Whales evolved from split-hooved land mammals. Very little is known about the animals that first ventured into the water, so drawings are entirely speculative.



This is one interpretation of what a Pakicetid may have looked like. They are sometimes described as the "first whales" because they were mostly land-based but the structure of part of their inner ear is very unusual and resembles only the ear structure of modern and fossil cetaceans. (Image: Carl Buell, courtesy of the Thewissen lab)

A 1983 interpretation of what Pakicetid may have looked like. This drawing was created from just a fossil skull, says Philip Ginergirch of the University of Michigan. (Image: AAAS)

After Pakicetids came Ambulocetids. They lived in swampy regions, could both walk on land and swim in water but may not have been very fast in either environment. They are sometimes thought of as having been crocodile-like in their habits (Image: Carl Buell) Advertisement

Another artist's interpretation of what Ambulocetids may have looked like (Image: Douglas H. Chadwick, Shawn Gould and Robert Clark)

Ambulocetids were followed by Protocetids. These mostly aquatic mammals still came onto land at certain stages of their lives – evidence of which lies in their strong hind legs. (Image: AAAS)

Protocetids stayed for the most part in the ocean, but kept within hundreds of metres of the shore. Like sea lions, they probably still came ashore, but would not have wandered very far inland (Image: John Klausmeyer, University of Michigan Museums of Natural History)

In 2000, researchers discovered this remarkable fossil of a pregnant female Protocetid which hints that they gave birth on land. The fetus is shaded in blue with orange teeth. The mother is in pink (Image: University of Michigan Museum of Paleontology)

Around 35 to 40 million years ago, Protocetids evolved into Basilosaurids (represented here) and Dorudontids. Both groups of animals were fully aquatic and had small rear flippers, remainders of their ancestors' hind legs. (Image: Arthur Weasley)

This virtually complete skeleton of Dorudon atrox, a Dorudontid, was excavated in Wadi Hitan, Egypt. (Image: Philip Gingerich)