Their body is made up of just one single cell. E.g. - Amoeba, Paramecium. The oldest forms of life are the unicellular organisms and they existed 3.8 billion years ago. Bacteria, Amoeba, protozia, viruses, diatoms are the five different types of unicellular organisms.This single cell regulates the various functions of the body.Unicellular organisms are invisible to the naked eye.heir body is made up of just one single cell. E.g. - Amoeba, Paramecium. The oldest forms of life are the unicellular organisms and they existed 3.8 billion years ago. Bacteria, Amoeba, protozia, viruses, diatoms are the five different types of unicellular organisms.