Similar structures that related species inherited from a common ancestor?

Phylogenetics, or the study of the relationships between organisms, is now almost entirely done based on genetic sequences. This requires, of course, a gene common to all organisms, and one of the first universal trees of life derived from genetic information used the small subunit ribosomal RNA (ssrRNA) to show that the last universal common ancestor (LUCA) existed somewhere at the origin of bacteria and archea, with the eukaryotes (which includes all animals) branching off of the archea later in time.

Apart from phylogenetics, however, the universality of the genetic code, the prevalence of L-stereoisomers of amino acids and the ubiquity of adenosine in biochemical processes all suggest either a common ancestor or a group of very similar ancestors as the origin to all modern life.