“…are characterized by the presence of so-called expansion segments (ES), which are additional RNA elements of various sizes incorporated into the universal prokaryotic rRNA core (Gerbi, 1996;Bowman et al, 2020; Figure 1). These ES increase the size and complexity of the respective rRNAs; however, recent analyses have provided evidence for the presence of such ES in both bacteria and archaea (Armache et al, 2013;Penev et al, 2020;Tirumalai et al, 2020;Stepanov and Fox, 2021). Although most of these sequence additions are limited in size and number (Armache et al, 2013;Penev et al, 2020;Tirumalai et al, 2020;Stepanov and Fox, 2021), larger ES, similar in size to those commonly observed in eukaryotes, have been recently described in the Asgard archaeal phylum (Penev et al, 2020), which is proposed to be the cradle of the eukaryotic lineage (Spang et al, 2015;Zaremba-Niedzwiedzka et al, 2017;.…”