Evidence indicates that whales descended from a
                        land animal, perhaps a primitive ungulate (hoofed
                        mammal) that may also have given rise to modern
                        ungulates. The earliest known whale fossils are 40
                        million years old, but many scientists estimate that
                        whales date from 60 million years ago. No fossil has
                        yet been found, however, that links the land-living
                        ancestors and the early cetaceans, and the reasons
                        for a mammal?s return to the sea are also not
                        known. Perhaps the move was in response to a need for more space, or for a new
                        food source, or to escape predation. 

                        Classification

                        Most smaller whales, plus all the
                        dolphins and porpoises, belong to the
                        suborder Odontoceti, the toothed whales.
                        Those more than 4 to 5 m (13 to 16 ft)
                        long are generally referred to as whales,
                        whereas smaller species are known as
                        dolphins or porpoises. Odontocetes have
                        teeth that are uniform in size and shape
                        or else are toothless, and they feed on
                        fish and invertebrates such as squid and
                        crustaceans; one species, the killer
                        whale, has a more varied diet that
                        includes seabirds and marine mammals. A few species are commercially valuable
                        as exhibit specimens in aquariums and oceanariums, and some of the smaller
                        whales are harvested to a limited extent. One toothed whale, the sperm whale,
                        sometimes known as a cachalot, is quite large: The male grows to a length of 18.3
                        m (60 ft), and the female grows to a length of 12.2 m (40 ft). It was heavily hunted in
                        the past. 

                        The rest of the larger whales belong to the suborder Mysticeti, the baleen whales. In
                        this group of ten species?all of which have been or are currently being
                        harvested?teeth have been replaced with large structures, known as baleen plates,
                        that hang like vertical venetian blinds from the upper jaw. The plates number 160 to
                        360 on each side, are frayed into bristles on their inner edges, and are used to
                        capture the plankton or krill on which the animals subsist. When feeding, a baleen
                        whale swims with its mouth open in order to engulf plankton and seawater by the
                        ton. Then, shutting its cavernous mouth and pressing its tongue against the back of
                        the baleen bristles, the whale forces the water out of its mouth, trapping the
                        plankton on a mat of overlapping baleen. 

                        Probably the largest animal ever to have lived is a baleen whale, the blue whale,
                        which has been measured up to 30.5 m (100 ft) in length, with a weight of more than
                        200 metric tons. Baleen whales tend to spend the summer in polar seas, where
                        plankton blooms provide abundant food. After months of heavy feeding they migrate
                        to temperate or tropical zones, often fasting there over the winter.