The Origin of Whales or the Evolution

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 has attached itself to the vertebral column. These adaptations blur the relationship that exists between whales and their closest living relatives.

However, whales, like humans, are mammals. They have lungs and breath air. They are warm-blooded, meaning they maintain a constant body temperature (37°C). Their offspring develop in the mother’s womb, are nourished by the placenta during pregnancy and are nursed after birth.

Ambulocetus, a four-legged whale whose legs were probably webbed, was likely able to both walk and swim, while bone analysis has revealed that it could live in fresh and salt water alike. 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 baleen and could not use echolocation. In just 5 million years, whale species have diversified, probably due to rapid ecological changes in the oceans.

Fifteen 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 crustacean species consumed by some whales also increased.

The first mysticetes measured 5 to 9 metres in length (approximately the size of today’s 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 – begin to form seasonal concentrations, sometimes separated by thousands of kilometres.

In this new reality, size becomes a characteristic subject to a strong selection pressure. Thus, the most corpulent individuals, with their greater reserves, are able to travel longer distances and take advantage of these recently-formed feeding grounds. As for the smaller ones, they gradually disappear and give 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 much more at risk of extinction in times of environmental change or crisis.

Whales are still evolving nowadays. Ecosystems are regularly modified and interactions between species are dynamic. Hence, there is always a need to adapt to changing conditions. Modern-day whales are subject to several evolutionary pressures such as climate change, decline in prey abundance and ocean pollution. Their large size makes them particularly vulnerable to lack of food.

Bigger, heavier, better divers… Whales are often thought to be superior to humans in many respects, but sometimes it is the loss of genes that is at the root of their adaptations. Genes linked to feeding, including those responsible for saliva production or those that code for the reabsorption of sodium by the kidneys, have disappeared because they have become useless to whales which, with a few exceptions, live in saltwater environments. Their lives at sea make the lubrication of food unnecessary and sodium deficiency virtually impossible.

As for rest, this activity in whales is influenced by the loss of the gene responsible for the production of melatonin, a well-known hormone that regulates the sleep cycle. This adaptation is probably due to the fact that in cetaceans, breathing is a voluntary activity and falling into a deep slumber would likely increase the risk of drowning in these marine mammals.

Other genes that have disappeared in cetaceans are those that are useful to terrestrial animals but which cause problems during diving due to the extreme pressures their bodies are subject to. Researchers have notably observed the disappearance of genes that facilitate blood clotting and those associated with fibrosis and other lung issues in some humans. Another gene that indirectly increased the risk of DNA mutations in diving cetaceans has also vanished from their genetic makeup.