Here's what we know:
* Early Synapsids: Mammals evolved from a group of reptiles called synapsids, which first appeared around 320 million years ago. Synapsids had features that were transitional between reptiles and mammals, such as a single hole in the skull behind each eye (the "synapsid" characteristic).
* Therapsids: Within the synapsid group, a suborder called therapsids appeared around 280 million years ago. Therapsids were more mammal-like, with features such as a more upright stance, larger brains, and more specialized teeth.
* Cynodonts: A group of therapsids called cynodonts emerged around 250 million years ago. Cynodonts had even more mammal-like features, including a secondary palate (separating the nasal passages from the mouth), a more efficient respiratory system, and fur.
* The First "True" Mammals: By the end of the Triassic period (around 200 million years ago), the first "true" mammals had evolved. These early mammals were small, nocturnal, and likely insectivores.
Key Point: The evolution of mammals was a long and complex process, and there isn't a clear line separating "non-mammals" from "mammals". Instead, we see a gradual transition over millions of years with various species possessing more and more mammal-like characteristics.