The earliest life on land may have been bacteria some 3.22 billion years ago. The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion–3.2 billion years ago. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.
Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life. The ancestors of bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. Phylogenetic tree of Bacteria, Archaea and Eucarya, with the last universal common ancestor at the root. These evolutionary domains are called Bacteria and Archaea. Although the term bacteria traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Once regarded as plants constituting the class Schizomycetes ("fission fungi"), bacteria are now classified as prokaryotes. Bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector, as well as in biotechnology, and the manufacture of antibiotics and other chemicals. Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. The most common fatal bacterial diseases are respiratory infections. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are beneficial, particularly the ones in the gut. Most are in the gut, and there are many on the skin. Humans and most other animals carry vast numbers (approximately 10 13 to 10 14) of bacteria. The study of bacteria is known as bacteriology, a branch of microbiology. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. Bacteria also live in symbiotic and parasitic relationships with plants and animals. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. The nutrient cycle includes the decomposition of dead bodies bacteria are responsible for the putrefaction stage in this process. Bacteria play a vital role in many stages of the nutrient cycle by recycling nutrients and the fixation of nitrogen from the atmosphere. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. They constitute a large domain of prokaryotic microorganisms.
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