Reading Practice 91

The evolutionary history of whales provides one of the most compelling examples of macroevolution—the process of major evolutionary change over long periods. Modern whales, classified as cetaceans, include two suborders: toothed whales, or Odontoceti, and baleen whales, or Mysticeti. Despite their fully aquatic lifestyle and fish-like appearance, whales are mammals, a fact evident in their need to surface for air, their warm-blooded nature, and the presence of vestigial structures linked to terrestrial ancestors.

Fossil evidence has been pivotal in reconstructing the evolutionary pathway of whales. One of the earliest known ancestors of modern whales is Pakicetus, a terrestrial mammal that lived approximately 50 million years ago. Fossilized remains of Pakicetus indicate that it was a four-legged, wolf-like creature with an elongated skull and teeth similar to those of modern whales. It likely lived near freshwater environments and hunted aquatic prey. This early stage demonstrates the transition from land to water.

Another key transitional species is Ambulocetus natans, often referred to as “the walking whale.” Fossils of Ambulocetus show that it was amphibious, with limbs capable of supporting its weight on land but also adapted for swimming. Its elongated body and strong tail suggest an early form of propulsion in water, hinting at the gradual shift toward an aquatic lifestyle. This species also marks the beginning of significant changes in the anatomy of the ears, facilitating underwater hearing—a crucial adaptation for modern whales.

As evolution progressed, Rodhocetus, another intermediate form, exhibited even greater aquatic adaptations. The limbs of Rodhocetus were further modified, making it more efficient in swimming, while the pelvis became less attached to the spine, a change associated with streamlined movement in water. By this stage, approximately 40 million years ago, the nostrils of these ancestors began migrating toward the top of the skull, an evolutionary step that would eventually result in the blowholes seen in modern whales.

The emergence of fully aquatic ancestors, such as Dorudon, represents a major milestone in cetacean evolution. Living around 37 million years ago, Dorudon was fully adapted to life in the ocean, with reduced hind limbs and a tail fluke that allowed for powerful swimming. Its diet, inferred from fossilized teeth, suggests that Dorudon was a predator, preying on fish and squid. The reduction of hind limbs and the development of tail propulsion reflect the culmination of millions of years of gradual adaptation.

Genetic evidence further supports the link between modern whales and their terrestrial ancestors. DNA analyses reveal that cetaceans share a close evolutionary relationship with modern hippos. This surprising connection is corroborated by molecular studies and similarities in the structure of ankle bones, suggesting that whales and hippos share a common ancestor that lived around 55 million years ago.

The evolutionary journey of whales illustrates the dynamic interplay between environmental pressures and biological adaptations. The transition from terrestrial to fully aquatic life required profound anatomical and physiological changes, including the reorganization of respiratory systems, the development of streamlined body shapes, and the enhancement of sensory systems for underwater navigation. These changes, documented through fossils and genetic evidence, underscore the complexity of evolutionary processes and the intricate history of life on Earth.