In
biology and
ecology, an organism (in
Greek organon = instrument) is a
living complex adaptive system of
organs that influence each other in such a way that they
function in some way as a stable whole.The
origin of
life on
Earth and the relationships between its major lineages are controversial. Two main grades may be distinguished, the
prokaryotes and
eukaryotes. The prokaryotes are generally considered to represent two separate
domains, called the
Bacteria and
Archaea, which are not closer to one another than to the eukaryotes. The gap between prokaryotes and eukaryotes is widely considered a major missing link in
evolutionary history. Two
eukaryotic organelles, namely
mitochondria and
chloroplasts, are generally considered to be derived from
endosymbiotic bacteria. A similar
symbiogenesis hypothesis has been proposed involving the origins of the cell nucleus, it is described as
viral eukaryogenesis. Fungi, animals and plants are examples of species that are eukaryote.More recently a
clade,
Neomura, has been proposed, by
Thomas Cavalier-Smith, which groups together the
Archaea and
Eukarya. Cavalier-Smith also proposed that the Neomura evolved from
Bacteria, more precisely from
Actinobacteria.
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