DPANN Archaea and CPR Bacteria: insights into early cellular evolution?

Cultivation-independent techniques have facilitated the discovery of a diversity of thus far uncultivated organisms and led to the identification of new branches within the tree of life. This has reshaped the general view of key evolutionary processes and challenged fundamental understandings of cellular evolution. Two diverse and phylogenetically relevant lineages are the Diapherotrites, Parv-, Aenigma-, Nano- and Nanohaloarchaeota (DPANN) archaea and the Candidate Phyla Radiation (CPR) bacteria, originally proposed to form sister groups of all other Archaea and Bacteria, respectively. Members of the DPANN and CPR have reduced genome and cell sizes and incomplete biosynthetic pathways. In addition, the co-cultivation of the first DPANN and CPR representatives with their respective hosts indicates that they comprise many symbionts. While initial work has suggested that these two prokaryotic groups could have important implications for understanding cellular evolution and the complexity of the ancestors of the Archaea and Bacteria, respectively, more recent data have indicated that they may be derived organisms or have evolved in parallel with their partner organisms. Here, we provide insights into debates on the phylogenetic placement of these radiations and discuss implications for cellular evolution and the last universal common ancestor (LUCA) in light of Earth history.This article is part of the discussion meeting issue 'Chance and purpose in the evolution of biospheres'.