SSU rRNA

Small subunit ribosomal ribonucleic acid (SSU rRNA) is the smallest of the two major RNA components of the ribosome. Associated with a number of ribosomal proteins, the SSU rRNA forms the small subunit of the ribosome.

SSU rRNA sequences are widely used for working out evolutionary relationships among organisms, since they are of ancient origin and are found in all known forms of life.

...
Wikipedia