Mikroorganizmi so si kot taksonomska skupina dokaj pozno utrli pot do lastne veje na skupnem drevesu življenja. Dolgo so na tem drevesu prevladovali predstavniki velikih organizmov, torej živali in rastlin. Danes pa moderne metode omogočajo hitro in zanesljivo pridobivanje informacij o mikroorganizmih, ki bivajo na najrazličnejših koncih našega planeta, ne da bi jih morali zato osamiti in gojiti v laboratorijskih razmerah, celo videti nam jih ni več potrebno. Z metagenomskim sekvenciranjem izbranih delov mikrobnih genomov, največkrat so to geni za ribosomske molekule, so raziskovalci v zadnjem desetletju pokazali, da na našem planetu biva neprimerno večje število bakterijskih in arhejskih vrst kot pa vrst evkariontskih večceličarjev in da velja podobno tudi za najvišje taksonomske kategorije, torej debla ali kraljestva. Univerzalno drevo življenja je torej precej drugačno, kot smo si ga slikali doslej, in v njem popolnoma prevladujejo mikroskopski, v največjem delu še (razen taksonomske informacije) popolnoma neznani organizmi. Velika bitja predstavljamo na tem drevesu le nekaj drobnih, skoraj zanemarljivih vej.

Microorganisms have managed to push their way onto the universal tree of life fairly late. For a long time the representatives of large organisms, i.e. animals and plants were dominating the trees of life. However, modern methods have made possible a fast and reliable retrieval of information concerning microorganisms, inhabiting various parts of our planet, without the need for isolation and cultivation of these organisms in in vitro conditions. Moreover, we don't even need to observe them anymore. Using the metagenomic sequencing of chosen parts of the microbial genomes, these are mostly the ribosomal genes, the researchers showed us within the last decade, that substantially more bacterial and archaeal species live on our planet as eucaryotic metazoans and similar is true at the highest taxonomic levels, i.e. phyla and kingdoms. Universal tree of life appears thus quite different from the trees that we have been seeing till now and is apparently completely dominated by microscopical, in the largest part completely unknown, apart from the pure taxonomical information, organisms. Large organisms represent only few and minor branches on such a tree.