For a more in depth article see here: http://www.eurekalert.org/pub_releases/2006-05/uof-haw052206.php
Orca (photo: Stefan Jacobs)
“More than 50 million years ago the ancestors of whales and dolphins were four-footed land animals, not unlike large dogs. They became the sleek swimmers we recognize today during the next 15 million years, losing their hind limbs in a dramatic example of evolutionary change.
Fossil evidence shows that whales lived on land – sharing a common ancestor with hippos, deer and camels. Their transition to an aquatic lifestyle occurred long before they eliminated their hind limbs. During the transition, their limbs became smaller, but they kept the same number and arrangement of hind limb bones as their terrestrial ancestors.
The actual loss of the hind limb occurred much further along in the evolutionary process, when a drastic change occurred to inactivate a gene, Sonic hedgehog, essential for limb development.
Researchers say the findings tend to support traditional evolutionary theory, a la Charles Darwin, that says minor changes over vast expanses of time add up to big changes. And while Sonic hedgehog’s role in the evolution of hind limbs in ancient whales is becoming apparent, it is still not fully defined.”
Sonic hedgehog (image: whozoo.org)