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Life originated as single-celled prokaryotes and later evolved into the more complex eukaryotes. In contrast to prokaryotic cells, eukaryotic cells are highly organised. Prokaryotes are the bacteria and archaea, while eukaryotes are the other life forms — protists, plants, fungi and animals.
The cellular components of prokaryotes are not enclosed in membranes within the cytoplasm, like eukaryotic organelles.Bacteria have microcompartments, quasi-organelles enclosed in protein shells such as encapsulin protein cages, [4] [5] while both bacteria and some archaea have gas vesicles.
However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry. Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.
Archaea were initially classified as bacteria, receiving the name archaebacteria (/ ˌ ɑːr k i b æ k ˈ t ɪər i ə /, in the Archaebacteria kingdom), but this term has fallen out of use. [5] Archaeal cells have unique properties separating them from Bacteria and Eukaryota. Archaea are further divided into multiple recognized phyla.
The tree of life. Two domains of life are Bacteria (top branches) and Archaea (bottom branches, including eukaryotes). The two-domain system is a biological classification by which all organisms in the tree of life are classified into two domains, Bacteria and Archaea.
Archaea are prokaryotic unicellular organisms, and form the first domain of life in Carl Woese's three-domain system. A prokaryote is defined as having no cell nucleus or other membrane bound-organelle. Archaea share this defining feature with the bacteria with which they were once grouped.
A major step forward in the study of bacteria came in 1977 when Carl Woese recognised that archaea have a separate line of evolutionary descent from bacteria. [257] This new phylogenetic taxonomy depended on the sequencing of 16S ribosomal RNA and divided prokaryotes into two evolutionary domains, as part of the three-domain system. [5]
Life originated as marine single-celled prokaryotes (bacteria and archaea) and later evolved into more complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. Protists are the eukaryotes that cannot be classified as plants, fungi or animals. They are mostly single-celled and microscopic.