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Before we directly delve into if humans are multicellular organisms, let us first understand what are multicellular and unicellular organisms.
The word 'multicellular organism' is self-explanatory to some extent; it refers to an organism that is made up of more than one cell. On the other hand, unicellular organisms are the exact opposite of multicellular organisms and refer to organisms that consist of merely one cell.
Unicellular organisms existed on Earth prior to the multicellular organisms, while the multicellular organisms have been formed on the basis of unicellular organisms. Interestingly, life on this blue planet didn't directly start as single-celled organisms. Molecules are the building blocks of a single cell which when creating copies of themselves and becoming organized, give rise to single-celled organisms. These cells then have the ability to produce copies of themselves which then give rise to multicellular organisms.
A multicellular organism can further coordinate multiple cell types to give birth to more complex organisms like humans and other animals. Evolution has been the key in the birth of a complex organism as with the passage of time, the multiple cells evolved to give rise to tissues with particular functions. These tissues then function interdependently to create a functioning complex organism.
The evolution from a planet inhabiting unicellular organisms to humans took an unfathomable amount of time with the first unicellular organism believed to have emerged around 4 billion years ago. The division between a unicellular organism and a multicellular organism is pretty easy to understand but there are exceptions where organisms are partially unicellular as well as multicellular. Organisms that are informally known as slime molds are the most common organism type to fall under this category.
Humans and bacteria are pretty different from each other when it comes to the cellular composition of their bodies. Bacteria is a single-cell organism and perhaps one of the earliest life forms to be present on Earth. Humans, on the other hand, are primary examples of complex life forms which are possible only in the case of multicellular organisms.
Bacteria is a unicellular organism that doesn't have a true nucleus and at the same time lacks properly developed and functioning organelles. One of the most important characteristics of single-cell organisms is that they are small in size and single-cell organisms such as bacteria exist in a variety of shapes ranging from rods to spirals. Interestingly, in these organisms there are no specialized cells like those in multicellular organisms, instead, there is only one cell that performs all the functions of reproduction, feeding, and respiration as well.
Bacteria, like most other unicellular organisms, reproduce asexually via the method of binary fission. Although two bacteria can come in contact with each other to transfer genetic material using pilus. Unlike most multicellular organisms, single-celled organisms can be found anywhere on Earth ranging from the planet's inner crust to water to hot natural springs and as well as in radioactive waste. Interestingly, some of these bacteria are also known to have a parasitic relationship with other animals, and plants.
Moving on to the human body, it is much more complex than bacteria and there are trillions of cells in a human body. Over the years, the cells have evolved to give rise to tissues and organs which have a specific function to run such a complex organism. At the same time, as we earlier saw, bacteria only have one cell which performs all the functions but in the case of humans, there are different cells that perform specialized functions such as the blood cells, muscle cells, skin cells, and nerve cells.
Multicellular life, whether it is of an animal or a human, is a combined, interdependent effort of all the different cells inside the body of the organism. Interestingly, even inside the human body, all the building blocks aren't multicellular, the cardiac muscle is typically unicellular and is made from multiple cardiac muscle sheets. Another reason why humans are multicellular organisms is their method of reproduction. Unlike single-celled organisms like bacteria, multicellular organisms like humans undergo reproduction to welcome new life on the planet.
There are a large number of multicellular organisms all around us ranging from your pet dog to the trees in your backyard. Any organism that is composed of many cells is regarded as multicellular, with the term 'multi' particularly referring to the multiple cells working together to ensure that the organism functions efficiently.
It was a division of labor and cell specialization that led to the birth of multicellular life. With the passage of time, a particular cell becomes specialized in carrying out a particular kind of task and thus has more efficiency than other cells to carry out that particular task. Due to this phenomenon, the particular cell depends upon the other cells to carry out the other tasks for the proper functioning of the organism.
One of the most distinctive features of organisms that are made up of many cells is, they are always visible to the naked eye and possess organs that carry out a particular function. Whether multicellular organisms are animals or plants, it is the combined coordinated efforts of all the cells, tissues, and organs within their body that allow the organism to function efficiently. Most of the life forms that we see around are multicellular such as cows, cats, plants, humans, and various other kinds of animals. On the other hand, some of the common examples of unicellular organisms include the likes of green algae, diatoms, Amoeba, Euglena, Nitrobacter, Nitrosomonas, and many others as well.
If we take a look at events that significantly contributed to the development of life on Earth, the transition from unicellular to multicellular might just be there near the top.
