Whales are descended from land animals of the order Artiodactyla (even-toed ungulates). They descended from the Indohyus, an extinct ungulate that resembled a chevrotain, about 48 million years ago. The earliest cetaceans, known as archaeocetes, entered the ocean some 49 million years ago and only completely evolved into aquatic animals 5–10 million years later. An archaeocete is characterized by the coexistence of primordial features not present in contemporary cetaceans, such as exposed legs or asymmetrical teeth, with anatomical characteristics unique to cetaceans. For survival in the maritime environment, their traits evolved. Their hearing system, which transmitted vibrations from the jaw to the earbone (Ambulocetus 49 mya), a streamlined body, and the development of flukes on the tail (Protocetus 43 mya), among other significant morphological alterations
Whales can feed, mate, give birth, nurse, and raise their young in the open ocean since they are totally aquatic organisms. Whales range in size from the dwarf sperm whale, which is only 2.6 meters (8.5 feet) long and weighs 135 kilograms (298 pounds), to the largest known mammal to have ever lived, the blue whale, which is 29.9 meters (98 feet) long and weighs 190 metric tons (210 short tons). The largest toothed predator on Earth is the sperm whale. There is sexual dimorphism in several whale species, where the females are bigger than the males.