Evolutionary innovations of the vertebrates.

This review concentrates on the greatest anatomical and morphological evolutionary innovations of the vertebrates. During evolution, many new species of vertebrates evolved and underwent modifications by developing new forms, structures and functions of tissues and organ systems. Evolutionary development of the chordates and vertebrates is herein examined in terms of innovations in their organ systems and organismal complexity. Phases during chordate and vertebrate evolutionary history with unusually high rates of increase in morphological complexity are discussed. These increases in complexity in particular chordates and vertebrates coincided with a likely genome duplication event, which resulted in a large increase in genome size and gene number in early vertebrates, and might indicate an increase in complexity. The Hox and Pax gene families are also discussed because both illustrate the relationships between organismal and molecular complexity. Most unique innovations of vertebrates caused major changes in their organismal complexity, and these changes provided new options for future evolutionary development.