Whales, dolphins and porpoises, together form the cetacea family. The cetaceans are a different type of mammal, that thrive in an aquatic habitat. They breathe through their lungs, and come up to the surface of the water to breathe. They also have bones in their fins that are very similar to the limbs of mammals. The cetaceans, unlike the fish and the amphibians, do not lay eggs for reproducing, but give birth to their offspring. Often these creatures have also been termed as leviathans. Scientists and experts have long been pondering on the problem of tracing the evolution of the cetacean family and in particular whales. This was a difficult task due to the lack of evidence in nature as well as the fossil gap (inconsistency in the fossil records). Recent studies of fossils found in Pakistan have given rise to a concrete theory about their stages of evolution. Most experts believe that whales are closely related to the hoofed animals (scientifically known as ungulates), Mesonychids. This species had a wolf-like appearance with hooves, and triangular teeth extremely similar to modern whales. The validity of this theory, however, has been questioned. The molecular phylogeny data collected has strongly indicated that the early whales living on land belonged to the mammal order of Artiodactyla, or even toed ungulates. Scientists nowadays also believe that early whales were very closely related to the hippopotamus, which also belonged to the Artiodactyla order. The stage of evolution that came next made the early whales take the form of mammals living in water as well as land, and display more of an amphibian behavior. Scientists believed this mammal to be the Pakicetus. The Pakicetus roamed the earth prominently during the Eocene epoch that occurred about 53 million years ago. Fossils of these animals indicate that they were large dog-like creatures and had hooves. The shape of the skull and the ear of these animals had a strong resemblance to the modern whale. The teeth of these animals were also similar to modern whales. Fossils of a similar creature, by the name Indohyus, were found in Kashmir. The Indohyus fossil distinctly displays its adaptability to aquatic life. It existed about 48 million years ago and was smaller than the Pakicetus. The fossils recently found in Pakistan belonged to a creature known as the Ambulocetus, which was believed to have evolved from the Indohyus and the Pakicetus. The Ambulocetus fossils show us that it was a long amphibious creature resembling a crocodile. This animal had better evolved feet for swimming as well as walking. The two animals that evolved from the Ambulocetus, the Protocetus and the Rodhocetus, resembled modern whales with respect to the nasal openings and the development of the vertebral column. The next step in the evolution of whales was when the Basilosaurus and the Dorudontids appeared, which were both fully developed marine animals belonging to the cetacean family. The first whale-like cetacean appeared about 33 million years ago and was known as the Squalodon. It had a distinct forehead like the modern whale and was capable of emitting sound pulses to map objects while traveling. The last stage in the development of the whales was the Baleen Whale, which finally evolved into the modern whales we see today.