
and red algae. The first evidence of multicellular organization, which is when unicellular organisms coordinate behaviors and may be an evolutionary precursor to true multicellularity, is from
cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum (biology), phylum of Gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), giving them their other name, "blue-green algae", t ...

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 cell A germ cell is any biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to as ...
s (i.e.,
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
and
egg An egg is the organic vessel containing the in which an develops until it can survive on its own, at which point the animal hatches. An egg results from of an . Most s, (excluding s), and lay eggs, although some, such as s, do not. eg ...

egg
cells), an issue that is studied in
evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research that compares the developmental processes of different organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''org ...
. Animals have evolved a considerable diversity of
cell types A cell type is a classification used to distinguish between morphologically or phenotypically distinct cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell ...
in a multicellular body (100–150 different cell types), compared with 10–20 in plants and fungi.


Loss of multicellularity

Loss of multicellularity occurred in some groups. Fungi are predominantly multicellular, though early diverging lineages are largely unicellular (e.g.,
Microsporidia Microsporidia are a group of spore-forming unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indi ...
) and there have been numerous reversions to unicellularity across fungi (e.g., ''
Saccharomycotina Saccharomycotina is a subdivision (subphylum) of the division (phylum) Ascomycota Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi o ...
'', ''
Cryptococcus ''Cryptococcus'', sometimes informally called crypto, is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus cl ...

Cryptococcus
'', and other
yeasts Yeasts are eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It ...
). It may also have occurred in some
red algae Red algae, or Rhodophyta ( , ; ), are one of the oldest groups of eukaryotic algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organism ...

red algae
(e.g., '' Porphyridium''), but it is possible that they are primitively unicellular. Loss of multicellularity is also considered probable in some
green algae The green algae (singular: green alga) are a large, informal grouping of algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to ...

green algae
(e.g., '''' and some
Ulvophyceae The Ulvophyceae or ulvophytes are a class of green algae, distinguished mainly on the basis of ultrastructural morphology, life cycle and molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, here ...
). In other groups, generally parasites, a reduction of multicellularity occurred, in number or types of cells (e.g., the
myxozoa Myxozoa (etymology Etymology ()The New Oxford Dictionary of English ''The'' () is a grammatical article Article often refers to: * Article (grammar) An article is any member of a class of dedicated words that are used with noun phras ...
ns, multicellular organisms, earlier thought to be unicellular, are probably extremely reduced
cnidarian Pacific sea nettles, ''Chrysaora fuscescens'' Cnidaria () is a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, ...

cnidarian
s).


Cancer

Multicellular organisms, especially long-living animals, face the challenge of
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumor A benign tumor is a mass of cells Cell most often refers to: * Cell (biolo ...

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 (
metazoan Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...
s) has often been described as a loss of multicellularity. There is a discussion about the possibility of existence of cancer in other multicellular organisms or even in
protozoa Protozoa (singular protozoon or protozoan, plural protozoa or protozoans) is an informal term for a group of Unicellular organism, single-celled eukaryotes, either free-living or Parasitism, parasitic, that feed on organic matter such as other mi ...

protozoa
. For example, plant galls have been characterized as
tumors A neoplasm () is a type of abnormal and excessive growth of Tissue (biology), tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissu ...

tumors
, but some authors argue that plants do not develop cancer.


Separation of somatic and germ cells

In some multicellular groups, which are called Weismannists, a separation between a sterile
somatic cell A somatic cell (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following period ...
line and a
germ cell A germ cell is any biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of all known organisms. Cells are the smallest units of life, and hence are often referred to as ...
line evolved. However, Weismannist development is relatively rare (e.g., vertebrates, arthropods, ''
Volvox ''Volvox'' is a Polyphyly, polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colony (biology), colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Anto ...

Volvox
''), as a great part of species have the capacity for
somatic embryogenesis Somatic embryogenesis is an artificial process in which a plant or embryo is derived from a single somatic cell A somatic cell (from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classic ...
(e.g., land plants, most algae, many invertebrates).


Origin hypotheses

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 essentially what
slime mold Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...

slime mold
s do. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a
coenocyteA coenocyte () is a multinucleate cell which can result from multiple nuclear divisions without their accompanying cytokinesis, in contrast to a syncytium, which results from cellular aggregation followed by dissolution of the cell membranes insid ...
. A membrane would then form around each nucleus (and the cellular space and organelles occupied in the space), thereby resulting in a group of connected cells in one organism (this mechanism is observable in
Drosophila ''Drosophila'' () is a genus of fly, flies, belonging to the family (biology), family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the character ...

