The cell is the smallest structural and functional unit of an organism. In Latin, the “cella” means the “small room”. The cell is known as the “building blocks of life” and the study of cells is called cytology or cell biology or cellular biology. The cells are made of cytoplasm that consists of nucleic acids and proteins. The dimensions of the cells are about one to 100 micrometres. It was discovered by Robert Hooke in 1665. There are two types of cells, they are prokaryotic and eukaryotic cells. The prokaryotic cells are unicellular organisms that lack a nucleus. The eukaryotic cells are single-celled organisms or multicellular organisms. These eukaryotic cells evolved from a community of prokaryotic cells. The multicellular organisms are formed in various ways such as cell division or by assembling single cells.
The macroscopic multicellular organisms evolved around the world about 600 million years ago. Maoyan Zhu at the Chinese Academy of Sciences located in Nanjing, along with his colleagues has reported that they discovered the well-preserved fossils found in northern China, where the length of these organisms is about 30cm long.
What is a Multicellular Organism?
An organism that consists of two or more cells is called multicellular organisms. All the species of plants, animals, fungi, red algae, green algae, and brown algae are multicellular organisms.
One of the hypotheses that tell about the origin of the multicellular organism is the aggregation of cells that have specific functions into a slug-like mass known as grex combined forms multicellular organisms.
The second hypothesis is that a primitive cell has become a coenocyte by the process of nuclear division. A membrane has been formed around each of these nuclei that resulted in the formation of a number of cells.
The third hypothesis is that a unicellular organism has divided to form two daughter cells in which at a particular stage the daughter cells fail to divide further and these cells develop into a specialized tissue performing different functions. This mainly happens in plant and animal embryos.
The multicellular organisms found at first were simple, didn't have bone, other body parts, or shell and they were not preserved in the fossil records. Now we got to know what a multicellular organism is. Now let us see the evolutionary history of these organisms.
The first evidence obtained for these organisms is from cyanobacteria that lived 3.5 billion years ago.
Loss of multicellularity has occurred in some of the groups of organisms. Fungi are unicellular organisms, but according to early diverging fungi are unicellular organisms. This has also happened in red algae.
The long-living multicellular organisms often face the challenge of cancer. This occurs only when the cells lack the capability of growth in their normal development. Cancer in multicellular organisms led to the loss of multicellularity. As per some researchers, the plant galls are considered tumours, but some researchers argue that the plant bodies do not get affected by cancer.
In some of the multicellular organisms, the separation of the sterile somatic cells and the germ cells has occurred.
Some of the Theories Suggest How Multicellular Cells Evolved:
Symbiotic Theory: Due to the symbiosis of different single-celled organisms that perform different roles the first multicellular organism has formed. But these organisms were dependent on each other; this further led to the incorporation of genomes to form a multicellular organism.
Cellularization Theory: It states that a unicellular organism that has numerous nuclei has developed the partitions by covering each of the nuclei with a membrane. This simple presence of multi nuclei was not sufficient to support the hypothesis.
Colonial Theory: It states that the symbiosis of organisms that belong to the same species has led to the formation of multicellular organisms. It is assumed the land is evolved and the multicellularity has occurred by joining or separating the cells, in some cases the multicellularity occurs when the cells fail to divide further. The main advantage of this theory is that this process of multicellularity has been found in 16 different phyla.
Oxygen Availability Hypothesis: The availability of oxygen on the earth acts as a limiting factor for the formation of multicellular organisms. This hypothesis defines the relationship between the availability of oxygen and the emergence of life.
Snowball Earth Hypothesis: It is a geological event in which the entire Earth has been covered with ice and snow. This theory suggests that the Cryogenian period acts as the catalyst to the formation of complex multicellular life.
Predation Hypothesis: This theory states that in order to get protection from the predators the single-celled organisms have evolved to form multicellular.
Characteristics of Multicellular Organisms
The Characteristics of the Multicellular Organisms are as Follows:
These are complex organisms that are made up of more than one cell.
These organisms are visible to the naked eye.
They are made up of different organs and the organ systems where each of these performs various functions.
They contain membrane-bound organelles as they belong to the eukaryotes.
The size of an organism increases with an increase in the number of cells in the body.
The division of labour takes place in between the cells.
We have learnt what is multicellular, characteristics, and history of these organisms. By this, we got to know that they have the advantages to increase their size without any limitations. Even though the individual cells die they have the capability of living for a longer lifespan. The complexity can be increased by the differentiation of the cells in an organism.