Pakicetus (Greek for "Pakistan whale"); pronounced PACK-ih-SEE-tuss
Shores of Pakistan and India
Early Eocene (50 million years ago)
Size and Weight:
About three feet long and 50 pounds
Small size; dog-like appearance; terrestrial lifestyle
If you happened to stumble across the small, dog-sized Pakicetus 50 million years ago, you'd never have guessed that its descendants would one day include giant sperm whales and gray whales. As far as paleontologists can tell, this was the earliest of all the prehistoric whales, a tiny, terrestrial, four-footed mammal that ventured only occasionally into the water to nab fish (We know that Pakicetus was largely landbound because its ears weren't well adapted to hearing underwater; in fact the structure of its inner ear is what gives it away as an early cetacean).
Perhaps because even trained scientists have a hard time accepting a fully terrestrial mammal as the ancestor of all whales, for a while after its discovery in 1983, Pakicetus was described as having a semi-aquatic lifestyle. (Matters weren't helped by a cover illustration on the journal Science, in which Pakicetus was depicted as a seal-like mammal diving after fish.) The discovery of a more complete skeleton in 2001 prompted a reconsideration, and today Pakicetus is deemed to have been fully terrestrial-in the words of one paleontologist, "no more amphibious than a tapir." It was only over the course of the Eocene epoch that the descendants of Pakicetus began to evolve toward a semi-aquatic, and then fully aquatic, lifestyle, complete with flippers and thick, insulating layers of fat.
One of the odd things about Pakicetus-which you can infer from its name-is that its "type fossil" was discovered in Pakistan, not normally a hotbed of paleontology. In fact, thanks to the vagaries of the fossilization process, most of what we know about early whale evolution derives from animals discovered on or near the Indian subcontinent; other examples include Ambulocetus (aka the "walking whale") and Indohyus.