Credit: Courtesy Williams Lab
The ribosome grew by accretion of new RNA onto old RNA in a process reminiscent of nested Russian dolls. The most ancient part of the ribosome contains small RNA fragments and is represented by the smallest doll. Ever more recent additions to the ribosome increased its functionality, and are represented on dolls of increasing size. The largest doll represents the ribosomal RNA that is shared by all current forms of life.