Most of the more obvious animals that live around us are vertebrates - animals with flexible backbones and internal skeletons. They are the mammals (which include humans), birds, reptiles, amphibians, and three types of fish.
We generally think of the vertebrates as divided into five main types of animals - mammals, birds, reptiles, amphibians, and fish. But the fish actually consist of three very different groups of animals that happen to live underwater, breathe with gills, and have similar body forms with fins rather than legs. So scientists classify the vertebrates into seven main groups.
These make up the vast majority of the phylum Chordata - animals with a spinal nerve cord reinforced by a tough, flexible rod called a notochord. In most vertebrates, the notochord is replaced by a bony spine at a very early stage in the animal’s life.
The first vertebrates, or animals with backbones, to evolve were fish. They now make up about half of all vertebrate species, and include many of the most spectacular animals on the planet.
Fish are vertebrates that have always lived underwater, since the evolution of their earliest ancestors around 530 million years ago. Other vertebrates, including whales and seals, also live in the water, but they must breathe air and they have many other features that show their ancestors were land animals. By contrast, a fish’s skeleton, internal organs, gills, skin, muscles, and senses are specialized for aquatic life, and they are amazingly efficient.
Although fish vary greatly in their size, shape, and habits - and even in their basic biology nearly all species of fish share some key features.
Amphibians are best known for the way that many species spend part of their lives in water and part on land. Indeed, the word amphibian means "double life."
Early amphibians were the first vertebrates (animals with backbones) to live on land. They evolved from fish that had developed the ability to breathe air, and they have retained a few key features of their aquatic ancestors. In particular, amphibian eggs do not have tough shells to stop them from drying out, so they usually have to be laid in water. The eggs hatch as aquatic larvae, which eventually turn into air-breathing adults.
Amphibians have evolved an amazing variety of adaptations to help them survive on land, making them the most diverse group of land vertebrates. Despite this, nearly all amphibians share some key features that have a big influence on their way of life.
With their beautiful feathers, fascinating habits, and amazing flying skills, birds are among the most colorful and intriguing of all animals.
Birds are the most familiar of all wild animals. They live all around us and are easy to watch as they search for food, build their nests, and raise their young. Like us, they use sight as their main sense and have a strong sense of color. But there is something else about birds that makes them particularly interesting. Recent research has proved that they are small, flying dinosaurs, descended from relatives of ancient hunters such as Velociraptor. Many of these long-extinct dinosaurs had warm, insulating feathers just like those of birds today, but the birds turned them to even better use - by taking to the air.
Birds are warm-blooded vertebrates. Their bodies are covered with feathers, they lay eggs, and most birds are able to fly.
Warm-bodied, often furry mammals are the animals that we find easiest to understand, and for a very good reason. We are mammals too, and share many of the features that have helped them thrive.
The first land mammals evolved from reptile-like ancestors at about the same time as the earliest dinosaurs, near the beginning of the vast span of time we call the Mesozoic Era. But while the big dinosaurs disappeared in the catastrophic extinction that marked the end of that era, around 66 million years ago, the mammals survived. They went on to colonize the entire planet, evolving adaptations that have allowed them to flourish in every possible habitat. There are mammals living everywhere, from the polar ice to the sun-scorched deserts, and from the highest mountain peaks to the deepest oceans.
All mammals are warm-blooded vertebrates (animals with internal skeletons) that feed their infant young on milk. All 5,400 species give birth to live young, except for the platypus and spiny anteater, which lay eggs.