<P> 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 . 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 . </P>
<P> 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 . 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 . </P>
microscopic organism that could be prokaryotic or eukaryotic in nature
microscopic organism that could be prokaryotic or eukaryotic in nature
unicellular organism
unicellular organism