<P> Once regarded as plants constituting the class Schizomycetes, bacteria are now classified as prokaryotes . Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane - bound organelles . 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 . These evolutionary domains are called Bacteria and Archaea . </P> <P> The word bacteria is the plural of the New Latin bacterium, which is the latinisation of the Greek βακτήριον (bakterion), the diminutive of βακτηρία (bakteria), meaning "staff, cane", because the first ones to be discovered were rod - shaped . </P> <P> 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 . 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 . However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal / eukaryotic lineage . The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion--3.2 billion years ago . </P> <P> 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. in ancient "amitochondrial" protozoa). Later, some eukaryotes that already contained mitochondria also engulfed cyanobacteria - like organisms, leading to the formation of chloroplasts in algae and plants . This is known as secondary endosymbiosis . </P>

When did the first bacteria appear on earth