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Multicellular organism

 

 
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Multicellular organism
 
 
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Multicellular organisms are organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
s consisting of more than one cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
, and having differentiated cells that perform specialized functions in the cell. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdom
Kingdom (biology)

In Biology taxonomy, kingdom or regnum is a taxonomic rank in either the highest rank, or the Rank below domain . Each kingdom is divided into smaller groups called Phylum ....
s Plant
Embryophyte

The embryophytes are the most familiar group of plants. They include trees, flowers, ferns, mosses, and various other green land plants. All are complex multicellular eukaryotes with specialized reproductive organs....
ae and Animal
Animal

Animals are a major group of 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....
ia (except for specialized organisms such as Myxozoa
Myxozoa

The Myxozoa are a group of parasite animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan....
 in the case of the latter).

y life was most probably single celled and multicellularity has appeared dozens of times in the history of Earth. The oldest known taxonomically resolved multicellular organism is a red algae, Bangiomorpha pubescens, found fossilized in 1.2 billion year old rock from the Ectasian
Ectasian

The Ectasian is the second geologic geologic period in the Mesoproterozoic Era and lasted from 1400 annum ago to 1200 Ma . Instead of being based on stratigraphy, these dates are defined chronometrically....
 period of the Mesoproterozoic
Mesoproterozoic

The Mesoproterozoic Era is a geology era that occurred between 1600 Ma and 1000 annum .The major events of this era are the formation of the Rodinia supercontinent, the breakup of the Columbia , and the evolution of sexual reproduction....
 era.

In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cell
Germ cell

Germ cells are progenitors of the gametes. These singled-out cells move through the gut to the developing gonads and undergo mitotic Cell proliferation followed by meiosis and Cellular differentiation into either eggs or sperm ....
s (i.e.






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C Elegans Stained
Multicellular organisms are organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
s consisting of more than one cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
, and having differentiated cells that perform specialized functions in the cell. Most life that can be seen with the naked eye is multicellular, as are all members of the kingdom
Kingdom (biology)

In Biology taxonomy, kingdom or regnum is a taxonomic rank in either the highest rank, or the Rank below domain . Each kingdom is divided into smaller groups called Phylum ....
s Plant
Embryophyte

The embryophytes are the most familiar group of plants. They include trees, flowers, ferns, mosses, and various other green land plants. All are complex multicellular eukaryotes with specialized reproductive organs....
ae and Animal
Animal

Animals are a major group of 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....
ia (except for specialized organisms such as Myxozoa
Myxozoa

The Myxozoa are a group of parasite animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan....
 in the case of the latter).

Evolutionary history

Early life was most probably single celled and multicellularity has appeared dozens of times in the history of Earth. The oldest known taxonomically resolved multicellular organism is a red algae, Bangiomorpha pubescens, found fossilized in 1.2 billion year old rock from the Ectasian
Ectasian

The Ectasian is the second geologic geologic period in the Mesoproterozoic Era and lasted from 1400 annum ago to 1200 Ma . Instead of being based on stratigraphy, these dates are defined chronometrically....
 period of the Mesoproterozoic
Mesoproterozoic

The Mesoproterozoic Era is a geology era that occurred between 1600 Ma and 1000 annum .The major events of this era are the formation of the Rodinia supercontinent, the breakup of the Columbia , and the evolution of sexual reproduction....
 era.

In order to reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cell
Germ cell

Germ cells are progenitors of the gametes. These singled-out cells move through the gut to the developing gonads and undergo mitotic Cell proliferation followed by meiosis and Cellular differentiation into either eggs or sperm ....
s (i.e. sperm
Sperm

The term sperm is derived from the Greek word sperma and refers to the male reproductive Cell . In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell....
 and egg
Ovum

An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization....
 cells), an issue that is studied in developmental biology
Developmental biology

Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, cellular differentiation and "morphogenesis," which is the process that gives rise to biological tissues, organ s and anatomy....
. Therefore, the development of sexual reproduction
Sexual reproduction

Sexual reproduction is characterized by processes that pass a Genetic recombination of Genetics material to offspring, resulting in Genetic diversity....
 in unicellular organisms during the Ectasian period is thought to have precipitated the development and rise of multicellular life.

Multicellular organisms also face the challenge of cancer
Cancer

Cancer is a class of diseases in which a group of cell display uncontrolled growth , invasion , and sometimes metastasis . These three malignant properties of cancers differentiate them from benign tumors, which are self-limited, do not invade or metastasize....
, which occurs when cells fail to regulate their growth within the normal program of development.

Hypotheses for origin


There are various mechanisms which are disputed as being the first responsible for the emergence of multicellularity, but it is difficult to say which is correct. This is because all the suggested mechanisms are viable, but establishing which was responsible for the first multicellular life requires mostly speculation.

One hypothesis is that a group of function-specific cells aggregated into a slug-like mass called a grex
Grex (biology)

A grex is a multicellular aggregate of amoeba of the groups Acrasiomycota or Dictyosteliida that can be formed when the amoeba run out of food....
, which moved as a multicellular unit. Another hypothesis is that a primitive cell underwent nucleus division, thereby becoming a syncytium
Syncytium

In biology, a syncytium is a large cell-like structure filled with cytoplasm containing many cell nucleus....
. 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 and specialised cells in one organism (this mechanism is observable in Drosophila
Drosophila

Drosophila is a genus of small fly, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit....
). A third theory is that, as a unicellular organism divided, the daughter cells failed to separate, thereby resulting in a conglomeration of identical cells in one organism which could each then specialize.

See also

  • Cellular differentiation
    Cellular differentiation

    In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of Tissue and cell types....
  • Eukaryote
    Eukaryote

    Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
  • Evolution of multicellularity
    Evolution of multicellularity

    The first organisms that existed are believed to have been unicellular. How organisms then became multicellular is a huge evolutionary step and is consequently under great debate....
  • Bacterial colony