There is nothing quite as breathtaking as a pod of humpback whales swimming in the open ocean. They call to each other with haunting songs, leap majestically into the air, and crash back into the waves, slipping effortlessly into the watery depths. Many people enjoy observing whales for their beauty, power, and grace. But not everyone knows that whales are more closely related to humans than to their fish neighbors. Humans and whales actually have a lot in common because both are mammals: creatures that are warm-blooded, breathe air, and give birth to live young. Scientists studying evolution believe that, once upon a time, the ancestors of both people and whales walked on all fours near the water’s edge.
The idea that the ancestors of whales lived on land was once a big problem for biologists. Even now, some religious scholars think the differences between marine mammals and land dwellers are too great to be explained by the theory of evolution. But recent developments in DNA research and fossil analysis have convinced the scientific community that the physical similarities in the fossil record cannot be ignored. For example, the bone structure of a whale’s flipper greatly resembles the jointed arrangement of bones in a human hand. Whales also have hip bones, and some specimens even have tiny leg bones—where their hind legs used to be—contained completely within their bodies. But researchers have only recently been able to compile this evidence and to understand the amazing changes this animal has undergone over time.
Over the past few decades, scientists have uncovered the fossils of hundreds of long-dead specimens that have proven to be some of the "missing links" between whales and their land-dwelling ancestors. With these fossils, scientists are trying to determine how small, furry land animals evolved into the dozens of species of whales we know today.
Scientists believe that life on planet Earth began in the oceans, over three billion years ago. Slowly, some organisms began to adapt to dry habitats, until the whole planet—the land and the water—was teeming with life. By around 65 million years ago, dinosaurs dominated the landscape. One popular theory states that the earth changed dramatically when an asteroid slammed into it, throwing up a massive dust cloud. This cloud cooled the planet, causing the extinction of dinosaurs and many other plants and animals. The surviving animals could now take over the newly abandoned territories. Within a few million years, mammals of all shapes and sizes were flourishing.
One of these survivors evolved into a hoofed animal about the size of a fox, which experts named Indohyus. This whale ancestor probably took its first steps into the water to avoid predators. Indohyus didn’t look like a whale, but the structure of its bones and teeth was similar to that of modern whales.
Within eight million years, a descendant of this creature had developed an improved ability to hunt in shallow water. When paleontologists working in Pakistan in 1979 found a fossilized skull of this wolf-sized animal, they named it Pakicetus, for "Pakistan whale." The Pakicetus skull had peculiarities in the bones of its ear that exist today only in whales. Another species was soon unearthed, thought to be around 49 million years old. The body of Ambulocetus, or "walking whale," shared underwater adaptations with Pakicetus. Still designed to prowl both the shore and the nearby shallow waters, Ambulocetus may have had help from webbed feet, short but powerful limbs, and a strong tail. Rodhocetus, another intermediary species, appeared around 42 million years ago. It was the size of a sea lion and could have looked like a crocodile with fur.
By the time of Rodhocetus, the ancient ancestors of whales were spending most of their time in the water, but they still moved the same way that dogs do today: they basically trotted through water. Eventually, the same forces that shaped fish—those forces involved in underwater movement—began to impact these mammals. This is why both whales and fishes have streamlined bodies. They use them differently, however: Whales swim by moving their tails up and down, while fish swim by moving their tails from side to side.
Another adaptation developing during the time of Rodhocetus was a new way to hear. Visibility was often limited in the water. As a result, these creatures began to depend more on sound than sight to find prey and avoid predators. The external ear openings of most land animals became less useful. Instead, the ancestors of whales began to use their lower jaws to focus sounds. The vibrations were then transmitted through their skulls to the auditory canal, a tube inside the ear.
The next known whale ancestors appeared forty million years ago. Further adaptations in body structure had made them more efficient in the water. Dorudon and Basilosaurus had lost almost all traces of their hind legs, and the ends of their tails flared out into horizontal flukes. Most of their fur had disappeared, making them faster swimmers. The fronts of their skulls had grown long, and their nostrils had become blowholes at the tops of their heads, allowing them to breathe easily while swimming. Through this gradual process, a species that resembled a modern whale had appeared by around 35 million years ago.
The adaptations, however, continued, as whales evolved into two separate groups: toothed whales and baleen whales. The toothed whales include sperm whales, orcas, dolphins, and porpoises. Toothed whales developed echolocation, a way to use sound waves to explore their surroundings. They produce whistles and clicks of different frequencies, and by listening for echoes bouncing off objects, they can identify the locations of other creatures and structures around them.
Baleen whales are larger than toothed whales and include humpback, blue, gray, and mink whales. Scientists think baleen whales lost the ability to use echolocation when they stopped chasing after large fish and other mammals and started eating tiny fish and plankton, which swim in large, slow-moving schools. The teeth of these whales transformed into baleen—long, thick curtains of hair-like material that filter food from the water. Although they cannot echolocate, many baleen whales sing complicated songs that can be heard for miles, and are being studied by experts to try to decipher their mysterious language.
Evolutionary biologists continue to study the unique characteristics of these marine mammals. Every year, more pieces of the puzzle of life fall into place, providing clues about how whales have adapted to their watery environment and changed over time from small, furry land-dwellers to the majestic animals that make their home in the oceans today.