What are bones? - where did our bones come from?
Welcome to Quatr.us Study Guides!

What are bones?

Fish skeleton
Skeleton of a fish

One-celled animals don't have any support system that holds them in a certain shape. Plants do have a support system, but it's the cellulose in the cell wall of each cell, rather than bones. Early multi-celled animals like jellyfish and sponges and worms also didn't have skeletons, but beginning with arthropods, about 550 million years ago, animals began to make specialized structures to support their bodies and give them a definite shape.

The earliest skeletons were exoskeletons - they were on the outside of the animal. Modern lobsters, crabs, and snails have exoskeletons. These hard shells formed on creatures during the

By about 510 million years ago, eels began to have their skeletons on the inside as well as on the outside (their scales). They evolved first notochords and then vertebrae to protect their delicate spinal cords. These early inside skeletons (endoskeletons) were made out of cartilage rather than bone. Sharks still have skeletons made out of cartilage.

By about 480 million years ago, some fish were beginning to have teeth, so they could eat other fish. To protect their heads, some fish evolved their teeth into skulls - the earliest skulls look like lots of tiny teeth! Fish also evolved two sets of fins - one near their heads, and the other about half-way down - to help them swim faster.

Frog skeleton
Frog skeleton
(thanks to Bone Clones)

During the Devonian period, about 400 million years ago, many seas were very shallow, and fish evolved to be able to live in very shallow water. Their fins developed into four legs, because walking worked better than swimming in these puddles, and they developed fingers to help them balance when they were walking.

Gradually these lungfish evolved into frogs, about 375 million years ago, and lived more and more on land. Their bodies were not supported by the water anymore, and they needed stronger bones to hold them up. How did the frog skeleton change from the fish skeleton? Can you see which parts match up?

Starting from the head, you can see that frogs have lost all their fish teeth. They didn't need teeth to eat the little insects they lived on. Like fish, frogs have hard skulls, but they have much bigger eye sockets, because they needed better eyes to catch the flying insects. Where fish have vertebrae and ribs, frogs have them too, but fewer of them.

More about skeletons
Learn by doing - Fish skeletons
Learn by doing - Vertebrae

Bibliography and further reading about skeletons:

Finger bones
Quatr.us home

LIMITED TIME OFFER FOR TEACHERS: Using this article with your class? Show us your class page where you're using this article, and we'll send you a free subscription so all your students can use Quatr.us Study Guides with no distractions! (Not a teacher? Paid subscriptions are also available for just $16/year!)
Please help other teachers and students find us: link to this page from your class page.
Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram or Twitter, or buy her book, Vandals to Visigoths.
Cite this page
  • Author: K.E. Carr
  • Title:
  • Site Name: Quatr.us Study Guides
  • Publisher: Quatr.us
  • Date Published:
Proud of your class page, homework page, or resource page? Send it in and win a Quatr.us "Great Page!" award!
Sign up for more free articles and special offers in Quatr.us' weekly newsletter:
We will never share your e-mail address unless you allow us to do so. View our privacy policy. Easy unsubscribe links are provided in every email.
Comment on This Article

Does your class page honor diversity, celebrate feminism, and support people of color, LBGTQ people, and people with disabilities? Let us know, and we'll send you a Diversity Banner you can proudly display!
Looking for more?
Quatr.us is loading comments...
(Comments will appear after moderation, if they are kind and helpful. Feel free to ask questions, and we'll try to answer them.)
Cite this page
  • Carr, K.E. . Quatr.us Study Guides, . Web. 25 March, 2017