However, we can say that the earliest known whale ancestors lived about 50 million years ago, during the Eocene epoch. These creatures, like Pakicetus and Indohyus, were small, four-legged mammals that lived in freshwater environments. They possessed some features that would later become characteristic of whales, such as a thick skull and a large tail.
Over millions of years, these ancestors gradually evolved, developing adaptations for an aquatic lifestyle. By about 40 million years ago, creatures like Ambulocetus and Rodhocetus emerged. They were semi-aquatic, spending time both on land and in water, and had developed features like webbed feet and a streamlined body.
Finally, around 34 million years ago, fully aquatic whales like Dorudon and Kutchicetus evolved. These creatures had lost their hind limbs, developed flukes (tail fins), and possessed other characteristics that allowed them to thrive in the ocean.
So, while there isn't a single "primitive whale" species that marks the beginning of whale evolution, it's safe to say that the process of whale evolution began around 50 million years ago and continued over millions of years, leading to the diverse array of whale species we see today.