A parsimonious reconstruction of predecessors. The phylogenetic tree is based on 16S rRNA sequence comparisons (after Fig. 2 of ). For all modern species, their species-type and evolutionary events leading most plausibly from the ancestral predecessor to the current genome content are added. The most probable species-type of predecessors for Crenarchaeota, Euryarchaeota, and Bacteria is given next to the grey circles. Abbreviations for events changing genomic content: Li1 or Lo2, (L)oss of the operon-based trpB1 or the non operon-based trpB2 gene, respectively. Ti1 (T)ranslocation of the operon based trpB1, Di1 (D)uplication of the operon-based trpB1. The colour of species names indicates the habitat: Hyperthermophiles are given in red, thermoacidophiles in orange, thermophiles in pink, mesophiles in green, halophiles in blue, and species living in a both hyperthermophilic and halophilic environment are given in purple.