Broadly speaking, animals can be classified into one of the following: mammals, birds, fish, reptiles, or amphibians.
Where it All Began
Scientists believe that the first living beings appeared on Earth more than three billion years ago. These were very simple amoeba-like creatures. Then, gradually, more complex creatures developed. Probably the invertebrates were first, then fish, amphibians, reptiles, birds, and finally mammals.
Scientists have pieced together the evolution of different animal classifications from layers of ancient rock formations. Preserved in each layer are traces of the life that existed on Earth at the time the rock was formed. These traces, pressed into the sediment, are called fossils.
The Ages
After the first vertebrates appeared, there were so many fish that this period of time is called the Age of Fishes. Then came the age of amphibians when great swampy forests covered much of the world. Next, came the Age of Reptiles and then the Age of Mammals.
When new animals appeared, the old ones didn’t always die out, but we do know some animals from the past that are already extinct. Perhaps the best known extinct animals are the dinosaurs which were the strongest force on Earth for millions of years.
What’s the Difference?
Mammals are warm-blooded, tend to have fur or hair, and feed their babies milk from the mothers’ glands. Most mammals bear live babies. Animals with feathers belong in another group: birds. Birds are also warm-blooded, but they lay eggs from which their babies hatch. All other vertebrates are cold-blooded. Cold-blooded means that the animals’ temperature changes to closely match the temperature of their environment. There are three groups of cold-blooded vertebrates: reptiles (like turtles and snakes); amphibians (like frogs); and fish.