Vertebrates are animals with backbones that comprise one subphylum of the phylum Chordata; the remainder of Chordata are the invertebrates that do not have backbones. Vertebrates encompass many of the familiar animals on Earth: birds; fishes; frogs; reptiles; snakes; and mammals, including we Homo sapiens. The vertebrate fossil record dates back some 500 million years ago to the upper Cambrian. Vertebrates first evolved in the ocean. The invertebrate ancestors of vertebrates had gill slits that were primarily used for filter feeding, with oxygen uptake being through their skin. These early vertebrates lacked jaws, as does extant hagfish and lampreys. Jawed vertebrates appeared 100 million years later in Silurian time. In terms of number of species, vertebrates are a tiny per cent of the living creatures.