Delving into the Depths of Fish Evolution

Fish were the first ever vertebrates on our planet, without them, we wouldn’t even exist. The first fish provided the basic body plans and shapes for millions of future species of animals. We pretty much owe our success to these early fish.

The earliest fish didn’t look much like the fish we know in today’s world. They first appeared after the Cambrian explosion which started around 542 million years ago. Before this explosion, most organisms were really simple cells.

These early fish had no backbones, and no jaws. They were small and worm-like and are known as Agnatha. Agnatha literally means ‘no jaws’ in Greek.

There are only two jawless fish which still exist; they are the Hagfish and the Lamprey. The Lamprey gives us a good idea of what these early fish would have looked like.


Towards the end of the Cambrian period, which lasted for around 50 million years, other eel type fish known as Conodonts, and small bony plated fish known as Ostracoderms, began to evolve and appear.

Conodonts had a notochord (a flexible rod) running through their body, similar to cartilage and were anywhere from 1 to 40cm in length. Ostracoderms were around 12 inches long and had gills designed specifically for breathing. Before this species, gills were used both for breathing and feeding.

The Pikaia was one of the next heavily studied fish, through its fossils. This fish like creatures has four essential features which are thought to be the prerequisite for vertebrates as we know them today. This leaf-shaped animal had:

– A head (obvious head, separate from the tail)

– Bilateral symmetry (same shape either side)

– V-shaped muscles

– A nerve cord running throughout its body

It is the nerve cord that laid the foundation for all future vertebrates. The cord in the Pikaia was not surrounded by a bone or tube so this fish cannot be considered a vertebrate, but instead was a chordate.

It is thought that the first true vertebrates were the Haikouichthys, and the Myllokunmingia, however – they still didn’t have jaws.

The jaw was an essential development which allowed fish to feed more easily, and become predators. Given that almost all of jawless fish are now extinct – it’s obvious to see why this was a crucial development for them.

The first jaw is thought to have evolved during the Silurian period around 430 million years ago, in Placoderms and spiny sharks. Placoderms were heavily armoured fish with bony plates; they ruled the waters during their short time. They were mostly small in size, but some reached lengths of 3-4 metres. They became extinct during the end of the Devonian period, around 80 million years later. Spiny sharks were only around 20cm in length and whilst they didn’t evolve and diversify as much as the Placoderms, they did survive for longer. They became extinct during the Permian period, 290 million years ago.

Also during the Silurian period, Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish) developed.

The Devonian Period stretched from 358-719 million years ago, and is also known as the age of the fish. This period saw a huge increase in diversification of fish as they evolved and developed into many different shape and sizes.

The Osteichthyes developed during this period into two main groups:

1. Ray finned (Actinopterygii)

2. Lobe-finned (Sarcopterygii)

Tetrapods (vertebrates with true legs) evolved from the lobe-finned fish, and they are the ancestors to all land-dwelling creatures.

Earth then experienced another mass extinction – the largest in the Earth’s history during the Permian-Triassic periods. It wiped out around 96% of marine species, but they bony fish were able to recover and diversify again.

Wrap Up

With over 29,000 species of bony fish currently alive, they make up around 95% of all fish and form the most diverse group of vertebrates on the planet.

The fish that we see in the oceans today are a result of millions of years of fascinating evolution.

Infographic link:


About the Author: 

Rob Woods is a third generation fish keeper and has raised freshwater fish since childhood. He currently has a 150 gallon freshwater tank and has educated over 250,000 people through his blog Fishkeeping World.

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