Felidae is the name of the biological family of cats; a member of this family is called a felid, or cat. 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. Extant felids belong to one of two subfamilies: Pantherinae (which includes the tiger, the lion, the jaguar, the leopard, and the snow leopard), and Felinae (which includes the cougar, the cheetah, the lynxes, the ocelot, the leopard cat, and the domestic cat). The first felids emerged during the Oligocene, about 25 million years ago. In prehistoric times, a third subfamily, the Machairodontinae, included the “saber-toothed cats”, such as the well-known Smilodon. Other superficially cat-like mammals, such as the metatherians Thylacosmilus and Thylacoleo, or the Nimravidae, are not included in Felidae despite superficial similarities. Felids are the strictest carnivores of the 13 terrestrial families in the order Carnivora, although the three families of marine mammals comprising the superfamily Pinnipedia are as carnivorous as the felids. Felids are obligate carnivores – they must consume meat to survive – and are sometimes referred to as hypercarnivores because of the much higher proportion of protein they require in their diet, much more than most other mammals.