Nature

The first form of life to develop on the Earth were microbes, and they remained the only form of life until about a billion years ago when multi-cellular organisms began to appear. Microorganisms are single-celled organisms that are generally microscopic, and smaller than ... the human eye can see. They include Bacteria, Fungi, Archaea and Protista. These life forms are found in almost every location on the Earth where there is liquid water, including in the Earth's interior... — en.wikipedia.org