Microbes - bacteria, archaea, fungi, algae, protozoa and viruses - have been around for at least 3,500 million years and were the only life forms on Earth for most of that time. As the Earth cooled, liquid water formed and the first microbial life appeared. The conditions on Earth in the beginning were very hostile so the first microbes probably resembled the archaea, as they were able to live in extreme environments such as the high temperature found on the cooling planet.
Around 2,800 million years ago, cyanobacteria, the largest and most diverse group of photosynthetic bacteria, probably appeared. This was an important development as these were the first organisms able to carry out aerobic photosynthesis. It is thought that cyanobacteria were responsible for raising the level of oxygen in the Earth’s atmosphere from less than 1 % to the 21 % of today. The presence of oxygen in the atmosphere allowed the evolution of new aerobic (oxygen using) species of microbes, which began to colonise every habitat on the planet.
Different species of cyanobacteria formed complex microbial communities with other types of microbes as they evolved, and these communities have left an extensive fossil record. They are fossilised in structures called stromatolites, dome-shaped mounds formed by the merger of mineral sediments into microbial mats.
Mammals and flowering plants are relative newcomers and only appeared around 100 million years ago.
Microbes affect every aspect of life on Earth. They have an amazing variety of shapes and sizes and can exist in a wide range of habitats from hot springs to the icy wastes of Antarctica and inside the bodies of animals and plants.