The Origin of the Cell

There exist many theories around the causation of biological diversification. More generally, it is found that speciation is dependent primarily on large-scale changes in lithospheric (geomorphological) complexity (Cracraft 1985). This is easily demonstrated by allopatric speciation being the preferred mode as well as tectonic changes being the main cause of geographic and ecological barriers' formation (Cracraft 1985). On top of this strictly geographical assumptions, diversification began within the species themselves through means of natural selection (Darwin 1859).

All living beings originate from a common cellular ancestor which earliest signs are documented at 3.5 to 4 billion years ago in marine environments (Benton 2016). With the evolution of organisms, modern biology identifies three main domains of life which originated from the diversification of species: Bacteria, Archea and Eukarya, which is actually a ramification of the Archean branch (Koonin 2010).

In particular, prokaryotic cells, which refers to bacteria and prokaryotic archea, represent the first ever living being as well as currently the most populated on Earth (Cohan 2008). The reasons for this great success are numerous: starting with the prokaryotic cell's membrane chemical composition which allows them to thrive in extreme environments up to the way they are enabled to absorb energy from several external sources, many factors allowed prokaryotes to succeed in the evolution of life (Cohan 2008). Considering as well their simple cellular composition, which results in an asexual reproduction by binary fission as well as the possibility of lateral-gene transfers, is a facilitating factor justifying their expansion into different geographical domains without many biological restrictions (Cohan 2008).

Prokaryotic cells are vital for the survival of other living beings, especially eukaryotes, as they play a critical role in the recycling of nutrients to be re-used and absorbed by other organisms, both by decomposing dead cells as well as supporting in vital functions for energy synthesis, for example in the animals digestive system (Cohan 2008).

The main theory supporting the birth of eukaryotic cells is endosymbiosis (Hillis 2020). According to this hypothesis, early eukaryotic cells were engulfed by phagocytosis, remained undigested and became part of a new functionality to the engulfing cell (Hillis 2020). Over time, the engulfed cell became one of the organelles of the organism, among these we derived mitochondria which is the energy factory of the eukaryotic cell providing ATP molecules through means of cellular respiration (Hillis 2020). The production of food and energy in the form of ATP along with maintaining the homeostasis (organisms' equilibrium) by eliminating waste represents the main functions and peculiarities of eukaryotic cells (Hillis 2020).

The ways that eukaryotic cells evolved from unicellular to multicellular is still highly debated. However, it is generally accepted that the first step towards multicellularity has been the clustering of cells, afterwards depending on the success or failure of the cluster, cells lost their evolutionary autonomy becoming dependant to one another (Lobby 2020).  


References:


Cracraft, Joel., (1985), Biological Diversification and Its Causes, Annals of the Missouri Botanical Garden 72(4)


Darwin, Charles, (1859), On the origin of species by means of natural selection, or preservation of favoured races in the struggle for life 


Benton, M.J.,  (2016), Origins of biodiversity, PLOS Biology 14(11)


Koonin, E. V. (2010), The Two Empires and Three Domains of Life in the Postgenomic Age, Nature Education 3(9)


Cohan, F.M., and Koeppel, A.F., (2008), The Origins of Ecological Diversity in Prokaryotes, Current Biology, Volume 18, Issue 21


Hillis, Heller, Hacker, Hall, Laskowski and Sadava, (2020), Chapter 26: The origin and diversification of Eukaryotes, Life: The Science of Biology


Eric Libby, William C. Ratcliff, (2014), Ratcheting the evolution of multicellularity: Traits that entrench cells in a group lifestyle may pave the way for complexity, Science 346(6208)

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