What is the earliest known species of cat to evolve?

It is thought that the oldest "true" cat was Proailurus. It lived in what is now Europe and Asia, around 25 million years ago. It was a small predator, slightly larger than a modern domestic cat.

Proailurus is thought to be to be a likely ancestor to Pseudaelurus, prehistoric cat that lived 20-8 million years ago. Pseudaelurus is the ancestor of today's cats, as well as the famous sabre-tooth cats.