The three domains of cellular life (simplified schemes)

(A) The unrooted three-domain tree derived by phylogenetic analysis of 16S rRNA and other information-processing genes. (B) The rooted three-domain tree. (C) The network of life. A: archaea; B: bacteria; E: eukaryote; LUCA: last universal common ancestor of cellular life-forms. The question mark indicates the uncertainty with respect to the biological features of the LUCA. The arrows in (C) denote horizontal gene transfer (HGT), and the gray shading indicates the possibility of rampant gene transfer at early stages of evolution, so distinct lineages of descent might not have been discernible. The curved arrows show HGT within each of the three domains of cellular life (limited in eukaryotes as shown by the dashed line).