One of the most powerful lines of evidence for common descent (the theory of evolution) is often referred to as molecular phylogenetics. In brief, it’s the use of protein sequences to discern how closely two species are related. As the National Center for Biotechnology Information explains it,
Evolutionary theory states that similarity among individuals or species is attributable to common descent, or inheritance from a common ancestor. Thus, the relationships established by phylogenetic systematics often describe a species’ evolutionary history and, hence, its phylogeny, the historical relationships among lineages or organisms or their parts, such as their genes.
These relationships are often expressed in what is called a phylogenetic tree, much like a family tree, but with each individual descendant being a different species that split off from an ancestral species, just as each individual splits off, in a sense, from its parents. These trees were initially built based upon fossil evidence and comparative anatomy, long before it was possible to sequence proteins and compare them. Based on the fossil evidence and comparative anatomy, the rough outline of the phylogenetic tree of life would have mammals and birds splitting off from the reptile branch, reptiles splitting off from the amphibian branch, amphibians splitting off from the fish branch, and so forth, all the way back to bacteria.
There are lots of smaller branches within each of those groups, of course. Within mammals, for example, you could further divide it into different types of mammals –