High-resolution reconstruction of a ribosome sheds light on evolutionary dynamics and tissue specificity.

is an important model organism for human health and disease, with foundational contributions to the understanding of gene expression and tissue patterning in animals. An invaluable tool in modern gene expression research is the presence of a high-resolution ribosome structure, though no such structure exists for Here, we present a high-resolution single-particle cryogenic electron microscopy (cryo-EM) reconstruction and molecular model of a ribosome, revealing a significantly streamlined animal ribosome. Many facets of ribosome structure are conserved in , including overall ribosomal architecture and the mechanism of cycloheximide, whereas other facets, such as expansion segments and eL28, are rapidly evolving. We identify uL5 and uL23 as two instances of tissue-specific ribosomal protein paralog expression conserved in , suggesting that ribosomes vary across tissues. The ribosome structure will provide a basis for future structural, biochemical, and genetic studies of translation in this important animal system.