As far as we can tell the fin-to-limb transition, the movement of fish onto land, took place just once. Movement of land animals back to water, on the other had, took place too many times to count, especially if we include early transitional forms like the polar bear.

The final OLLI class on evolutionary transitions focused on cetaceans (whales and dolphins) but we started out with a brief survey of land animals that have gone back to sea—obviously a far easier transition than the movement of fish onto land.

Some of the animals that went back to the oceans are extinct today—the plesiosaurs, icthyosaurs, and  mosasaurs of the Mesozoic. Other reptiles that went back to the ocean are alive today: sea turtles and sea snakes, for instance. If birds are counted as reptiles, penguins also have to be included in this group.

Mammals have returned to the sea several times, and the sirenians, the pinnipeds and the cetacians are the most obvious as they have adapted to water so much that life on land would be difficult. Others that are in the process of adapting include the sea otters and polar bears.

Interestingly, the process of adaptation has generally involved several stages, none of which include the ability to breathe water. Even the deepest-diving of whales, though they can hold their breaths for up to an hour, must come to the surface to breathe.

The first stage of adaptation to the water is simply streamlining and swimming ability. In cold water, insulation in the form of waterproof fur and/or a thick fat layer is also necessary. The polar bear is at this stage. Breeding takes place on sea ice, and the cubs are born on land.

The sea otter can hold its breath a little longer (up to five minutes) and breed and pup in water Its rear legs have been modified for swimming to such an extent that the sea otter finds locomotion on land difficult, though not impossible.

The pinnipeds (seals, sea lions and walrus) are even more clumsy on land—their limbs have been greatly modified into flippers, and their lungs are highly adapted for long periods of breath-holding and recoverable collapse under water pressure. However, they must leave the water to breed and pup, and the pups are not born knowing how to swim.

The sirenians (manatees and dugongs) are unusual in being herbivores. They have lost their hind legs, but their tails have been modified into paddles. The surviving species require shallow, warm water and are totally aquatic. They are also highly endangered due to habitat loss and coastal development. The recently extinct Stellar’s sea cow, however, must have been able to tolerate cold water.

The cetaceans are all carnivores. Some (baleen whales) are filter feeders, consuming large amounts of very small prey. Others (toothed whales, including dolphins and narwhals) hunt larger prey.

Surprisingly, their closest modern relatives are artiodactyls—a group that today includes cows, sheep, moose and hippos. Most of these modern animals are herbivores or in some cases (pigs) omnivores. So the transition from land animals to whales involved a number of changes: herbivory to carnivory; land to water, and fresh water to salt water.

Transitional fossils are still being found, though the most fruitful sites (Pakistan and Afghanistan) are not very hospitable for fossil-hunters these days. It does seem that the transition to water was via wading, then swimming and finally to estuarine systems and the deep ocean. The first (Eocene) episode of the DVD, Walking With Prehistoric Beasts, shows an early transitional form called Ambulocetus (walking whale.) I’m not sure about how well it could hold its breath, but the reconstruction is interesting.

Whales were fully adapted to marine conditions by the late Eocene. Again, Walking with Prehistoric Beasts does a good job of fleshing out Basilosaurus and Dorudon, early forms of whales. Basilosaurus in particular was a far more efficient carnivore than its surviving kin, which are more likely to be descendants of some cousin of Dorudon.