Animals characterized by a segmented spinal column are called vertebrates. In biology, there are five classes of vertebrates, one of which is called mammals. What are the other four?

The animal kingdom is incredibly diverse, but one of the most fundamental ways biologists classify animals is by the presence of a backbone. The five major classes of these vertebrates showcase a remarkable evolutionary journey. Alongside the familiar mammals, the four other groups that complete this classification are fishes, amphibians, reptiles, and birds. These groups encompass a vast range of species, from the tiniest minnow to the largest dinosaur's modern-day relative.

Each class is defined by distinct characteristics that reflect its place in the evolutionary tree. Fishes, the earliest vertebrates, are adapted for aquatic life, breathing through gills. Amphibians, like frogs and salamanders, represent a key transition from water to land, typically starting life with gills before developing lungs. Reptiles, with their protective scaly skin and leathery eggs, were the first to fully conquer terrestrial environments. Birds are unique for their feathers, hollow bones built for flight, and their direct lineage from dinosaurs.

While fishes, amphibians, and reptiles are ectothermic, or "cold-blooded," relying on their surroundings to regulate body temperature, birds and mammals independently evolved to be endothermic, or "warm-blooded." This ability to generate their own internal heat allowed them to thrive in a wider range of climates around the globe. Together, these five classes represent the major branches of the vertebrate family.