Microbes

The first form of life to develop on the Earth were microbes, and they remained the only form of life on the planet until about a billion years ago when multi-cellular organisms began to appear.[48] Microorganisms are single-celled organisms that are generally 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 the interior of rocks within the planet.[49] Their reproduction is both rapid and profuse. The combination of a high mutation rate and a horizontal gene transfer[50] ability makes them highly adaptable, and able to survive in new environments, including outer space.[51] They form an essential part of the planetary ecosystem. However some microorganisms are pathogenic and can post health risk to other organisms.