Chordates are animals that have a dorsal nerve chord. The largest subgroup in the Phylum Chordata is the Vertebrata, those chordates that encase their nerve chord or spinal chord in hard bony vetebrae. The smaller subgroups in the phylum are sometimes put in their own separate Phyla but if we look at them we can see how our group evolved to become the biggest and most dominant animals on earth.
Let's start with a little worm-like animal called Amphioxus. Although it superficially resembles all the other invertebrate worm types, it has a special characteristic that sets it apart: its notochord. This dorsally runnng nerve chord is the same as the one that develops in the embryos of all the vertebrates and eventually becomes the spinal chord of the adult animal. The evolutionary sequence appears to be: worm-like animal with a dorsal nerve chord to early fish. The fish radiated out in all directions once they evolved fins for swimming. The first fish had cartilaginous coverings on their nerve chords for protection. Their descendants are the modern sharks and rays. Other fish developed bone and a hard skeleton beginning with the vertebrae, which not only provided protection for the all important nerve chord but also gave internal strength that eventually allowed amphibians, reptiles, mammals and birds to leave the water and conquer the terrestrial environment.
The difference between Amphioxus and the rest of the invertebrate worms occurs at a fundamental level. All worms have a head end where a mouth, sensory organs and some kind of nervous system develop. But in most groups of worms, the digestive system runs from the mouth through a dorsal tube back to the anus. The nerve centre at the head end extends into a ventral nerve chord that runs back through the body to send sensory information and instructions for muscles to move back and forth. This worm-like body develops early in the embryological sequence from the original ball of cells that is produced when the fertilised egg divides over and over.
In most invertebrates the ball of cells first develops a hole called a blastopore and then a head develops around this hole which becomes the mouth. The cells migrate and differentiate to form a worm shape with head, body and tail from the ball of cells with the blastopore becoming the mouth. Except in Amphioxus, the Chordates and the Echinoderms (starfish and sea cucumbers). In these groups a second hole develops to become the mouth. THis leads to the development of a worm that is upside down and backwards to the other worms with a dorsal nerve chord and a ventrally running digestive tract. Just think about a prawn and a mouse. They both have a head but the prawn's digestive tract runs along its back, as anyone who has cleaned shrimp or prawns knows. The animal's nerve chord runs along the bottom. In the mouse, the backbone holds the dorsal nerve chord and the mouth leads to a ventrally running esophagus with the stomach being on the bottom side of the animal.
So Chordates all share a common embryological characteristic with the Phylum Echinodermata and they are thus known as Deuterostomes to separate them from the Protostomes, which contains all the diverse Phyla that we call the Invertebrates. Deuterostomes must have made their evolutionary change and diverged from the rest hundreds of millions of years ago when multicellular life was just beginning and most multicellular forms were still just colonies of cells like the modern day Volvox. Deuterostomes made a fundamental change, perhaps one that was a mutation at the time but it led to a fundamentally different path of development which had profound influences on the type of animals that could evolve. Most importantly it led to an endoskeleton instead of an exoskeleton.
The top Protostome group is the phylum Arthropoda. These animals have not only a ventral nerve chord but they have an exoskeleton that must be shed for the animal to grow and which limits the upper sizes possible for individuals. The Arthropods may dominate in terms of numbers of species but the Vertebrates definitely win in the size competition. No insect could ever rival a Blue Whale! Chordates did this by building off the dorsal nerve chord, making it not only the channel for nervous control of the body but also the structural centre of the body. With a internal vertebral column, muscles could radiate out to a large body and the internal support structures for limbs could be hung on it. It is the endoskeleton that allows Vertebrates to get so big and thus become the dominant members of their ecosystems.
We are Chordates. We share this phylum with the sharks, bony fish, amphibians, reptiles, birds and mammals. And we all start our development in the same way. We develop first the ventral nerve chord and the worm-like shape and then limbs and a tail. Eventually our tail and gill slits disappear and we develop into human beings. But traces of our evolutionary journey still show in our embryological development linking us to all the other members of the great Phylum Chordata.