The root of the tree

The node gathering all species is called the root of the tree, and is normally used to give a biological and evolutionary interpretation of the tree beyond a pure relative comparison among the species. This node, when present, corresponds to the last common ancestor of all species considered in the tree. It tells you what are the earliest diverging nodes and therefore the “direction” of the evolution.

In the example below, you have the exact same tree rooted (right) and unrooted (left). In this example the root is splitting the first diverging clade (gathering “a” and “b”) and the rest of the taxa.