The beginning of multicellular Eukaryotes is one of the most important event in earth’s history regarding Eukaryotic life. As we can observe, most if not all animals, plants, and fungi are multicellular Eukaryotes therefore multicellularity is a key component of the early evolution regarding complex life. However, multicellularity nor Eukaryotic cell suddenly occur at once. In fact there is an order regarding the events that occurred and there are many theories as to how unicellularity evolved to multicellularity.
To understand how multicellular Eukaryotes evolved, we first have to look at its ancestors. Multicellularity can be followed in this order unicellular ïƒ colonial ïƒ multicellular. The prefix “uni” in unicellular means one therefore unicellularity means “one cell”. Unicellular organism like Amoebas are made up of one cell, they are not dependent on any other cells and they reproduce asexually (Waggoner, 2001). Colonial organisms can be thought of as the connecting bridge between unicellularity and multicellularity. Colonial organisms such as Volvox are a collection of unicellular organisms coming together to form a structure that is beneficial to all of the cells. (Cooper GM, 2000) However if we remove one cell from this colony it is capable of fully surviving. This is the clear distinction between colonial and multicellular organisms. Lastly Multicellular organisms such as plants and animals contain many different specialized cells that work and communicate together within the organism and if we remove a cell that cell cannot survive on its own. (Cooper GM, 2000)
In addition, Eukaryotes have been thought to have evolved from Prokaryotes. The first Prokaryote fossils were discovered 3.8 billion years ago (Cooper GM, 2000) then the first Eukaryotic cells were thought to have evolved around 2.7 billion years ago in the Proterozoic time period and one billion years later multicellular Eukaryotes appeared. (Cooper GM, 2000) There are a few emerging theories as to how Prokaryotes evolved to Eukaryotes and most scientist agree it is through endosymbiosis. Endosymbiosis theory can be explained as one Prokaryotic cell such as bacteria engulfing another Prokaryotic cell and that cell escapes digestion and therefore lives inside the cell. Now this creates a symbiotic relationship where the engulfed cell provides energy and nutrients and in return the engulfed cell receives protection. (Waggoner, 2001)
The difference between Eukaryotes and Prokaryotes is that Eukaryotes have a nucleus, membrane bound organelles, linear DNA, and are typically multicellular. Conversely Prokaryotes lack a nucleus, organelles, linear DNA (have circular DNA), and are typically unicellular and smaller. (Waggoner, 2001)
After the events mentioned above, the next event that occurred was the change from unicellularity to multicellularity which is fundamental to all plants, animals, and fungi. The theories to how these unicellular cells evolved to multicellular cells is still debated but there are few emerging theories.
Theories are not the only thing that is being debated by scientists. The number of times multi cellularity has evolved is also debated based on the definition one uses. According to Karl Niklas (2014) a plant biologist from Cornell University If one defines multicellularity as an aggregation of cells “it can be said that it has been evolved at least in 25 lineages.” (Para.3) However if one defines it more strictly such as requiring the cells to communicate, cooperate and connect then it can be said to have “evolved once in animals, three times in fungi, six times in algae, and multiple times in bacteria.” (para.3) the reason for the variety of the number of times multicellularity has evolved is due to natural selection. The mechanism that was used to achieve multicellularity could have been in many ways but if the result is an evolved multicellular organism that functions better and has better fitness than the unicellular organism then this organism will be favored. In other words the traits are being naturally selected not the mechanism. (Niklas, 2014)
There is a certain criteria for multicellularity to evolve according to Karl Niklas. The first step for evolution of multicellularity is the cells must be genetically compatible to some extent. Secondly the group of cells that are formed must work together in a coordinated way to reproduce more cells until a distinct organism forms. The pathways of achieving multicellularity among the different kingdom differs which means the process of multicellularity is an example of convergent evolution. (Niklas, 2014)
The most plausible theory of the origins of multicellular Eukaryotes is the colonial theory which states that a unicellular organism of the same species comes together to form an aggregate and eventually a sphere which eventually folds to make tissue. (Niklas & Newman, 2013) In more detail, unicellular organisms of the same species exist and reproduce independently. Then they form an aggregate which is like a clump of unicellular organisms together because they benefit from coming together and cooperating. Then the cells line up in a more hollow shape with polarity on each side of a cell similar to the cell membrane in cells today. This helps keep the internal environments relatively stable. Lastly selective pressure leads to a separation and specialization of the somatic and germ cells in which through invagination of germ cells produce differentiated tissue. In fact this similar to the embryonic development of many multicellular organisms such as humans.
A second theory is called the cellularization theory which states the failure of cytokinesis following nuclear divisions is the cause of multicellularity. (Niklas & Newman, 2013) To be more specific cells exist and reproduce independently and then when they form an aggregate they form a syncytial (cell with multiple nuclei) because of incomplete cell division. Then after the Coenocyte, membranes form between the nuclei allowing one nucleus per cell and lastly these cells differentiate to form different types of cells such as germ cells. This is very similar to colonial theory except for the fact that it has a more complex transition in the beginning (single cell to multinucleate cell) compared to colonial theory and therefore it is less likely to have occurred. But this type of mechanism can be seen in algae, many fungi, and even fruit fly.
Lastly the symbiotic theory states that different species of cells come together and work together because of mutual benefits from aggregation and evolve until they become codependent. (Waggoner, 2001) More specifically, cells from two different species first live and reproduce independently. Then they come in contact and they have a mutualistic relationship meaning they both benefit from being physically associated. After time they become codependent meaning they could not survive alone and at some point there is a fusion of the genomes which produces a new species and a new germ line. However the problem with this theory is there is no explanation of how the germ and somatic cells are produced nor any explanation of how fusion of two different genomes could occur without leaving any evidence. However the process of symbiosis of cells is observable in lichen.
Although the origin of multicellularity is still unknown, the advantages of multicellularity is the driving force behind the evolution of multicellularity. Multicellularity allows organism to exceed the size limits which helps defend against predators therefore increase survivability which would be favored. In addition, multicellularity increases the complexity of organisms by allowing specialization of cells which gives rise to other complex structures such as tissues and organs and it can help the organism keep a stable environment or homeostasis. (Waggoner, 2001)
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Niklas K. J., Cobb E. D., Crawford D. R. (2013).The evo-devo of multinucleate cells, tissues, and organisms, and an alternative route to multicellularity.Evol. Dev.15466–474 10.1111/ede.12055
American Journal of Botany. (2014, January 25). From one cell to many: How did multicellularity evolve?.ScienceDaily. Retrieved April 06, 2015 from www.sciencedaily.com/releases/2014/01/140125172414.html
Waggoner, Ben. Eukaryotes and Multicells: Origin. University of Central Arkansas. Macmillan Publishers Ltd, Nature Publishing Group, n.d. Web. 07 Apr. 2015. <http://faculty.uca.edu/benw/reprints/1640.pdf>.
Niklas, K. J., & Newman, S. A. (2013). The origins of multicellular organisms.Evolution & Development,15(1), 41-52. doi:10.1111/ede.12013