The Land-Living Ancestors of Whales
Evolution of primitive cetaceans
∘ Cetaceans are mammals
∘ The primitive whales
∘ The origin of modern cetaceans
∘ Searching for more ancestors and modern relatives of whales
Cetaceans are mammals
Whales, dolphins, and porpoises are aquatic mammals classified inside of the infraorder Cetacea.
They are characterized by having a fusiform body which makes them more hydrodynamic, they tend to be large, and they are exclusively carnivorous [1].
There are over 86 known living cetacean species (grouped into 42 genera and 14 families), that can be found in some rivers and in all the world’s oceans [2, 3].
The group of cetaceans includes the largest known animal species in the history of the planet, the blue whale (Balaenoptera musculus, averaging 27 m in length, 136,000 kg), but it also has some very small representatives, such as the vaquita porpoise (Phocoena sinus, averaging 1.4 m in length, 42 kg).
Modern cetaceans are so adapted to aquatic life that some people mistakenly associate them with fish, but they are a phylogenetically distant and more recent group.
The primitive whales
The oldest known cetaceans are the pakicetids (extinct animals of the family Pakicetidae), which lived in the floodplains of present-day South Asia during the Eocene (middle Paleogene), 50 million years ago [4].
Pakicetids didn’t look similar to modern cetaceans. Indeed, they were externally more similar to a wolf with a long nose and they were no more amphibious than a tapir [5].
Pakicetids were identified as primitive cetaceans due to the presence of some characteristic features of their inner ear that only occur in modern whales [5, 6].
Pakicetids are classified along with other extinct primitive cetaceans (such as ambulocetids, remingtonocetids, protocetids, and basilosaurids), within the parvorder Archaeoceti.
Archaeoceti species show clear transitional morphological forms between terrestrial and aquatic animals and represent the earliest known cetacean radiation that occurs in the Paleogene, from the Eocene to the Oligocene (50 to 23 million years ago) [7].
These transitional changes turn the evolution of cetaceans into one of the most representative and well-documented examples of macroevolution.
The origin of modern cetaceans
Modern cetaceans originated 30 million years ago in the Oligocene. During this period, several glaciations extinguished the archaeocetes that had lived in the warm climates of the Eocene [8].
The changing conditions of the planet in the Oligocene made the polar and subpolar regions very productive and, in turn, caused many marine animals to evolve to feed on plankton [8].
During this time there was a drastic divergence in the feeding strategies that originated modern groups of cetaceans, the Odontocetes (Toothed whales,) and Mysticetes (Baleen whales) [8].
Filter feeding was so beneficial as it allows baleen whales to efficiently obtain large energy resources and made it possible to achieve a larger body size [9].
On the other hand, toothed whales developed echolocation. A characteristic that allowed them to better adapt to the changing environment and give them ecological advantages over archaeocete cetaceans.
Echolocation made it easier for toothed whales to hunt for prey and allow them to dive deeper (where there is little light) and exploit new food sources [10].
Searching for more ancestors and modern relatives of whales
Fossils are important because they document evolution that occurred over thousands of years in the past.
In Darwin’s time (around 1859), it was already expected that it would be possible to find transitional forms between terrestrial and aquatic animals in primitive cetaceans.
It was until 1983 (more than a century later), that pakicetids were found and it is quite certain that in the future, we will still find more transitional forms in the fossil record.
The evolution of cetaceans is far from fully understood. There are still unresolved questions about the evolution of whales [1].
Why did cetaceans enter the oceans?
The availability of rich new food sources has been proposed as a reason for the cetacean entry into the water, but this is unlikely, given that primitive cetaceans already lived in very shallow freshwater [1].
This in turn leads to another important question.
Which group of mammals are cetaceans descended from?
Genetics gives us some clues to solve this mystery.
Molecular and morphological evidence, recently suggested that raoellids (extinct family Raoellidae) are an evolutionary nexus between primitive cetaceans and other modern groups of mammals [5, 11, 12, 13].
The raoellids of the genus Indohyus appear to be one of the oldest non-cetacean relatives of whales [11].
These raccoon-sized creatures spent much of their time in the water, but they had a diet that was nothing like that of any primitive cetacean. It is suggested that the incentive for these animals to enter the water may have been a change in their diet and to hide from predators [11].
This could have been one of the first steps in the evolution of whales toward the aquatic environment.
These contemporary phylogenetic studies also link cetaceans with hippos (family Hippopotamidae), suggesting that they are the closest modern relatives and share a common ancestor that lived about 55 million years ago [12, 13].
This is an example of how different disciplines (Paleobiology and Genetics) can complement each other and help us understand different parts of the same history.
I wonder if Darwin imagined hippos and whales to be relatives.
Since hippos have an amphibian lifestyle, it doesn’t sound crazy, but it doesn’t sound obvious either. I actually didn’t know it at the moment to write this story.
I was surprised that whales are not related to other marine mammals like pinnipeds (seals) or sirenians (manatees). This is a great example of the convergent evolution (independent evolution of similar characteristics in unrelated species).
As we are able to understand the evolutionary history of whales, we will not only learn about the past, but we will also be able to learn more about how they would respond to current patterns of climate change and find better ways to improve their conservation.
You can find more information about the evolution of cetaceans at the following links: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
Let me know your opinion in the comments.
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