The “vertebrates” refers to all animals with a vertebral column or backbone. In addition, most living vertebrates also possess jaws, teeth, and paired fins or limbs, which are used for swimming, climbing, hunting, and interacting with the environment. These features are unique to vertebrates and distinguish them from invertebrates, the vast majority of all animals that lack these characteristics. Fossils of the earliest vertebrates help us understand how these features originated and how they evolved and diversified over time.
All vertebrates are chordates – a species (or sub-kingdom) of eukaryotic (non-fungal) animals containing all living organisms except fungi and algae. Chordates are characterized by an internal skeleton composed of bone or cartilage, a central nervous system divided into the brain and spinal cord, and not more than two pairs of paired limbs. Vertebrates are further distinguished by a notochord, an elastic rod that runs the length of their spines and is covered by bone during fetal development.
In addition, the notochord supports the internal structures and protects the spinal cord from damage. It is believed that the emergence of the vertebrate body plan was driven by the specialized cell type known as the neural crest. This cell population has multipotency and can differentiate into cells that give rise to various skeletal and sensory tissues. The neural crest also forms the rudiment of the vertebral column.
Evidence suggests that the notochord was covered by bony material in early chordates, forming a backbone in some species. The formation of the backbone is associated with several other significant changes, including an expansion of the anterior neuroepithelium into a brain, the appearance of a new sensory system, and the evolution of a new mode of nutrition involving heterotrophic feeding.
Cambrian fossils from China show several well-developed features that suggest the ancestor of today’s fish, amphibians, reptiles, birds, and mammals had a vertebral column. These include Myllokunmingia, a fish that appears in the Lower Cambrian Maotianshan shales and is characterized by an extremely long, narrow rostrum or snout, which supports sharp teeth; Metaspriggina, another Cambrian fish with a very short rostrum; and Haikouichthys, an extraordinary Cambrian fish that had a remarkably complete skull and braincase.
The earliest tetrapods (four-legged vertebrates) evolved about 375 million years ago from lobe-finned fish ancestors like Eusthenopteron. Interestingly, the earliest tetrapods and lobe-finned fish shared similar features, including the same pattern of one big bone (your humerus) connected to smaller bones (your radius and ulna).
The ancestors of frogs and mammals evolved from these tetrapods around 300 million years ago. The earliest amphibians were the first vertebrates to leave the water and live on land, but they had to return to the water to reproduce. Later, the earliest reptiles and birds evolved from amphibian-like ancestors that could lay eggs with internal fertilization.