For around a billion years, this planet was inhabited by only unicellular organisms such as green algae but once multicellular organisms came into existence, organisms such as dinosaurs walked this planet. Interestingly, the transition from unicellular organisms to multicellular organisms did not take a lot of time but the further evolution into such complex life systems took a considerable amount of time.
It is a simple phenomenon- it is easy for a predator to attack a single organism but when these organisms combine into a pack, it becomes difficult for the predator to break it down. Scientists believe that a similar approach led to the evolution of multicellular life forms from unicellular organisms. When attacked by a predator, the unicellular organisms became unified to defend themselves which gave birth to complex multicellular organisms.
This transition from unicellular to multicellular life form is believed to have taken place around 1.5 billion years ago. There were a series of events that led to the introduction of multicellular organisms to Earth, the first of which was the cells forming cell groups by adhering to each other which significantly increased the survival rate of the cells. But at the same time, although the formation of cell groups increased the cells' defenses against predators, it lowered its reproduction rate. The formation of multicellular life forms from unicellular thus did not take a lot of time but its evolution into efficiently functioning organisms took a considerable amount of time. It took generations for multicellular organisms to become adept at reproducing in comparison to unicellular organisms.
Interestingly, cell division or mitosis is an integral part of the life of multicellular organisms. The process of cellular division performs three primary functions with the first one being the formation of sperms and eggs which lead to reproduction in multicellular organisms. It is these germ cells that take care of this process. The other functions performed by cellular division are to grow and repair tissues in multicellular animals and reproduce new unicellular organisms. Interestingly, the process of mitosis which is known for producing identical cells is used for the two latter functions performed by the process of cell division.
In layman's terms, cells can be described as tiny units of life which manufacture their own energy and can self replicate if and when needed.
Inside the cell, there are a number of structures which are known as organelles that perform a specific function. In simpler terms, these organelles are like the transport systems, warehouses, power plants, and factories inside the cell which help it function. Let us take a closer look at these different parts of the cells and the functions that they perform.
Talking about a cell, we have to first address the nucleus, which is regarded as the headquarter of the cell. Usually, all the cells have one nucleus whose primary function is to store the DNA of the cell and make the cell function regarding whether it should grow, or divide, or even die. The next cell part is the plasma membrane whose primary task is to prohibit the watery substances from entering the cell. The plasma membrane basically acts as the guard of the cell. The cytoplasm is another important part of the cell, which can be identified as the water-like substance which surrounds the nucleus and where all the organelles are present inside the cell.
Majority of the reactions which take place inside the cell all take place in the cytoplasm. Moving on to another indispensable part of the cell is the endoplasmic reticulum which performs an important task of processing molecules within cells and then transporting it to a particular location. The endoplasmic reticulum primarily folds and transports proteins from one part of the cell to the other as per requirement. Interestingly, within the cell, there are two kinds of endoplasmic reticulum, the smooth endoplasmic reticulum, and the rough endoplasmic reticulum.
Commonly known as the powerhouse of the cell, the mitochondria are undoubtedly one of the most important parts of a cell. Being the cell's powerhouse, the mitochondria are known for converting food that the human consumes into energy that the cells require. It is also known for storing calcium and playing an important role in the process of apoptosis. Another important part of the cell is the Golgi apparatus which is primarily known for collecting items once they are processed by the endoplasmic reticulum and then letting the cell use these items either inside the cell or outside as the case may be. All the individual cells within an organism perform various kinds of functions whether it is the skin cells or the nerve cells but their basic composition is more or less the same to perform the primary cell functions.
Human cells are a type of animal cell and animal cell is a kind of eukaryotic cell. Eukaryotic cells are largely divided into two categories, animal cells, and plant cells. The plant cells have more organelles than animal cells such as chloroplasts, large vacuole, and a cell wall. Animals do not require chloroplasts as they do not make their own food like plants.
Human cells can be described as those cell types which are found in the human body and perform functions such as storage and secretion among many others. There aren't a lot of differences between human cells and animal cells as the human cell is a type of animal cell except for the fact that human and animal cells differ in size from each other.
The primary function performed by human cells includes providing a structure to the body, taking nutrients from the food consumed, and then converting these nutrients into energy. These cells also contain hereditary material about the body which is used in the process of reproduction. In the case of animals, the cells produce energy and then store it for carrying out all the necessary body processes. At the same time, animal cells are also responsible for making proteins which play a pivotal role in metabolism.
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