Single-celled organisms are only composed of single cells, which often aggregate into cell colonies. The earliest living things on the earth were formed about 3.5 billion years ago to 4 1 100 million years ago. Prokaryotes are the most primitive creatures, such as bacteria and cyanobacteria, which appear in warm water. Unicellular organisms include all archaea, eubacteria and many protozoa. According to the old classification, there are many animals, plants and fungi, most of which are multicellular organisms. Amoebas are counted as unicellular animals, but some of its species are counted as myxomycetes. Flagellated flagellates, such as euglena, are sometimes classified as unicellular algae or unicellular animals.

Single-celled organisms are mainly divided into nucleated and seedless single cells. Nucleated organisms such as paramecium are typical nucleated single-celled organisms. Nucleated unicellular organisms mainly include nucleus, cytoplasm and organelles.