Drosophila
). 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 later develop specialized tissues. This is what plant and animal
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to a unicellular organism. All species of animals, Embryophyte, la ...

embryo
s do as well as colonial
choanoflagellate The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals. Choanoflagellates are collared flagellates having a funnel shaped collar of interconnect ...
s. 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 '' ( Poecilosclerida) File:Geodia barretti.jpg, ''Geodia barretti'' (Tetractinellida) Demosponges (Class Demospongiae) are the most diverse Class (biology), class in the phylum Porifera. They include 76.2% of all species of sponges with nearl ...
, which may have left a chemical signature in ancient rocks. The earliest fossils of multicellular organisms include the contested ''
Grypania ''Grypania'' is an early, tube-shaped fossil from the Proterozoic eon. The organism, with a size over one centimeter and consistent form, could have been a giant bacterium Bacteria (; common noun bacteria, singular bacterium) are a type o ...
spiralis'' and the fossils of the black shales of the
Palaeoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6  Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
Francevillian Group Fossil B Formation in
Gabon Gabon (; ), officially the Gabonese Republic (french: République gabonaise), is a country on the west coast of . Located on the , Gabon is bordered by to the northwest, to the north, the on the east and south, and the to the west. It has ...

Gabon
( Gabonionta). The Doushantuo Formation has yielded 600 million year old microfossils with evidence of multicellular traits. Until recently,
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

phylogenetic
reconstruction has been through
anatomical Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, mo ...

anatomical
(particularly
embryological Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo An embryo is the early stage of development of a multicellular organism. In general, in organism In biology, an organism (from Ancient Greek, Greek: ὀργ ...
) similarities. This is inexact, as living multicellular organisms such as
animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, ...

animal
s and
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s are more than 500 million years removed from their single-cell ancestors. Such a passage of time allows both divergent and
convergent Convergent is an adjective for things that wikt:converge, converge. It is commonly used in mathematics and may refer to: *Convergent boundary, a type of plate tectonic boundary * Convergent (continued fraction) * Convergent evolution * Convergent s ...
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:


The symbiotic theory

This theory suggests that the first multicellular organisms occurred from symbiosis (cooperation) 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 independently, eventually leading to the incorporation of their genomes into one multicellular organism. Each respective organism would become a separate lineage of differentiated cells within the newly created species. This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Heteractis magnifica, Riterri sea anemones. In these cases, it is extremely doubtful whether either species would survive very long if the other became extinct. However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as a single species. Although such symbiosis is theorized to have occurred (e.g., mitochondria and chloroplasts in animal and plant cells—endosymbiosis), it has happened only extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during mitosis of the host species. For instance, the two or three symbiotic organisms forming the composite lichen, although dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more.


The cellularization (syncytial) theory

This theory states that a single unicellular organism, with multiple Cell nucleus, nuclei, could have developed endomembrane, internal membrane partitions around each of its nuclei. Many protists such as the ciliates or slime molds can have several nuclei, lending support to this hypothesis. However, the simple presence of multiple nuclei is not enough to support the theory. Multiple nuclei of ciliates are dissimilar and have clear differentiated functions. The macronucleus, ''macro''nucleus serves the organism's needs, whereas the micronucleus, ''micro''nucleus is used for sexual reproduction with exchange of genetic material. Slime molds syncytium, syncitia form from individual amoeboid cells, like syncitial tissues of some multicellular organisms, not the other way round. To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium.


The colonial theory

The Colonial Theory of Haeckel, 1874, proposes that the symbiosis of many organisms of the same species (unlike the #The symbiotic theory, symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some, it is presumed land-evolved, multicellularity occurs by cells separating and then rejoining (e.g., slime mould, cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division. The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation. The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla. For instance, during food shortages the amoeba
Dictyostelium ''Dictyostelium'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also ...

Dictyostelium
groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protista are Volvocaceae, such as Eudorina and
Volvox ''Volvox'' is a Polyphyly, polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colony (biology), colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Anto ...

Volvox
, the latter of which consists of up to 500–50,000 cells (depending on the species), only a fraction of which reproduce. For example, in one species 25–35 cells reproduce, 8 asexually and around 15–25 sexually. However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed "pluricellular" rather than "multicellular".


