Evolution

indohyus_by_buell

50 million years ago

The history of whales, like that of our planet, is in part written in the rocks. The first cetaceans appeared some 50 million years ago, after dinosaurs had already gone extinct. Their ancestor is an ancient artiodactyl, meaning a four-legged, even-toed hoofed (ungulate) land mammal, adapted for running. Today’s artiodactyls (for example, cows, pigs, camels, giraffes and hippopotami) and cetaceans thus descend from a common ancestor. They are so closely related that some scientists propose merging them into a single order: cetartiodactyls.

Land-roaming whales

Pakicetus_trans

The path that this land mammal took on its return to the sea is a difficult one to trace. On one hand, paleontologists have long believed that mesonychids, a group of now extinct ungulates, were the most probable ancestors of cetaceans. On the other hand, various genetic and molecular clues point to a close link between cetaceans and artiodactyls, particularly the hippopotamus. However, in 2001, paleontologists found the “missing link”: skeletons of animals possessing typical ankles of an artiodactyl and a typical skull of a cetacean. The skeletons, discovered in Pakistan, are those of the Pakicetus, the oldest known cetacean. Unlike today’s whales, this animal was not aquatic and its ankles are testimony to its running ability. With this discovery, mesonychids could be eliminated as potential ancestors of cetaceans. Nevertheless, the links between the hippopotamus and cetaceans remain unclear. Are hippopotami more closely related to whales than other artiodactyls? Specialists have not reached a consensus on this question.

Back to the water

It is believed that descendants of these first land cetaceans gravitated increasingly to an aquatic environment, in some cases to seek refuge from danger or to feed, initially on vegetation and then exclusively on animals. As today’s artiodactyls are all herbivores, biologists believe that cetaceans evolved because certain herbivores of this group appear to have changed their diets to become carnivores. This hypothesis is not universally accepted, however. Some scientists speak instead of an omnivorous ancestor. The path taken is still unclear and several pieces of this complex puzzle remain to be discovered.

Living in the water

The whales that we know today are extraordinarily well adapted to life in the water. Millions of years in the sea have favoured transformations to facilitate life in this new environment. Nostrils have evolved into blowholes and are now located at the top of the head. Hind limbs have disappeared and front limbs have transformed into fins. The body has lost its fur and nearly all of its hair. It is streamlined. A horizontal, powerful propeller of a tail attached itself to the vertebral column. These adaptations blur the relationship that exists between whales and their closest living relatives. Nevertheless, whales are mammals like us. They have lungs and breath air. They are warm blooded, meaning that they maintain their internal temperature at a constant 37°C. Their young develops in the uterus of the mother, who feeds it directly through the placenta. They nurse their calves.

Illustrations of Indohyus and Pakicetus: © Carl Buell