The First Vertebrates

The vertebrate animals include mammals, birds, reptiles, amphibians and fish (pictured left). They all have a backbone that encloses their spinal cord. They also have, at some stage in their life history, gill slits in the region of the head.

The earliest vertebrates

The earliest vertebrates were fishes. Fossil remains of the oldest fishes are found in rocks from Ordovician times, about 480 million years ago. It is not clear from which animals the fishes evolved. One possible clue comes from the way vertebrates develop while they are embryos. Their hard backbone forms around a softer stiffening rod called a notochord.

Amphioxus

One of the simplest creatures with a notochord and gill slits is a small sea animal called Amphioxus. It has a large number of very fine gills. Similar gills are also found in the acorn worms, which burrow in the mud and sand around some of our coasts. The acorn worms' microscopically small larvae swim by means of bands of tiny wavering hairs called cilia.

Almost identical larvae are found in the echinoderms, a group of invertebrates which includes the starfish, sea urchin, brittle-star and sea cucumber. So it is possible that the ancestors of these spiny skinned animals gave rise to the ancestors of the fishes.

Amphioxus

How fishes evolved

How did this change from echinoderm to fish ancestor take place? We can only guess that about 500 million years ago there was a kind of little worm, only two or three centimetres long, swimming through the sea. It had developed gills and a simple sort of spinal nerve. After millions of years, one of its descendants grew a fold or ridge of skin along its back which helped it to swim and turn even faster. Slowly the worms of this type grew larger and more complicated. Eventually creatures similar to our present day Amphioxus developed.

Some of the descendants of Amphioxus began to grow harder outer coverings which protected them against attacks from other creatures of the sea-bottom that tried to eat them. These ancestral fishes had no jaws but they were heavily armoured, with their bony skeleton protecting their head and the front part of their body, and thick bony scales over the rest of the body. Although these animals could no longer swim quite so fast, their hard outer coverings protected them even more. One of the best known of these heavily armoured, jawless fish is Cephalaspis.

Cephalaspis has its eyes on top of its head, suggesting that it
probably lived on the sea-bottom, sucking food from the mud.

Lampreys and hagfish

Today there are still some strange parasitic creatures called lampreys and hagfish which have basket-like supports around their gills, paired fins and no jaws. They are believed to be descendants of the ancient jawless fishes.

Although the jawless fishes survived into the Devonian period, they were joined by other primitive fishes called placoderms. The name means 'plated skins', a reference to the body scales many of them had. Like the jawless fishes, many of the placoderms also had bony armour over their bodies, but they differed in having movable jaws with teeth, and paired fins like modern fishes.