Finally freed from dinosaur repression, we synapsid mammals evolved
rapidly in many directions - and will remain in charge at least until the
next disaster (natural or manufactured).
Fish in shallow waters learned to breath air as a matter of survival. A variety of lobe finned fish took a liking to land (plenty of big bugs to eat and no sharks) and evolved into a "Tetrapod Amphibian".
The Tetrapods branched into several lines, and the lineage to which we and
all other mammals belong is the Synapsids. While the famous Permian
Synapsid Dimetrodon used a large fin for temperature regulation, later
Synapsids took cooling and heating internal, becoming "warm blooded" and
developing fur coats. This differs from the dinosaurs who also became warm
blooded but developed feathers instead.
This chart shows descendents of the Tetrapods (for simplicity extinct (thus inedible) and minor lines and some intermediate phases have been omitted).