There are approximately 39 trillion bacterial cells in the human microbiota as personified by a "reference" 70 kg male 170 cm tall, whereas there are 30 trillion human cells in the body.
The study of bacteria is known as bacteriology, a branch of microbiology.
There are typically 40 million bacterial cells in a gram of soil and a million bacterial cells in a millilitre of fresh water.
The ancestors of modern bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago.
For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.
Bacteria were among the first life forms to appear on Earth, and are present in most of its habitats.
Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep portions of Earth's crust.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.In March 2013, data reported by researchers in October 2012, was published.Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes.Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea.This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya (sometimes in highly reduced form, e.g. Later, some eukaryotes that already contained mitochondria also engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants.