The cats. Felids are the most strictly carnivorous of the sixteen mammal families in the order Carnivora. The most familiar felid is the domestic cat, which first became associated with humans about 10,000 years ago, but the family includes all other wild cats including the big cats.  Discounting extinct species, the living felids belong to either the Pantherinae (lion, tiger, jaguar, and leopard), and Felinae (cougar, cheetah, lynxes caracal, and domestic cat).  The first felids emerged dabout 25 million years ago. In prehistoric times, there was a third group called the Machairodontinae, which included the saber-toothed cats such as the well known Smilodon.   There are 40 known species of felids in the world today. They originated in Asia and spread across continents by crossing land bridges. Ancient cats evolved into eight main lineages that diverged in the course of at least 10 migrations (in both directions) from continent to continent via the Bering land bridge and Isthmus of Panama. The Panthera species are the oldest and the Felis species are the youngest.   Felids are purely carnivorous animals, subsisting almost entirely on other vertebrates. Aside from the lion, they are solitary. Most are secretive animals, often nocturnal, and live in relatively inaccessible habitats. Around three-quarters of cat species live in forested terrain, and they are generally agile climbers. However, felids may be found in almost any environment, with some species being native to mountainous terrain or deserts. Wild felids are native to every continent except Australia and Antarctica.  The various species of felid vary greatly in size. One of the smallest is the Black-footed Cat at between 35-40 cm in length, while the largest and most powerful is the Siberian tiger. Compared with many other mammals, they have relatively short faces, and good binocular vision.  The fur of felids takes many different forms, being much thicker in those species that live in cold environments, such as the Snow Leopard. The colour of felids is also highly variable, although brown to golden fur is common in most species, often marked with distinctive spots, stripes, or rosettes. Many species also have a \"tear stripe,\" a black stripe running from the corner of each eye down the side of the nose.  The tongue of felids is covered with horny papillae, which help to rasp meat from their prey. Almost all felids have fully retractable claws (one exception is the cheetah). Cats have five toes on their forefeet and four on their hindfeet, reflecting their reliance on griping and holding down their pray with their claws.  Felids have relatively large eyes, situated to provide binocular vision. Their night vision is especially good, due to the presence of a tapetum lucidum, which reflects light back inside the eyeball, and gives cat eyes their distinctive shine.  The ears of felids are also large, and in the smaller cats especially sensitive to high-frequency sounds. Felids have a highly developed sense of smell, although not as much so as in canids. This is further supplemented by the presence of a vomeronasal organ in the roof of the mouth, allowing the animal to taste the air. The use of this organ is associated with the Flehmen response, in which the upper lip is curled upwards.  Felids possess highly sensitive vibrissa (whiskers) set deep within the skin, and provide the cat with sensory information about the slightest air movement around it. For this reason they are very helpful for a nocturnal hunter. Most felids are able to land on their feet after a fall, an ability which relies on vision and the sense of balance acting together.  The canine teeth are large, reaching exceptional size in the extinct saber-tooth species. The upper third premolar and lower molar are adapted as carnassial teeth, suited to tearing and cutting flesh.  The jaws of felids can only move vertically. This prevents them from being able to chew, but makes it easier for their powerful masseter jaw muscles to hold struggling prey.   Text after Wikipedia, 3rd August 2008.