The vertebrates include fish (Chapter 9), reptiles and birds (Chapter 10), mammals (Chapters 11-12), and humans (Chapter 13). They have advanced sensory systems, internal bone structure, large brain, complex nervous systems, and organs that allow them to grow large and live on land.. All vertebrates descended from early vertebrate jawless lampreys that were discovered in the Chengjiang Lagerstatten (517 Ma). This chapter focuses on the origin of the early vertebrates as well as the evolution of jawed fish and bony fish from the Cambrian to Devonian periods.
When organisms or organs have a common origin, they are said to be homologous. Linda Holland, one of the world’s leading experts in chordate evolution, described the many homologous physiological and genetic characteristics that show that the cephalochordates approximate the status of chordate phylum prior to the appearance of the vertebrates.
There are many nonhomologous characteristics in vertebrates that are not in cepholchordates or other invertebrates. The appearance of so many unique characteristics at one time in the fossil record is unprecedented in the previous or subsequent evolution of animals. Subsequent evolution of complex capabilities such as advanced hearing and sight in mammals was in general a gradual process.
Neuralation is the folding process by which the neural plate becomes the neural tube in cephalochordates and vertebrates. In vertebrates, the neural crest forms over the neural tube. The neural crest divides in stem cells, which move to different parts of the body and establish the unique vertebrate organs such as the nervous system, heart, and kidney.
Many research studies in the last few decades compared vertebrate DNA to that of cnidarians, amphioxus (cephalochordate), annelid worms, and other animals in order to determine the evolutionary relationships at the base of the animal kingdom. There are three important findings. First, many genes in bilaterians are found in cnidarians. Second, a similar patterning of genes surrounding the vertebrate and annelid nerve cords (Figure 9‑16) indicates that the nerve cord evolved prior to the separation of deuterostomes (chordates) and protostomes.
The fossil record shows a gradual evolution of jawed and then bony fish from jawless fish (518 Ma). The evolution of the jaw took 80 million years. The jawed fish diversified into bony, cartilaginous, and lobe-finned fish by the Early Devonian Period, 20 million years later. Amphibians began moving to land after 60 million years at the end of the Devonian.
This section shows a debate between leading paleontologists. The topic is the timing of origin of the animal kingdom. The fossil record and DNA studies indicate that the animal kingdom began with sea anemones at the end (560 Ma) of the Avalon Explosion. Coelomates (arthropods, mollusks, annelid worms, etc...) do not appear prior to the Cambrian in 541 Ma, indicating that the divergence between protostomes and deuterostomes took place after 541 Ma; however, molecular clocks indicate that the divergence between protostomes and deuterostomes took place long before the Avalon Explosion.
in the second half of the fifth age, Moses described the supernatural creation of elongated monsters, creatures that move (like a fish), and every winged bird, all of which are vertebrates.
Vertebrates. Credit: Prehistoricplanes. Used here per CC BY-SA 4.0