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.