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It's complicated:
New Evidence for Genome-Wide Duplications at the Origin of Vertebrates Using an Amphioxus Gene Set and Completed Animal Genomes
http://genome.cshlp.org/content/13/6a/10…
Origin and early evolution of vertebrate skeletonization
http://onlinelibrary.wiley.com/doi/10.10…
From the American to the European amphioxus: Towards experimental Evo-Devo at the origin of chordates
http://www.ijdb.ehu.es/web/pdfdownload.p…
An aboral-dorsalization hypothesis for chordate origin
http://onlinelibrary.wiley.com/doi/10.10…
Genome duplication and the origin of the vertebrate skeleton
http://people.biology.ufl.edu/mjcohn/Pub…
The integumentary skeleton of tetrapods: origin, evolution, and development
http://www.ncbi.nlm.nih.gov/pmc/articles…
New Evidence for Genome-Wide Duplications at the Origin of Vertebrates Using an Amphioxus Gene Set and Completed Animal Genomes
http://genome.cshlp.org/content/13/6a/10…
Origin and early evolution of vertebrate skeletonization
http://onlinelibrary.wiley.com/doi/10.10…
From the American to the European amphioxus: Towards experimental Evo-Devo at the origin of chordates
http://www.ijdb.ehu.es/web/pdfdownload.p…
An aboral-dorsalization hypothesis for chordate origin
http://onlinelibrary.wiley.com/doi/10.10…
Genome duplication and the origin of the vertebrate skeleton
http://people.biology.ufl.edu/mjcohn/Pub…
The integumentary skeleton of tetrapods: origin, evolution, and development
http://www.ncbi.nlm.nih.gov/pmc/articles…
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According to an analysis of living animals and the fossil record, it happened this way:
Single celled animals evolved into simple colonies of undifferentiated cells, which then led to multicellular animals with cell specialization. Nerve cells evolved, and they prove to be advantageous, so advantageous that practically all multicellular animals have them, because the frist animal that came up with nerves became the common ancestor of a vast majority of animals on earth. In some groups of animals, nerves evolved into an interconnected network. In some of these animals with nerve networks, the notochord, which is a bundle of nerves evolved. As one can see, in each of these stages of evolution, the animals are favored by natural selection, and the parts that evolved are adaptive.
The notochord remains unprotected in some living chordates. In others, however, the notochord is enclosed by a vertebral column. Here is how the vertebral column probably evolved. Among early chordates, a group of fossils that puzzled scientists for decades evolved. These were the conodonts. These were tooth-like microfossils with no body parts attached to them. Then, one day, a conodont fossil was found with an impression with its body and they turned out to be eel-like animals with teeth that allowed them to eat. The body parts were identifiable as chordates as there are fins, chevron shaped muscles and a notochord. So, teeth or bone like elements apparently first evolved in the conodonts, and were probably used for feeding.
Single celled animals evolved into simple colonies of undifferentiated cells, which then led to multicellular animals with cell specialization. Nerve cells evolved, and they prove to be advantageous, so advantageous that practically all multicellular animals have them, because the frist animal that came up with nerves became the common ancestor of a vast majority of animals on earth. In some groups of animals, nerves evolved into an interconnected network. In some of these animals with nerve networks, the notochord, which is a bundle of nerves evolved. As one can see, in each of these stages of evolution, the animals are favored by natural selection, and the parts that evolved are adaptive.
The notochord remains unprotected in some living chordates. In others, however, the notochord is enclosed by a vertebral column. Here is how the vertebral column probably evolved. Among early chordates, a group of fossils that puzzled scientists for decades evolved. These were the conodonts. These were tooth-like microfossils with no body parts attached to them. Then, one day, a conodont fossil was found with an impression with its body and they turned out to be eel-like animals with teeth that allowed them to eat. The body parts were identifiable as chordates as there are fins, chevron shaped muscles and a notochord. So, teeth or bone like elements apparently first evolved in the conodonts, and were probably used for feeding.
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