The miacids were mammals that lived during the Eocene epoch between 55.8 and 33.9 million years ago. Some were small, weasel-like, predominantly tree-dwelling creatures, while others were more cat – or doglike and spent more time on the ground.
The miacids evolved into the first “true” carnivores, which appeared about 48 million years ago and can be divided into two groups.
One group, the Old World viverravines, subsequently gave rise to the nimravids, or “false” saber-tooth cats, and the catlike carnivores (including hyenas, civets, and mongooses). The second group, the New World miacines, eventually gave rise to the doglike carnivores, including wolves and bears.