an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution. it is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on earth. viruses are still one of the largest reservoirs of unexplored genetic diversity on earth. giant viruses the giant mimivirus largest known virus, tupanvirus, named after tupã, the guarani supreme god of creation viruses normally range in length from about 20 to 300 nanometers. this can be contrasted with the length of bacteria, which starts at about 400 nanometers. there are also giant viruses, often called giruses, typically about 1000 nanometers (one micron) in length. all giant viruses belongto phylum nucleocytoviricota (ncldv), together with poxviruses. the largest known of these is tupanvirus. this genus of giant virus was discovered in 2018 in the deep ocean as well as a soda lake, and can reach up to 2.3 microns in total length. the discovery and subsequent characterization of giant viruses has triggered some debate concerning their evolutionary origins. the two main hypotheses for their origin are that either they evolved from small viruses, picking up dna from host organisms, or that they evolved from very complicated organisms into the current form which is not self-sufficient for reproduction. what sort of complicated organism giant viruses might have diverged from is also a topic of debate. one proposal is that the origin point actually represents a fourth domain of life, but this has been largely discounted. prokaryotes main article: marine prokaryotes marine bacteria pelagibacter ubique, the most abundant bacteria in the ocean, plays a major role in the global carbon cycle. vibrio vulnificus, a virulent bacterium found in estuaries and along coastal areas electron micrograph showing a species of the widespread cyanobacteria synechococcus. carboxysomes appear as polyhedral dark structures.