Dear Dr. Universe,
Why do we have spines?
— Jessie, 10, Covina, Calif.
Your spine is more than just a long line of bones. It’s the secret to jumping for joy, the base for all your best dance moves. Every time you run, climb, walk, and play, your spine is right there with you.
“Without a spine, our ability to move would be completely different,” Edward Johnson said.
Johnson teaches Human Anatomy in the School of Biological Sciences at Washington State University. He is very curious about how all the parts of your body work together.
All creatures with spines are called “vertebrates,” including humans. They get this name from their vertebrae: the special bones that make up the spine. Your vertebrae are different shapes and sizes, but they all connect together.
These bones’ odd shape points to their purpose. Vertebrae have spiky parts sticking out, which click together and help them stack in a line. They also give places for muscles and ligaments to attach, which keep the spine straight and allow you to move.
“It’s kind of like sailboats that have a mast, with ropes that go down and help support it,” Johnson said. “Vertebrae are the same, with muscles and ligaments supporting and helping hold them in line.”
Vertebrae also have a hole in their middle, housing your spinal cord. Your spinal cord runs through them, carrying messages from your brain to the rest of your body. But it’s very sensitive and fragile. Your vertebrae have the important job of protecting it.
As a human, your spine gives your body rigid structure as it moves. But spines aren’t as common as you might think.
The first animals didn’t have spines. Even today, 90 percent of animals don’t have them. Think about beetles and spiders. Instead of bones inside their bodies, a hard covering called an exoskeleton protects their soft, gooey parts.
But vertebrates are different. They share both a spine and a common history. The story begins with the notochord: a tube full of fluid, and the backbone’s ancestor.
Millions of years ago, worm-like animals had the very first notochords. When the first fishes emerged, they also had a notochord. But they also had new, tiny bones: the first vertebrae.
In some animals, the notochord stuck around. Sharks and ancient fishes still have this very old combination of both notochord and vertebrae together. But in other animals, big changes in the spine happened over millions of years. Vertebrae got bigger and more important, and the notochord got smaller.
As a human, that includes you. You actually had a notochord for a while, while you were still growing inside your mother. But by the time you were born, your notochord had mostly disappeared. Vertebrae grew instead. Only parts of the notochord got left behind, in the squishy pads sitting between your vertebrae.
So the next time you bust a move, think about fish. Even though they can’t groove like you, you’ve got something in common: a notochord in your past, and a spine supporting your every move.
Have a science question? Ask Dr. Wendy Sue Universe, WSU’s resident science cat and writer, by email at Dr.Universe@wsu.edu.
