The basic theme of whale evolution is the development of large animals from much smaller ancestors, and nowhere is this more evident than in the case of multi-ton sperm and gray whales, whose ultimate forebears were small, dog-sized prehistoric mammals that prowled the riverbeds of central Asia 50 million years ago. Perhaps more intriguingly, whales are also a case study in the gradual evolution of mammals from fully terrestrial to fully marine lifestyles, with corresponding adaptations (elongated bodies, webbed feet, blowholes, etc.) at various key intervals along the way.

Until the turn of the 21st century, the ultimate origin of whales was shrouded in mystery, with scarce remains of early species. That all changed with the discovery of a huge trove of fossils in central Asia (specifically, the country of Pakistan), some of which are still being analyzed and described. These fossils, which date from only 15 to 20 million years after the demise of the dinosaurs 65 million years ago, prove that the ultimate ancestors of whales were closely related to artiodactyls, the even-toed, hooved mammals represented today by pigs and sheep.

The First Whales

In most ways, Pakicetus (Greek for "Pakistan whale") was indistinguishable from other small mammals of the early Eocene epoch: about 50 pounds or so, with long, dog-like legs, a long tail, and a narrow snout. Crucially, though, the anatomy of this mammal's inner ears closely matches that of modern whales, the main "diagnostic" feature that places Pakicetus at the root of whale evolution. One of Pakicetus' closest relatives was Indohyus ("Indian pig"), an ancient artiodactyl with some intriguing marine adaptations, such as a thick, hippopotamus-like hide.

Ambulocetus, aka the "walking whale," flourished a few million years after Pakicetus and already displayed some distinctly whale-like characteristics. Whereas Pakicetus led a mostly terrestrial lifestyle, occasionally dipping into lakes or rivers to find food, Ambulocetus possessed a long, slender, otter-like body, with webbed, padded feet and a narrow, crocodile-like snout. Ambulocetus was much bigger than Pakicetus and probably spent a significant amount of time in the water.

Named after the region of Pakistan where its bones were discovered, Rodhocetus shows even more striking adaptations to an aquatic lifestyle. This prehistoric whale was genuinely amphibious, crawling up onto dry land only to forage for food and (possibly) give birth. In evolutionary terms, though, the most telling feature of Rodhocetus was the structure of its hip bones, which weren't fused to its backbone and thus provided it greater flexibility when swimming.

The Next Whales

The remains of Rodhocetus and its predecessors have been found mostly in central Asia, but the larger prehistoric whales of the late Eocene epoch (which were able to swim faster and farther) have been unearthed in more diverse locations. The deceptively named Protocetus (it wasn't really the "first whale") had a long, seal-like body, powerful legs for propelling itself through the water, and nostrils that had already begun to migrate halfway up it forehead, a development foreshadowing the blowholes of modern whales.

Protocetus shared one important characteristic with two roughly contemporary prehistoric whales, Maiacetus, and Zygorhiza. The front limbs of Zygorhiza were hinged at the elbows, a strong clue that it crawled onto land to give birth, and a specimen of Maiacetus (meaning "good mother whale") has been found with a fossilized embryo inside, positioned in the birth canal for terrestrial delivery. Clearly, the prehistoric whales of the Eocene epoch had a lot in common with modern giant tortoises!

The Giant Prehistoric Whales

By about 35 million years ago, some prehistoric whales had attained gigantic sizes, bigger even than modern blue or sperm whales. The largest genus yet known is Basilosaurus, the bones of which (discovered in the mid-19th century) were once thought to belong to a dinosaur, hence its deceptive name, meaning "king lizard." Despite its 100-ton size, Basilosaurus possessed a relatively small brain and didn't use echolocation when swimming. Even more important from an evolutionary perspective, Basilosaurus led a fully aquatic lifestyle, birthing as well as swimming and feeding in the ocean.

Contemporaries of Basilosaurus were much less fearsome, perhaps because there was only room for one giant mammalian predator in the undersea food chain. Dorudon was once thought to be a baby Basilosaurus; only later was it realized that this small whale (only about 16 feet long and half a ton) merited its own genus. And the much later Aetiocetus (which lived about 25 million years ago), though it weighed only a few tons, shows the first primitive adaptation to plankton feeding; small plates of baleen alongside its ordinary teeth.

No discussion of prehistoric whales would be complete without a mention of a fairly new genus, the aptly named Leviathan, which was announced to the world in the summer of 2010. This 50-foot-long sperm whale weighed "only" about 25 tons, but it seems to have preyed on its fellow whales along with prehistoric fish and squids, and it may have been preyed on in turn by the largest prehistoric shark of all time, the Basilosaurus-sized Megalodon.