Humans, cats, frogs, birds and fish are all members of the Phylum Chordata. The Phylum Chordata is a classification of living creatures characterized by the presence of a notochord, a section of nerves running down the back of the organism.
Phylum Chordata encompasses a large group of various species that are broken down into subphylum. Though each subphylum differs significantly, they all share the similarity of having a spinal cord and accompanying evolutionary features that allow for a higher brain function. The name Chordata is derived specifically from the notochord, which houses all of the important nerves that connect the brain to the rest of the body. Mammals, such as humans and cats, have a skeletal structure that hinges around the presence of the notochord. Fish and birds have a similar skeletal structure with the backbone playing a major role in the development and abilities of the organism.
Other distinguishing characteristics of the Phylum Chordata are the pharangeal slits, which are openings near the head of the organism. In the earliest forms of chordates, these were used for feeding or filtering water. They can be seen in fish in the form of gills and in humans and birds in the form of ears.