The evolution of the whale has long been shrouded in mystery. Fossil remains have demonstrated features that suggest that the large mammals that we know as sea-dwellers actually once lived on land. This idea is not so far-fetched considering that whales possess large, developed brains, breath oxygen, are warm-blooded and give birth to live young, all of which are characteristics shared with most land mammals.
Experts believe that ancient "whales" had legs and were well suited for life on land. When these ancient mammals made their way back to the sea they shed their legs and their bodies adapted to an aquatic lifestyle. Interestingly, this is the reverse of what is thought to have happened millions of years ago, when the very first mammals are believed to have crawled from the depths of the sea to become land dwellers.
Not all of the details of whale evolution are clear and remain the subject of study. However, the transformation from land dwellers to sea dwellers is considered a certainty, as a great number of fossils have been uncovered in the past few decades, which demonstrate this amazing transition.
One of the most important discoveries concerning whale evolution was made in Pakistan in 1978 by paleontologist Phil Gingerich. These fossil remains resembeled wolf-sized carnivores (creodonts) that roamed the earth between 60 and 37 million years ago. However, the skull of these land-dwellers also had traits that were similar to those of the oldest known whale species (archaeocetes). These new fossil findings, named Pakicetus, demonstrated the transition between land dwelling mammals and fully aquatic animals. The most notable evidence of this came from an examination of the ear region of the creodonts, which showed adaptations not found in other land-dwelling animals of the time. This suggests an evolutionary tie to modern-day whales, whose ear regions are highly-modified for underwater hearing.
Other relevant findings include the remains of an amphibious animal called Ambulocetus, whose front limbs featured both fingers and small hooves. However, the hind feet of the animal were adapted for swimming and it had a tail which would further help it to manuever through water. Skelatal analysis showed that the Ambulocetus was able to function on land as well as in the water.
Another link in the chain is the Rhodocetus, whose remains suggests an evolution towards a marine lifestyle. This animal had more stable neck, a feature observed in other aquatic animals, and in an extreme form in modern-day whales. Like the Pakicetus, the ear region of the Rhodocetus is also better suited for hearing underwater. Finally, it's legs are detached from the pelvis, evidence of a severance from the life of land-dweller.
Yet another important finding that demonstrates the land-to-sea transition is the Basilosaurus. By 40 million years ago, this aquatic animal was fully adapted to a life at sea, with a long, flexible body and sturdy flippers. However, the Basilosaurus still displayed one remnant of a land-dwelling existence. The aquatic animal possessed hind legs. However, the limbs were small, weak and obviously non-functioning for land transportation.
Experts do not suggest any of these animals to be directly related to the whales of today. However, they may still reside somewhere within the same family tree. More importantly, they allow us to observe the gradual adaptations to aquatic life which led to modern whales.
Information regarding whale evolution obtained from http://www.pbs.org