Figure 2: Distribution of the two gene clusters involved in vitamin B12 biosynthesis among the Thermotogae phylum. The corrinoid synthesis gene cluster contains genes for the first part of the de novo B12 synthesis pathway and the cobinamide salvage gene cluster contains genes that synthesize vitamin B12 from cobinamides, incomplete B12 molecules. Together these two gene clusters complete the de novo B12 biosynthesis pathway. Presence of a gene cluster is denoted by (+) and absence is denoted by (−). The most parsimonious explanation for the extant presence/absence patterning for the cobinamide salvage gene cluster is one gain at the root of the phylum and three losses marked by blue and (+) and (−) and for the corrinoid synthesis gene cluster one gain marked by a red (+). This suggests the cobinamide salvage pathway was present in the ancestor of the Thermotogae phylum and the genes for complete de novo synthesis were gained in a later event by the Thermosipho lineage.