The fossil record of vertebrates unequivocally supports the hypothesis that vertebrates have evolved through time, from their first records in the early Paleozoic Era about 500 million years ago to the great diversity we see in the world today. The hypothesis has been strengthened by so many independent observations of fossil sequences that it has come to be regarded as a confirmed fact, as certain as the drift of continents through time or the lawful operation of gravity. Paleontology relies for its evidence on two different but historically related fields, biology and geology.
Evolution is the central organizing principle of biology, understood as descent with modification.
Evolution is equally basic to geology, because the patterns of rock formations, geomorphology, and fossil distributions in the world make no sense without the underlying process of change through time. Sometimes this change has been gradual, and sometimes it has been characterized by violent upheaval. These processes can be seen on the Earth today in the forms of earthquakes, volcanoes, and other tectonic phenomena. Vertebrates have also evolved at a variety of rates, some apparently gradual, and some apparently rapidly. Although the fossil record is not complete, and our knowledge of evolution will always be less than entire, the evidence for the progressive replacement of fossil forms has been adequate to support the theory of evolution for over 150 years, well before genetic mechanisms of evolutionary change were understood.