The origin of mammals.

Despite a very popular theory, mammals are not descended from reptiles. Their ancestors were medium-sized animals called synapsids. Synapsids have existed since the days of the dinosaurs, in appearance they resembled Komodo monitor lizards. However, they had a woolen or hair cover, teeth of different functions and structure (in lizards, all teeth are uniform), differences in the circulatory and respiratory systems. Over time, they became warm-blooded and milk-secreting glands appeared on their skin.