In biology, an organism is any contiguous living system (such as animal, fungus, micro-organism, or plant). In at least some form, all types of organisms are capable of responding to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole.
An organism may be either unicellular (a single cell) or, as in the case of humans, comprise many trillions of cells grouped into specialized tissues andorgans. The term multicellular (many cells) describes any organism made up of more than one cell. An organism may be either a prokaryote or aeukaryote.
Prokaryotes are represented by two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound compartments called organelles (such as mitochondria in animals and plantsand plastids in plants and algae, all generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples ofkingdoms of organisms within the eukaryotes.
In 2002 Thomas Cavalier-Smith proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved fromBacteria, more specifically from Actinobacteria. See the article: Branching order of bacterial phyla