English: "a, The syntrophic/fermentative host archaeon is suggested to degrade amino acids to short-chain fatty acids and H2, possibly by interacting with H2-scavenging (and indirectly O2-scavenging) SRB (orange; see Supplementary Note 6). b, The host may have further interacted with a facultatively aerobic organotrophic partner that could scavenge toxic O2 (the future mitochondrion; red). Continued interaction with SRB could have been beneficial but not necessarily essential; dotted arrows indicate the interaction; see Supplementary Note 7. c, Host external structures could have interacted (for example, mechanical or biological fusion with the aerobic partner to enhance physical interaction and further engulf the partner for simultaneous development of endosymbiosis and a primitive nucleoid-bounding membrane. d, After engulfment, the host and symbiont could have continued the interaction shown in b as a primitive type of endosymbiosis. e, Development of ADP/ATP carrier (AAC) by the endosymbiont (initial direction of ATP transport remains unclear; see Supplementary Note 9). f, Endogenization of partner symbiosis by the host through delegation of catabolism and ATP generation to the endosymbiont and establishment of a symbiont-to-host ATP channel."