If you look below the fold, you can see two molecules locked in a tight embrace. These molecules or their closely related cousins can be found in any cell because their ability to evolve is slowed by their need to interact with each other in the right way.
In an earlier post, I asked:
Who are they?
One partner is a small bit of 16S ribosomal RNA, about 56 nucleotides to be precise. The other partner is S15, one the proteins in the ribosome.
If we could look inside the bacteria that made these, we would see lots of other proteins binding to these two partners within a molecular machine. Ribosomes and ribosomal RNA are essential for making new proteins and all living things must have them. They do vary a bit in structure between three kingdoms of life; procaryotes, archeabacteria, and eucaryotes; but they can still be recognized by their sequences.