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Molecular phylogenetics has provided insight into the evolution and interrelationships of the three domains of life. The division between prokaryotes and eukaryotes reflects two very different levels of cellular organization; only eukaryotic cells have an enclosed nucleus that contains its DNA, and other membrane-bound organelles including ...
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0 μm in diameter. [1] [page needed] A prokaryotic cell has three regions:
The history of life was that of the unicellular prokaryotes and eukaryotes until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period. [ 17 ] [ 26 ] The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges , brown algae , cyanobacteria ...
The mechanism, now ubiquitous in living cells, powers energy conversion in micro-organisms and in the mitochondria of eukaryotes, making it a likely candidate for early life. [ 168 ] [ 169 ] Mitochondria produce adenosine triphosphate (ATP), the energy currency of the cell used to drive cellular processes such as chemical syntheses.
The conclusion was that eukaryotes evolved from archaea, specifically Crenarchaeota (eocytes) and the results "favor a topology that supports the eocyte hypothesis rather than archaebacterial monophyly and the 3-domains tree of life." [26] A study around the same time also found several genes common to eukaryotes and Crenarchaeota. [33]
The most recent research does not support the classification of the eukaryotes into any of the standard systems. In 2009, Andrew Roger and Alastair Simpson emphasized the need for diligence in analyzing new discoveries: "With the current pace of change in our understanding of the eukaryote tree of life, we should proceed with caution."
Life originated as marine single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic.