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Fish are likely to be the oldest vertebrate group on Earth.And the most primitive kind of fish is that of the jawless class, also known as 'Agnatha,' which means the same thing; many of these also possessed a kind of body armour formed by scales and massive plaques. In fact, current theories state that it is quite likely that fish are the evolutionary result of annelid worms trying to protect themselves from predators.

These annelids worms were formed by rings and could move freely in the oceans of the Ediacaran and Cambrian periods. One example of such annelids is 'Dickinsonia,' from the Ediacaran period; this worm could achieve lengths of up to one metre, and naturally, was a good prey for any hunting animal of that time.

The first predators were very different from what we have in mind now as our definition of carnivorous animals but ironically, their feeble preys seem to have been the ancestors of almost all animal life forms today. These predators are long gone and little is known of them, but their sudden appetites and the emergency evolutionary response that followed is what eventually produced the biomass that we can see today.

From these worms, the various vertebrates and invertebrates that appeared during the Cambrian evolved into very distinct groups; however, in the beginning of that change, both vertebrates and invertebrates would have looked quite similar and differences would have been very small. On one side, jawless fish appeared, and on the other, arthropods like trilobites.

Jawless fish gathered their food by suctioning thorough an open mouth that reminds of a vacuum cleaner; this is one of the elements that seem to confirm that these early vertebrates were related to annelid worms. And regarding the rings that we mentioned, and that are characteristic of annelids, from them came the first plaques and scales that began evolving first as a somewhat crude armour, and later into something more streamlined.

Over time, these life forms evolved and get more specialised; during the Ordovician period, fish did not look quite like we see today, but slowly during the Silurian and Devonian, they acquired the general shape that we now associate with these animals. Sharks and rays represent the next stage in fish evolution: they have a cartilaginous body and jaws; this group is known collectively as 'Gnathostomes,' meaning or indicating that they have jaws.

The oldest shark found corresponds to the Silurian; it was about 2 metres long and has been named 'Cladoselache.'

Almost all families of primitive fish are now extinct. Lampreys are a rare exception: these descend directly from the early, jawless fishes, although by now they have also evolved and show several significant differences, something that anyone could reasonably expect after five hundred million years of evolution.

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