You have seen that the animal kingdom is divided up into a number of major groups. Each group is usually referred to as a phylum. Man belongs to the phylum Chordate. All animals belonging to this group must possess a notochord, that is, a structure like a spine at some stage during their life cycle.
In some animals, such as the graptolites that have already been discussed, this notochord remains in its elementary state. In the case of a group of chordates known as the Craniata, this noteochord develops into a support for the body, and the skull or cranium which protects the central nervous system. As you might have guessed by now, the Craniata includes all the birds, mammals, and so on, and of course, man himself.
The Chordata are thought to be very closely related to the Echinodermata; the group of animals without backbones which includes sea urchins and starfish. This can be proved by the great similarity of certain chemicals contained in the bodies of both groups. It is certain that all of the ancestors of the chordates lived in the sea.
The earliest vertebrates-animals with backbones were probably as long ago as 510 million years. Their remains have been found in Cambrian rocks in Arizona in North America. They are fragments of the scales of creatures, called agnathans, which were fishes that had no jaws or teeth.
These small animals had skeletons of cartilage and they were covered by amour in the form of bony plates, or scales, and it was these that were found in Arizona. The oldest known vertebrate fossils known in Britain come from central Scotland. They are about 420 million years old. That first vertebrate remains are, therefore, parts of primitive fish, and it is thought that they lived in the place where they are found.
It is interesting to compare these remains with the bones of modern jawless fish. Modern jawless fish do not have effective kidneys because the salts in their bodies are in balance with their surroundings. It is reasonable to suppose that the same was true for their early vertebrate ancestors.
This sounds even more likely when we take into account that there is more energy in the form of food available in the sea so that evolution is more likely to come about. The first backbone simply acted as a support for the fish’s muscles, but later it evolved to protect the brain. The first fish were armored fish.
The backbones should not really be called bones at all, because they are made of cartilage, and bone merely provided the fish with protective armour. From that time onwards vertebrates evolved very speedily indeed.
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