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 the simple single celled organisms have sometimes been referred to as Monads. 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.