It may come as a surprise to many that whales and dolphins were once terrestrial mammals not too different from the modern-day wolf.

The ancestors of whales and dolphins were four-footed land animals that fished in shallow seas, and lost their hind limbs in one of the most dramatic examples of evolutionary change. The Ontario Science Center is currently running the Whales / Tohorā exhibition that has an impressive display of fossilized whale remains, which depict their gradual metamorphosis from shallow-shore to deep-sea.

By looking at numerous ancestral fossils, paleontologists have identified that whales lived on land and shared a common ancestor with hippos, camels and deer. However, the transition of whales into the sea occurred long before they lost their hind limbs. Their limbs gradually shrank over 15 million years due to a genetic change that affected limb size during fetal development.

Scientists believe that the first whale, Pakicetus lived on land with a cetacean-like ear structure and a fish-based diet. After Pakicetus came Ambulocetus, that lived in swampy regions, resembled the modern crocodile and could walk on land and swim in water. Ambulocetus were followed by Protocetids, who were mostly aquatic mammals that still came onto land at certain stages of their lives. Around 35 to 40 million years ago, Protocetids evolved into Basilosaurids and Dorudontids; both groups of animals were fully aquatic and had small rear flippers, which were remainders of their ancestors’ hind legs.

The complete loss of hind limbs occurred much further along in the evolutionary process, when a drastic change led to the inactivation of a gene (called the Sonic Hedgehog) essential for limb development during fetal period.

Traces of vestigial organs can sometimes still be seen today in the form of resurfaced limb rudiments in whales and other animals with a history of limbs, but not in sharks who were always aquatic. While the word “evolution” may sound Lamarckian or like a progressive phenomenon, why a mammal would leave land to go back into its ancestral sea seems rather regressive — this is because the Darwinian theory of evolution by natural selection has no order.

These magnificent mammals have come a long way and deserve their place in our vast oceans. Unfortunately, whales have almost been hunted to extinction by humans in past centuries and many whale species have yet to recover. In 1986, the International Whaling Commission adopted a moratorium on commercial whaling. However, Japan continues to hunt endangered species such as Humpbacks, Blue and Minke whales for alleged ‘scientific’ purposes.

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Image Source: Image 1, Image 2