why are organisms placed into the domain of eukarya
Domain (biology) In biological taxonomy, a domain (Latin: regio[1]), also superkingdom or empire,[2] is the highest taxonomic rank of organisms in the three-domain system of taxonomy designed by Carl Woese, an American microbiologist and biophysicist. According to the Woese system, introduced in 1990, the tree of life consists of three domains: Archaea, Bacteria, and Eukarya.[1] The first two are all prokaryotic microorganisms, or single-celled organisms whose cells have no nucleus. All life that has a nucleus and membrane-bound organelles, and multicellular organisms, is included in the Eukarya. Stefan Luketa in 2012 proposed a five "dominion" system, adding two more to the above.[3]
Unicellular organism A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of more than one cell. Historically, simple unicellular organisms have been referred to as monads, though this term is also used more specifically to describe organisms of the genus Monas and similar flagellate ameboids.[1] The main groups of unicellular organisms are bacteria, archaea, protozoa, unicellular algae, and unicellular fungi. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8–4 billion years ago.[2][3]