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Why is ribosomal RNA useful for studying more ancient evolutionary relationships?

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A diagram showing the tentative evolutionary relationships among hominid species over the past four million years, with skull images and labeled species names connected by solid and dotted lines to indicate proposed evolutionary pathways. — phylogeny Tentative phylogenetic scheme for the evolution of the human lineage. Solid bars indicate the time ranges during which species are thought to have existed, based on fossil evidence. Dotted lines signify evolutionary relationships between hominin species that have been proposed on the basis of the fossil evidence.

Ribosomal RNA (rRNA), particularly the 16S rRNA gene, is a vital tool for studying ancient evolutionary relationships because it is present in all living cells and evolves at a very slow pace.12

Because the ribosome is essential for protein synthesis, its structure is highly constrained. Any significant mutation in the rRNA sequence can disrupt the assembly or function of the ribosome, often resulting in cell death. Consequently, many positions within the rRNA molecule change very slowly over time, preserving a record of evolutionary history that dates back billions of years.21

While some regions of the molecule undergo rapid changes—allowing researchers to distinguish between closely related species—the highly conserved regions provide a reliable "molecular clock" for comparing organisms across broad taxonomic levels, such as the divergence between Bacteria, Archaea, and eukaryotes. This stability makes rRNA an invaluable marker for mapping the deepest branches of the tree of life.12