page_title: Last universal common ancestor
section_title: Location of the root
hierarchical_section_title: Last universal common ancestor / Location of the root
caption_attribution_description: English: reductive acetyl-CoA pathway in bacteria Deutsch: reduktiver Acetyl-CoA-Weg (Wood-Llungdahl-Weg) in Bakterien
context_page_description: The last universal common ancestor or last universal cellular ancestor, also called the last universal ancestor, or concestor, is the most recent population of organisms from which all organisms now living on Earth have a common descent, the most recent common ancestor of all current life on Earth. LUCA is not thought to be the first life on Earth, but rather the only type of organism of its time to still have living descendants.
  While there is no specific fossil evidence of LUCA, it can be studied by comparing the genomes of all modern organisms, its descendants. By these means, a 2016 study identified a set of 355 genes most likely to have been present in LUCA. The genes describe a complex life form with many co-adapted features, including transcription and translation mechanisms to convert information from DNA to RNA to proteins. The study concluded that the LUCA probably lived in the high-temperature water of deep sea vents near ocean-floor magma flows.
  Studies from 2000–2018 have suggested an increasingly ancient time for LUCA.
context_section_description: The most commonly accepted tree of life, based on several molecular studies, has its root between a monophyletic domain Bacteria and a clade formed by Archaea and Eukaryota. However, a very small minority of studies place the root in the domain Bacteria, either in the phylum Firmicutes or state that the phylum Chloroflexi is basal to a clade with Archaea and Eukaryotes and the rest of Bacteria (as proposed by Thomas Cavalier-Smith).
  Research by William F. Martin (2016) genetically analyzed 6.1 million protein-coding genes and 286,514 protein clusters from sequenced prokaryotic genomes of various phylogenetic trees, and identified 355 protein clusters that were probably common to the LUCA.  The results "depict LUCA as anaerobic, CO₂-fixing, H₂-dependent with a Wood–Ljungdahl pathway (the reductive acetyl-coenzyme A pathway), N₂-fixing and thermophilic. LUCA's biochemistry was replete with FeS clusters and radical reaction mechanisms." The cofactors also reveal "dependence upon transition metals, flavins, S-adenosyl methionine, coenzyme A, ferredoxin, molybdopterin, corrins and selenium. Its genetic code required nucleoside modifications and S-adenosylmethionine-dependent methylations." The results are "quite specific": they show that methanogenic clostridia was a basal clade in the 355 lineages examined, and that the LUCA may therefore have inhabited an anaerobic hydrothermal vent setting in a geochemically active environment rich in H₂, CO₂, and iron.
  These findings could mean that life on Earth originated in such hydrothermal vents, but it is also possible that life was restricted to such locations at some later time, perhaps by the Late Heavy Bombardment. The identification of these genes as being present in LUCA has also been criticized, as they may simply represent later genes which migrated via horizontal gene transfers between archaea and bacteria.
caption: The LUCA used the Wood–Ljungdahl or reductive acetyl–CoA pathway to fix carbon.
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