The history of carnivores can be traced back to protozoa hundreds of millions of years ago, when amoebas fed on other single-celled organisms. Throughout subsequent life history, carnivores have continuously evolved alongside the rest of the biological world. At every stage of life history, carnivores have been an indispensable part of the Earth's ecosystem.
Hyena
By the Late Cretaceous, although massive, ferocious carnivorous dinosaurs like Tyrannosaurus Rex still dominated terrestrial ecosystems, a small, mouse-like eutherian mammal called Trichodontidae had quietly diverged from primitive insectivores and joined the ranks of carnivores in Asia. By the Early Paleocene, these animals, classified as Paleocarnivora, had developed into many different species. However, these Paleocarnivores remained quite primitive in many ways, primarily in the absence of carnassial teeth specifically designed for tearing meat. It wasn't until the Eocene that some Paleocarnivores began to develop carnassial teeth, consisting of the first upper molar and second lower molar, or the second upper molar and third lower molar.
As ancient carnivores continued to evolve, a more advanced type diverged from a Paleocene species—the ancient cat-like mammals—whose carnassial teeth consisted of the fourth upper premolar and the first lower molar. From this point, a new type of carnivore—the Carnivora mammals—emerged. This forward-shifted carnassial tooth structure made tearing meat more efficient, and coupled with a series of other advanced physical structures, Carnivora mammals began to develop. After most ancient carnivores became extinct at the end of the Eocene, they completely replaced the ancient carnivores from the Oligocene onwards. Only the Hyaenodontidae lineage of ancient carnivores survived into the early Pliocene.
