Everyone is familiar with the riddle: Which came first, the chicken or the egg? This sometimes leads to discussions about bird evolution. But most people don't realize this type of question is common in paleontology. For example: Which came first, the meat-eating whale ancestor or the plant-eating one?

A Tough Question

Answering that question has not been easy. Early fossil finds suggested whales might be related to the mesonychids, an extinct group of wolf sized, hoof toed carnivores. These mammals had triangular teeth similar to whales.

But when genetic analysis began, that idea was put to the test. You see, scientists comparing DNA of whales to other mammals found that whales were most closely related to plant eating ungulates. This group includes sheep, antelopes and pigs. As a matter of fact, the closest living relative to whales is the hippopotamus. So, how is it that modern whales are meat eaters?

Unraveling The Riddle

Fossil discoveries have begun to unravel the riddle of whale evolution, providing paleontologists with clues. Indohyus was a fox sized, even toed ungulate found in India. This herbivore was adapted to living in the water like whales, with a bony wall around its middle ear for hearing underwater.

When scientists added Indohyus to the collection of more than eighty living and fossil groups and analyzed their genetic relationships and potential evolutionary histories, they found Indohyus like fossils were more likely ancestors for modern whales than mesonychids.

Riddle Solved? Not yet.

Indohyus could explain the whale's relationship with hippos, but not the change from herbivore to carnivore, the next step of whale evolution. Of course, like with any good mystery, only new fossil discoveries and time will tell.

Read More: Relationships of Cetacea (Artiodactyla) among mammals: Increased taxon sampling alters interpretations of key fossils and character evolution (plosone.org) and Whales originated from aquatic artiodactyles in the Eocene epoch of India (PDF)