Animals evolved through a process known as evolution which is the change in a species' characteristics over several generations. Evolution can be caused by mutation, migration, natural selection, genetic drift, and non-random mating. Animals are organisms from the Kingdom Animalia which are multicellular eukaryotes. Some invertebrates evolved a rigid rod along the length of their body. This rod is called a notochord. The notochord gives the body support and shape. It also provides a place for muscles to attach. It counterbalances the pull of the muscles when they contract. Animals with a... Show more

Animals evolved through a process known as evolution which is the change in a species' characteristics over several generations. Evolution can be caused by mutation, migration, natural selection, genetic drift, and non-random mating. Animals are organisms from the Kingdom Animalia which are multicellular eukaryotes.

Some invertebrates evolved a rigid rod along the length of their body. This rod is called a notochord. The notochord gives the body support and shape. It also provides a place for muscles to attach. It counterbalances the pull of the muscles when they contract. Animals with a notochord are called chordates. All of them are placed in Phylum Chordata. Living invertebrate chordates include tunicates.    
The earliest vertebrates evolved around 550 million years ago. It happened when some chordates evolved a backbone to replace the notochord after the embryo stage. They also evolved a cranium, or bony skull. The cranium enclosed and protected the brain. The earliest vertebrates probably looked like hagfish.    

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