The Synzoospore theory

Some authors suggest that the origin of multicellularity, at least in Metazoa, occurred due to a transition from temporal to spatial cell differentiation, rather than through a gradual evolution of cell differentiation, as affirmed in Haeckel’s Gastraea, Gastraea theory.


GK-PID

About 800 million years ago, a minor genetic change in a single molecule called guanylate kinase protein-interaction domain (GK-PID) may have allowed organisms to go from a single cell organism to one of many cells.


The role of viruses

Genes borrowed from viruses and mobile genetic elements (MGEs) have recently been identified as playing a crucial role in the differentiation of multicellular tissues and organs and even in sexual reproduction, in the fusion of egg cell and sperm. Such fused cells are also involved in metazoan membranes such as those that prevent chemicals crossing the placenta and the brain body separation.Eugene V. Koonin
Viruses and mobile elements as drivers of evolutionary transitions
In: Philos Trans R Soc Lond B Biol Sci., 2016 Aug 19, doi:10.1098/rstb.2015.0442
Two viral components have been identified. The first is syncytin, which came from a virus. The second identified in 2007 is called EFF1, which helps form the skin of ''Caenorhabditis elegans'', part of a whole family of FF proteins. Felix Rey, of the Pasteur Institute in Paris has constructed the 3D structure of the EFF1 protein and shown it does the work of linking one cell to another, in viral infections. The fact that all known cell fusion molecules are viral in origin suggests that they have been vitally important to the inter-cellular communication systems that enabled multicellularity. Without the ability of cellular fusion, colonies could have formed, but anything even as complex as a sponge would not have been possible.


The Oxygen Availability Hypothesis

This theory suggests that the oxygen available in the atmosphere of early Earth could have been the limiting factor for the emergence of multicellular life. This hypothesis is based on the correlation between the emergence of multicellular life and the increase of oxygen levels during this time. This would have taken place after the Great Oxidation Event (GOE) but before the most recent rise in oxygen. Mills concludes that the amount of oxygen present during the Ediacaran is not necessary for complex life and therefore is unlikely to have been the driving factor for the origin of multicellularity.


Snowball Earth Hypothesis

A snowball Earth is a geological event where the entire surface of the Earth is covered in snow and ice. The most recent snowball Earth took place during the Cryogenian period and consisted of two global glaciation events known as the Sturtian glaciation, Sturtian and Marinoan glaciation, Marinoan glaciations. Xiao suggests that between the period of time known as the "Boring Billion" and the Snowball Earth, simple life could have had time to innovate and evolve which could later lead to the evolution of multicellularity. The snowball Earth hypothesis in regards to multicellularity proposes that the Cyrogenian period in Earth history could have been the catalyst for the evolution of complex multicellular life. Brocks suggests that the time between the Sturtian Glacian and the more recent Marinoan Glacian allowed for planktonic algae to dominate the seas making way for rapid diversity of life for both plant and animal lineages. Shortly after the Marinoan, complex life quickly emerged and diversified in what is known as the Cambrian explosion, Cambrian Explosion.


Predation Hypothesis

The Predation Hypothesis suggests that in order to avoid being eaten by predators, simple single-celled organisms evolved multicellularity to make it harder to be consumed as prey. Herron et al  performed laboratory evolution experiments on the single-celled green alga, ''C. reinhardtii'', using paramecium as a predator. They found that in the presence of this predator, ''C. reinhardtii'' does indeed evolve simple multicellular features.


Advantages

Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: single cells with increased size have a decreased surface-to-volume ratio and have difficulty absorbing sufficient nutrients and transporting them throughout the cell. Multicellular organisms thus have the Competition (biology), competitive advantages of an increase in size without its limitations. They can have longer lifespans as they can continue living when individual cells die. Multicellularity also permits increasing complexity by allowing Cellular differentiation, differentiation of cell types within one organism. Whether these can be seen as advantages however is debatable. The vast majority of living organisms are single cellular, and even in terms of biomass, single cellular organisms are far more successful than animals, though not plants. Rather than seeing traits such as longer lifespans and greater size as an advantage, many biologists see these only as examples of diversity, with associated tradeoffs.


See also

* Bacterial colony * Embryogenesis * Fauna Europaea * Organogenesis * Unicellular organism, Unicellular organisms


References


External links


Tree of Life Eukaryotes
{{DEFAULTSORT:Multicellular Organism Developmental biology Evolutionary biology Articles containing video clips