The ancestors of mammals diverged from ancient

The ancestors of mammals diverged from ancient synapsids (reptiles).

Explanation:

The mammals have known to be evolved from a group of reptiles. These group of reptiles were known as synapsids. In fact, some scientists consider mammals as the only living synapsids. The close relativity between reptiles and mammals can hence be seen because they have common ancestors. The synapsids were known to inhabit the earth during the Permian Period.

The scientists, however, do not know from which reptiles exactly did the first mammals grow.