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| Evolution of prokaryotes |
The current model of the evolution of the first living organisms is that these were some form of prokaryotes, which may have evolved out of protobionts. In general, the eukaryotes
are thought to have evolved later in the history of life. However, some
authors have questioned this conclusion, arguing that the current set
of prokaryotic species may have evolved from more complex eukaryotic
ancestors through a process of simplification. Others have argued that
the three domains of life arose simultaneously, from a set of varied
cells that formed a single gene pool. This controversy was summarized in
2005.
T here is no consensus among
biologists concerning the position of the eukaryotes in the overall
scheme of cell evolution. Current opinions on the origin and position of
eukaryotes span a broad spectrum including the views that eukaryotes
arose first in evolution and that prokaryotes descend from them, that
eukaryotes arose contemporaneously with eubacteria and archeabacteria
and hence represent a primary line of descent of equal age and rank as
the prokaryotes, that eukaryotes arose through a symbiotic event
entailing an endosymbiotic origin of the nucleus, that eukaryotes arose
without endosymbiosis, and that eukaryotes
arose through a symbiotic event entailing a simultaneous endosymbiotic
origin of the flagellum and the nucleus, in addition to many other
models, which have been reviewed and summarized elsewhere.
The
oldest known fossilized prokaryotes were laid down approximately 3.5
billion years ago, only about 1 billion years after the formation of the
Earth's crust. Even today, prokaryotes are perhaps the most successful
and abundant life-forms.[citation needed] Eukaryotes only appear in the
fossil record later, and may have formed from endosymbiosis of multiple
prokaryote ancestors. The oldest known fossil eukaryotes are about 1.7 billion years old. However, some genetic evidence suggests eukaryotes appeared as early as 3 billion years ago.
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