The first whales appeared 50 million years ago, well after the extinction of the dinosaurs, but long before the first human appeared. Their ancestor is most likely an ancient artiodactyl, a four-legged, even-toed hoofed land mammal, adapted for running. Hence cetaceans share a common ancestor with modern-day artiodactyls such as cows, camels and hippos.
The ancestor of today’s whales, the first cetacean, is believed to be Pakicetus, a quadruped measuring 1-2 meters long. Fossils discovered in Pakistan indicate that the animal had typical artiodactyl ankles and a typical cetacean skull. Unlike today’s whales, the species was not aquatic, and its ankles are testimony to its running ability. It is still considered a cetacean, due to the morphology of its inner ear, which resembles cetaceans’ ears and is unlike that any other mammal. The shape of its teeth suggests that Pakicetus was carnivorous, like modern-day whales.
The descendants of the first land-roaming cetaceans are believed to have increasingly gravitated to an aquatic environment, in some cases to seek refuge from danger or to feed. As today’s artiodactyls are all herbivores, biologists believe that cetaceans evolved when some of the herbivores in this group appear to have changed their diets to become carnivores.
Ambulocetus, a four-legged whale ancestor, whose feet were probably webbed, was probably able to both walk and swim. Bone analysis has revealed that it could live in both fresh and salt water. Its inner ear was adapted to life in an aquatic environment.
Today’s whales are divided into two major groups: odontocetes, or toothed whales, and mysticetes, or baleen whales. Their common ancestor, which lived about 34 million years ago, probably lacked baleens and could not use echolocation. In just five million years, whales species diversified, probably due to rapid ecological changes in the oceans.
15 million years ago, another rapid diversification of cetacean species occurred, as ocean cooling changed the currents. At the same time, the number of mollusk and crustaceans species consumed by some whales also increased.
The first mysticetes measured 5 to 9 meters in length (approximately the size of a minke whale). Whales are believed to have reached the size we know today only about 4,5 million years ago. The sudden growth of mysticetes coincides with the cooling of the climate and the formation of large ice caps in the northern hemisphere. In spring and summer, ice-trapped nutrients are released into open water and accumulate near the coasts. Driven by currents, plankton accumulate in these areas and grow substantially when they come into contact with nutrient-rich waters. Plankton – until then widely dispersed in the ocean – began to form seasonal concentrations, sometimes separated by thousands of kilometers.
In these changed conditions, size became a characteristic that was subject to a strong selection pressure. Thus, the most corpulent individuals, with their greater reserves, were able to travel longer distances and take advantage of these recently formed feeding grounds. As for the smaller ones, they gradually disappeared and gave way to the era of the giants. Being bigger has many benefits. Giants are more likely to be at the top of the food chain and therefore considerably less susceptible to predation. On the other hand, they are at much greater risk of extinction in times of environmental change or crisis.
Now what? Whales are still evolving today. Ecosystems are regularly modified and interactions between species are dynamic. Hence there is always a need to adapt to changing conditions. Whales are subject to various evolutionary pressures such as climate change, decline in prey abundance, noise-pollution and chemical pollution. Their large size makes them particularly vulnerable to lack of food.