Since the 1970's it was thought that whales had descended from Eocene carnivores that looked doglike. However, "DNA hybridization and other tests sujested that whales had descended from artiodactuls (that is, even-toed herbivores, such as antelopes and hippos…" (National Geographic)
In 2000, at an archeological site in Pakistan, it was confirmed that whales and antelopes were closely related when "an anklebone" was found "from a four-legged whale dating back 47 million years, that closely resembled the homogous anklebone in an artictyl." (National Geographic)
At appx. 40 million years ago, Basilosaurus became one of the first mammals to become "fully adapted to an aquatic enviroment [it] was swimming the ancient seas, propelled by its sturdy flippers and [a] long. flexible body" (http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html)
A former student of Gengerich's, Hans Thewissen, found a slightly formed of webbed feet, legs suitable for either walking or swimming with webbed feet and a long toothy snout. Thewissen called it Ambulocetus natans , or the "walking and swimming whale".(National Geographic)
Gengerich's and his team turned up several more including, hocetus balochistanensis, which was a sea creature , its legs more like flippers, its nostrils shifted backward on the snout, halfway to the blowhole position on a mordern whale.
In 1978, paleontologist Phil Gingerich discovered a 52-million-year-old skull in Pakistan that resembled fossils of creodonts — wolf-sized carnivores that lived between 60 and 37 million years ago, in the early Eocene epoch. But the skull also had characteristics in common with the Archaeocetes, the oldest known whales. The new bones, dubbed Pakicetus, proved to have key features that were transitional between terrestrial mammals and the earliest true whales. (http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html)
