the origin of eukaryotic cells. free template from nowadays we have known that cells can be...

The Origin of Eukaryotic Cells

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Nowadays we have known that cells can be conveniently divided into two groups : prokaryotic and eukaryotic cells.And people already presumed that how the prokaryotic cells start their lives .

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The earliest prokaryotic ancestors were presumed to have been anaerobic( 厌氧的 ) heterotrophic （异养的） cells .

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• Anaerobic because they derived their energy from food matter without employing molecular oxygen .

• heterotrophic because they were unable to synthesize organic compounds from inorganic precursors ,but instead had to obtain preformed organic compounds from their environment.

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But what is the origin of the eukaryotic cells?

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Most people have been agreed that prokaryotic cells arose before eukaryotic cells and gave rise to eukaryotic cells .

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• This opinion can be verified directly from the fossil( 化石 ) record ,which shows that prokaryotic cells were present in 2.7 billion years ago .(It is 1 billion years earlier than the eukaryotic cells appear.)

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• This point is follow from the the fact that 2 types of cells have to be related to one another because they share many complex traits that could not have enough independently in different organisms.

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About 1970,Lynn Margulis change peoples concept through his work at Boston University .He put forward a hypothesis （假说） which is referred to as the endosymbiont theory :The certain organelles of a eukaryotic cell—most notably the mitochondria and chloroplasts —had evolved from smaller prokaryotic cells that had taken up residence in the cytoplasm of a larger host cell .

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• Step one : A large ,anaerobic ,heterotrophic prokaryote ingested a small ,aerobic prokaryote.

• Step two : Resisting digestion within the cytoplasm ,the small aerobic prokaryote took up residence as a permanent endosymbiont ,and a colony of these composite cell was produced .

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• Step three : The cell whose ancestors had formed through the sequence of symbiotic events just described could have given rise to a line of cells that evolved other basic characteristics of eukaryotic cells ,including a system of membranes ,a complex cytoskeleton ,and a mitotic（有丝分裂） type of cell division .

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• Step four : The cell which possessed these various internal compartment would have been an ancestor of a heterotrophic eukaryotic cell .

• Step five : The acquisition of another endosymbiont could have converted an early heterotrophic eukaryote into an ancestor of photosynthetic eukaryotes .

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About all of this ,we known that groups of eukaryotes either possess mitochondria or show definitive evidence they have evolved from organisms that possessed these organelles .

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If we want to say that the modern eukaryotes and most modern prokaryotes evolved from two different ancient groups,it is conceivable under these circumstances （情况） :a particular group of living prokaryotes —descendants of the same group that gave rise to the first prokaryote .

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How does one determine evolutionary relationships among organisms that have been separated in time for billions of years?

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Because the difference between organisms in the sequence of amino acid that make up a protein or the sequence of nucleotides that make up a nucleic acid are the result of mutations of DNA that have been transmitted to offspring ,mutations can accumulate in a given gene at a relatively constant rate over long periods of time .

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Consequently ,comparisons of amino acid or nucleotide sequences can be used to determine how closely organisms are related to one another.

结论：我们可以通过对两种有机体中氨基酸或者核苷酸序列的比较，从 而得到它们之间亲缘的远近关系 。