The discipline that looks at how
organisms are
related to each other. Reconstruction, basically, of the 'Tree of Life' - where each current creature is a leaf (including us, there is no ladder here) and past creature (trilobites, dinosaurs, etc) are branches.
Increasingly molecular nowadays, with PCR and other sophisticated techniques. Study of evolution at the protein and gene level can give quite a different picture to that given by large scale features.