The origin and evolution of the ribosome is central to our understanding of the cellular world. Most hypotheses posit that\r\nthe ribosome originated in the peptidyl transferase center of the large ribosomal subunit. However, these proposals do not\r\nlink protein synthesis to RNA recognition and do not use a phylogenetic comparative framework to study ribosomal\r\nevolution. Here we infer evolution of the structural components of the ribosome. Phylogenetic methods widely used in\r\nmorphometrics are applied directly to RNA structures of thousands of molecules and to a census of protein structures in\r\nhundreds of genomes. We find that components of the small subunit involved in ribosomal processivity evolved earlier than\r\nthe catalytic peptidyl transferase center responsible for protein synthesis. Remarkably, subunit RNA and proteins coevolved,\r\nstarting with interactions between the oldest proteins (S12 and S17) and the oldest substructure (the ribosomal ratchet) in\r\nthe small subunit and ending with the rise of a modern multi-subunit ribosome. Ancestral ribonucleoprotein components\r\nshow similarities to in vitro evolved RNA replicase ribozymes and protein structures in extant replication machinery. Our\r\nstudy therefore provides important clues about the chicken-or-egg dilemma associated with the central dogma of\r\nmolecular biology by showing that ribosomal history is driven by the gradual structural accretion of protein and RNA\r\nstructures. Most importantly, results suggest that functionally important and conserved regions of the ribosome were\r\nrecruited and could be relics of an ancient ribonucleoprotein world
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