Whales did not just appear from the depths of the ocean. They have an intriguing evolutionary story. This story shines a light on what makes them so unique and well-adapted for their watery world. In this post, we explain the evolution of whales, from tiny ungulates to the killer whale.
It All Began on Land…
Around 50 million years ago, the Age of Dinosaurs had long since ended. Instead, large mammals began to dominate the environment. This is the Cenozoic era or the Age of Mammals.
During this time, the ancestral relative to whales could be found on land! A cat-sized mammal with four legs, a tail, and a long snout could be seen paddling through shallow waters, hunting for invertebrates, and foraging for plants. Through fossil records, scientists identified this mammal, named a raoellid, as an even-toed ungulate. Even-toed ungulates are animals with hoofed feet, which bear weight equally on two of five toes. More familiar mammals in this group include pigs, hippos, and deer. These even-toed ungulates are the closest living land relative to whales today.
Pakicetus, the first cetacean
After raoellidae, the story of whales takes several chapters to transition from a semi-aquatic lifestyle to fully aquatic. The first cetacean is believed to be Pakicetus, a little mammal measuring 1 to 2 meters long. It had lungs, as well as four limbs. Scientists believe the Pakicetus ate fish and other small animals. They also believe it was a semi-aquatic animal, spending some time on land, and some time in the water. It was identified as the first cetacean due to the structure of its inner ear, that resemble those of modern whales.
The next chapters include Ambulocetidae, Remingtonocetidae, Protocetidae, and Basilosauridae. The fluke is an important aquatic adaptation that aids in propulsion and locomotion. It appeared in the Protocetidae chapter. In addition to the appearance of a fluke, there is a change in the general body shape. Over time, the body becomes more oval-shaped or streamlined. At the same time, the pelvis, or hips, begins to shift the shrinking rear legs into a backward direction. These adaptations create a more hydrodynamic mammal in the water.
Introducing the King Lizard
The 17 meters (56 feet) long, snake-like Basilosaurus (or “king lizard”) became widespread in the world’s oceans. The streamlined body, elongated snout, pectoral fins, and fluke made the Basilosaurus and efficient swimming mammal. The rear legs have continued to reduce in size, and are no longer used for locomotion. Lastly, a distinct whale adaptation is a blowhole (or nostril) located at the top of the head. This adaptation allows for quicker and easier breathing at the surface than nostrils located at the tip of the snout. The Basilosaurus begins this transition with nostrils located mid-snout.
In Current Times…
Today, whales belong to a group called cetaceans. They all have slick, streamlined bodies, pectoral fins instead of front legs, powerful flukes, a blowhole located at the top of their heads, and no rear legs. You cannot really see the family resemblance between them and their ancestors. But a piece of their evolutionary story remains; and it is floating inside their bodies! The next time you see a whale skeleton, look for mysterious “free-floating” bones. These levitating bones are evidence that whales once had hips and walked on land! Scientists call them vestigial (or left-over) structures. An example of a vestigial structure in humans is wisdom teeth.
The Cetacean Division
Cetaceans are categorized into two types – odontocetes and mysticetes. Whether a whale is an odontocete or mysticete depends on their body structures and adaptations. Odontocetes (or toothed-whales) have teeth. Common odontocetes include sperm whales, pilot whales, and orcas. Odontocetes can also be identified by their singular blowhole and their use of echolocation to hunt and communicate. Odontocetes are also generally smaller than mysticetes.
Around 30 million years ago, mysticetes split off from odontocetes. Instead of teeth, mysticetes developed baleen plates. Baleens are long strips of keratin, the same material as our fingernails, that form a curtain hanging from the upper jaw. To feed, mysticetes suck water into their mouths, then push the water back out through the baleen. The baleen act as a filter, keeping their food in their mouth for subsequent ingestion. It’s just like using a colander to strain cooked spaghetti from pasta water!
Mysticetes (or baleen whales) are identified by having two blowhole openings and a lack of echolocation. Mysticetes are also generally larger than odontocetes.
You can click here to read more about blowholes.
From a little ungulate to 90 species!
Each cetacean species is unique and fulfills an important place in the ocean ecosystem, all 90 of them! By learning how they evolved and how their adaptations help them survive, we can better understand and protect these amazing animals in the future.
For further reading:
Fact to remember: Are whales dolphins or are dolphins whales? And I thought orcas were dolphin, but they’re also called killer whales? I’m so confused.
This is a common confusion. Dolphins are cetaceans that belong to the Delphinidae family under the odontocete umbrella (because they have teeth, as seen above). So, all dolphins are whales, but not all whales are dolphins. Similarly, orcas (also known as killer whales) are in the Delphinidae family and are both dolphins and whales.
Mackenzie Preble obtained her B.S. in marine biology from UC Santa Cruz and M.S. in marine science from Hawaii Pacific University. She has a passion for wildlife rehabilitation and science communication.
Through many volunteer and internship positions, Mackenzie has had the privilege to tag wild northern elephant seals, rehabilitate sick and injured pinnipeds, assist in whale strandings, and teach marine conservation to students and the public. She hopes to continue this work in the future.