groups of mammals because of their deep-sea habitat

Whales are descendants of land-dwelling mammals of the artiodactyl order (even-toed ungulates). They are related to the Indohyus, an extinct chevrotain-like ungulate, from which they split approximately 48 million years ago.[21][22] Primitive cetaceans, or archaeocetes, first took to the sea approximately 49 million years ago and became fully aquatic 5–10 million years later. What defines an archaeocete is the presence of anatomical features exclusive to cetaceans, alongside other primitive features not found in modern cetaceans, such as visible legs or asymmetrical teeth.[23][24][25][11] Their features became adapted for living in the marine environment. Major anatomical changes included their hearing set-up that channeled vibrations from the jaw to the earbone (Ambulocetus 49 mya), a streamlined body and the growth of flukes on the tail (Protocetus 43 mya), the migration of the nostrils toward the top of the cranium (blowholes), and the modification of the forelimbs into flippers (Basilosaurus 35 mya), and the shrinking and eventual disappearance of the hind limbs (the first odontocetes and mysticetes 34 mya).[26][27][28]

Whale morphology shows several examples of convergent evolution, the most obvious being the streamlined fish-like body shape.[29] Other examples include the use of echolocation for hunting in low light conditions — which is the same hearing adaptation used by bats — and, in the rorqual whales, jaw adaptations, similar to those found in pelicans, that enable engulfment feeding.[30]

Today, the closest living relatives of cetaceans are the hippopotamuses; these share a semi-aquatic ancestor that branched off from other artiodactyls some 60 mya.[11] Around 40 mya, a common ancestor between the two branched off into cetacea and anthracotheres; nearly all anthracotheres became extinct at the end of the Pleistocene 2.5 mya, eventually leaving only one surviving lineage – the hippopotamus.[31]

Whales split into two separate parvorders around 34 mya – the baleen whales (Mysticetes) and the toothed whales (Odontocetes).[32][33][34]Whales have torpedo-shaped bodies with non-flexible necks, limbs modified into flippers, non-existent external ear flaps, a large tail fin, and flat heads (with the exception of monodontids and ziphiids). Whale skulls have small eye orbits, long snouts (with the exception of monodontids and ziphiids) and eyes placed on the sides of its head. Whales range in size from the 2.6-metre (8.5 ft) and 135-kilogram (298 lb) dwarf sperm whale to the 34-metre (112 ft) and 190-metric-ton (210-short-ton) blue whale. Overall, they tend to dwarf other cetartiodactyls; the blue whale is the largest creature on Earth. Several species have female-biased sexual dimorphism, with the females being larger than the males. One exception is with the sperm whale, which has males larger than the females.[35][36]

Odontocetes, such as the sperm whale, possess teeth with cementum cells overlying dentine cells. Unlike human teeth, which are composed mostly of enamel on the portion of the tooth outside of the gum, whale teeth have cementum outside the gum. Only in larger whales, where the cementum is worn away on the tip of the tooth, does enamel show. Mysticetes have large whalebone, as opposed to teeth, made of keratin. Mysticetes have two blowholes, whereas Odontocetes contain only one.[37]

Breathing involves expelling stale air from the blowhole, forming an upward, steamy spout, followed by inhaling fresh air into the lungs; a humpback whale's lungs can hold about 5,000 litres (1,300 US gal) of air. Spout shapes differ among species, which facilitates identification.[38][39]

All whales have a thick layer of blubber. In species that live near the poles, the blubber can be as thick as 11 inches (28 cm). This blubber can help with buoyancy (which is helpful for a 100-ton whale), protection to some extent as predators would have a hard time getting through a thick layer of fat, and energy for fasting when migrating to the equator; the primary usage for blubber is insulation from the harsh climate. It can constitute as much as 50% of a whale's body weight. Calves are born with only a thin layer of blubber, but some species compensate for this with thick lanugos.[40][41]

Whales have a two- to three-chambered stomach that is similar in structure to those of terrestrial carnivores. Mysticetes contain a proventriculus as an extension of the oesophagus; this contains stones that grind up food. They also have fundic and pyloric chambers.[42]