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Bats, sharks, and frogs have something in common with elephants, eagles, and even humans – each one has a backbone. A backbone is made of a series of separate bones called vertebrae. Joints between the vertebrae allow the bones to move, so the backbone is flexible. Animals that have a backbone are called vertebrates.
There are about 40,000 different kinds of vertebrates on Earth today and they’re all descended from the very first simple vertebrate creatures that lived on Earth about 500 million years ago. Scientists group vertebrates based on common characteristics.