Hydrobacteria and Terrabacteria were inferred to have diverged approximately 3 billion years ago, suggesting that land (continents) had been colonized by prokaryotes at that time.

[1] Together, Hydrobacteria and Terrabacteria form a large group containing 97% of prokaryotes and 99% of all species of Bacteria known by 2009, and placed by Battistuzzi and Hedges in the proposed taxon Selabacteria, in allusion to their phototrophic abilities (selas = light).

[4][5] The definition of two major divisions within the domain Bacteria, Hydrobacteria and Terrabacteria, has come largely from rooted phylogenetic analyses of genomes.

The following tree is redrawn from one of those two recent studies,[4] showing the phylogeny of bacterial phyla and superphyla, with the position of Fusobacteria being unresolved and DST being the closest relative of Terrabacteria: "Gracilicutes," which was described in 1978 by Gibbons and Murray,[9] is sometimes used in place of Hydrobacteria.

Also, it did not follow the three-domain system, claiming instead that the lineage of eukaryotes + Archaea is nested within Bacteria as a close relative of Actinomycetota, a tree not supported in any molecular phylogeny.