Many are attracted to the unparalleled beauty of whales and dolphins. Like dancers, they all seem so at home in their ocean and river depths. But it wasn’t always that way.
Whales and dolphins, or more easily referred to as cetaceans, were not always water-dwellers. They were some of the very few creatures who evolved seemingly backwards; as most organisms evolved to leave the seas, the ancestors of cetaceans did so to return.
What are Modern Cetaceans?
As mentioned above, “cetaceans” is an all-inclusive term for whales, dolphins, and porpoises. There are two living subgroups (and one extinct, as will be explained below): odontoceti (toothed whales, dolphins, and porpoises) and mysticeti (baleen whales). All cetaceans but four species of freshwater dolphins are marine – they live in salt water.
When looking at skeletons of cetaceans, it’s clear that fins and flukes were once limbs that were better adapted to movement on land. While modern cetaceans no longer have hindlimbs, their pectoral fins are modified forelimbs. Instead of depending on sight and smell for perceiving their environments, cetaceans rely on their sense of echolocation, or a super-heightened sense of sound.
Where did Cetaceans Come From?
Though there are many arguments regarding the true ancestors of modern cetaceans, many paleontologists agree that whales and dolphins evolved from a hoofed, land-dwelling mammal. Some argue in favor of a more dog-like ancestor, but it’s generally accepted that the ancient whales evolved back into the sea.
The third subgroup of Cetaceans is called archaeoceti, which are extinct (or pseudoextinct, meaning there are no living species left, but have likely evolved into newer species). Within this subgroup is the archaic whale that fascinates paleontologists the most: Rodhocetus. Rodhocetus, a whale species caught in the transition between land- and water-dwelling, existed during the Eocene period.
Since water preserves fossils well, and since whale bones are large enough to find fairly easily, full skeletons of Rodhocetus have been uncovered. This discovery revealed a creature with associated hind- and forelimbs, though it is clearly related to modern cetaceans by the minor modifications to those limbs which would enhance water movement.
Putting Together the Cetacean Evolution Puzzle
Professor of paleontology at the University of Michigan, Philip Gingerich, has written many papers on the evolution of whales, focusing especially on the archaeoceti suborder of cetaceans. He argues that “molecular evidence from living animals is consistent with this historical perspective in recognizing a close relationship of whales and artiodactyls [hoofed land mammals].”
Rodhocetus has characteristics of a larger group of land mammals called artiodactyls, hoofed land mammals with an even number of toes. Paleontologists believe artiodactyls to have given evolutionary rise to the modern hippopotamus. And since Rodhocetus shared so many characteristics with these artiodactyls (the most important of which are in the ankle bones of the two types of skeletons), many scientists, including Gingerich, believe this to be good evidence of a link between artiodactyls, the transitional Rodhocetus, and modern cetaceans.
Even with compelling evidence, the transition from land mammal to marine mammal is still a mystery to paleontologists. Many continue to suggest either dog-like or even deer-like ancestry. Little is known about the early whales, such as whether they had fur or not (much like the dinosaur feathers debate), but with the discovery of new fossils, paleontologists may one day conclusively know how exactly the hypnotizing cetaceans came to be.
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