Animals with backbones are called vertebrates, and all vertebrates have a well-developed brain and a living endoskeleton, among other things. An endoskeleton is one that is made from bone or cartilage and grows and survives inside the body, rather than on the outside.
All mammals are vertebrates. The skeleton of mammals contains both bone and cartilage and most have a cranium, or skull, which encloses the brain. Vertebrates also have a muscular system that attaches to the endoskeleton, making movement possible.
Animals with backbones also have a closed circulatory system, which means the animal's blood is completely enclosed in the body.
Scientists believe the first vertebrate came along around 525 million years ago. It surfaced during the Cambrian period and, unlike other animals of the time period, the Myllokunmingia had a well defined tail, head and backbone. Other animals during that time were more eel-like in appearance.
Examples of vertebrates include humans, dogs, cats and most fish and frogs.
By contrast, invertebrates have no backbone and often lack a solid bone structure of any kind. Examples of invertebrates include octopuses, snails, crabs and insects.
Although it may not seem possible, snakes are actually vertebrates. A snake has a backbone that is attached to tiny ribs all along its body. This type of bone structure allows it to be flexible and move in ways other animals can't.