Organism

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 and organs. The term multicellular (many cells) describes any organism made up of more than one cell.

All organisms living on Earth are divided into the eukaryotes and prokaryotes based on the presence or absence of true nuclei in their cells. The prokaryotes represent 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 compartmentalization called organelles (such as mitochondria in animals and plastids in plants, both generally considered to be derived from endosymbiotic bacteria). Fungi, animals and plants are examples of kingdoms of organisms that are eukaryotes.

In 2002 Thomas Cavalier-Smith proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved from Bacteria, more specifically from Actinobacteria. See Branching order of bacterial phyla.