The Evolution of Life and the Emergence of Fish

As with most things, it is often useful to go back to the beginning, to fully appreciate the point where you are now. For fish, as for us humans, the beginning began roughly four billion years ago.

At this point in time, our planet would have seemed completely alien to us today. The continents would have been arranged in some unrecognisable form and the seas, which had just formed from clouds of water vapour, would have been warm. The thin atmosphere, barren of much of today’s oxygen, would have allowed extreme amounts of ultraviolet light to penetrate it – far too much for modern life to have survived. This allowed rampant electrical storms to rage around the globe, stabbing the land and seas with bolts of lightning. Yet it was from within this harsh environment that life emerged. The foreign mixture of gases in the environment – combining with the earth’s cooling water vapours, the sun’s ultraviolet light and the copious amount of electrical discharge in the form of lightning – caused complex molecules such as amino acids, sugars and nucleic acids to form.

These are the building blocks of protein. Upon the arrival of these molecules, life took its first giant step towards appearing. Over millions of years, this process continued.

The Beginning…

The presence of these molecules in the early seas grew, until finally, through a process that is still shrouded in mystery, a crucial molecule appeared: deoxyribonucleic acid, known in the common tongue as DNA.

One readily available, convenient and constant source of energy was the sun.

DNA is special for two main reasons. The first is that it acts as a master plan to manufacture amino acids. The second is that it can replicate itself. The latter is especially important, as when a process is repeated over and over again, some part of it will undoubtedly go wrong. When this happens, the replica is no longer a replica; it is, in fact, entirely different. Bacteria often engage in this process in a similar fashion; when this happened with some of the earliest organisms, it represented the very first step in evolution. Originally, the first forms of bacteria fed on the carbon compounds that had accumulated in the seas over the preceding millions of years. However, through the processes of replication, mistakes were made and new forms of bacteria began taking advantage of the various resources that were available. One readily available, convenient and constant source of energy was the sun. Instead of consuming energy from exterior sources such as food, like we do, these early forms of bacteria produced their own energy within the confines of their cell walls, using a process we call photosynthesis. In this chemical reaction, carbon dioxide reacts with hydrogen to make glucose, which is then used as energy. Early in the history of photosynthesis, the process relied on hydrogen that was gassed off in massive amounts from volcanic eruptions. Consequently, these early photosynthesising life forms that made use of volcanically produced hydrogen were destined to remain isolated, living only near volcanic action.

First arrivals

However, with the passing of time, new forms of life known as cyanophytes emerged. These primitive forms of algae contained a chemical agent called chlorophyll, which allowed them to take advantage of a much more readily available source of hydrogen: water. This was another giant milestone in the evolution of life. Water is chemically known as H2O, which represents one molecule of hydrogen and two molecules of oxygen. When photosynthesis occurred and the CO2 reacted with hydrogen, the oxygen that was previously bonded to the hydrogen as water was simply gassed off. Over the ensuing millions of years, the oxygen that this process left behind began to build up in the atmosphere. This provided the planet with the ozone layer, a thick shield around the earth that limits the amount of ultraviolet rays the earth receives from the sun. It also supplies the earth with the oxygen-rich atmosphere we now rely on to survive.

This situation existed for billions of years. Unicellular organisms such bacteria, algae and protozoa ruled the globe. Through their diversification, amoeba emerged. Today, these tiny microorganisms are often found in ponds and murky pools; however, they were once at the forefront of evolution and through them, more forms of life evolved.

Multicellular life

Many of these new forms of life were multicellular in nature. The cells that compose multicellular organisms often specialize and communicate with each other, making more complex forms of life possible. A multitude of theories explain how multicellular life evolved. One of the strongest is that it was all due to these single-celled amoebas. It has been proposed that a group of cells formed a slug-shaped mass known as a Grex. A Grex is, very simply, a mass of cells that have gathered together and that then behave as a single organism. Generally, a Grex is caused by pheromones released by a stressed amoeba. This is essentially a ‘call to arms’ which attracts other cells. Once together, the cells differentiate themselves according to their positions. Some will become spores, while others will form the actual structure of the Grex. It is possible that this was how multicellular life took its first steps. Due to the success of this cellular cooperation, additional single-celled organisms began to interact and form more complex beings, such as sponges and jellyfish.

These beings evolved for millions of years, until the pre-Cambrian period, when an aquatic organism lived in a state of permanent immobility evolved a free-swimming larval stage. Over millions of years of evolution, the larval stage became increasingly mobile and became able to reproduce so that the organism’s adult phase gradually disappeared. It is then thought that these free-swimming organisms then became conodonts – Latin for ‘cone tooth’. Conodonts formed the earliest vertebrates. Very little is known of conodonts except that they were ‘eel-like’ and many authors think they may have had a rudimentary backbone.

The arrival of fish

For ease of naming, the fish that evolved at this point – and that exist now – are categorised into several classes. Originally the first fish to evolve were the Ostracoderms, which came into existence during the Cambrian period, around 510 million years ago. Being from the Agnatha class, these fish were jawless and had bony armour. These fish did not have paired fins. They were bottom-dwellers that fed on invertebrates and organic debris and they would have swum in a ‘tadpole-like’ fashion. There is still some debate about how the Ostracoderms relate to modern jawless fish but fish belonging to the Agnathaa class dominated for around 100 million years, until jawed fish emerged. These jawed fish were split into four evolutionary lines: the Placoderms which are now extinct, the Acanthodians which are also extinct, the Chondricthyes and the Osteichthyes. When the Placoderms emerged, they had heavy armour with articulated plating that protected the head and thorax. Placoderms emerged during the late Llandovery epoch of the early Silurian period and they quickly rose to prominence as the most diverse group of fish during the Devonian period.

For at least 70 million years they reigned in freshwater and marine habitats, where this now-extinct order exploited a variety of ecological niches, being amongst the first jawed vertebrates (Gnathostomata) their jaws most likely evolved from gill arches. Evolution continued to march on for millions of years, as nature’s remorseless scythe allowed only the most successful individuals to survive and pass on their genes and traits to their offspring.

The emergence of modern forms

Through this process of evolution, two classes emerged that would prove to dominate the world’s aquatic habitats for the preceding 400 million years. One was the Chondricthyes, commonly known as cartilaginous fish. They emerged in the late Silurian era and quickly became the apex predators of the deep. Chondricthyes are truly amazing creatures. Evolution gave them a cartilaginous skeleton that allows them to avoid utilising bone – a helpful trait in the battle against ‘sinking’. This is because cartilage is less dense than bone; when one combines this lightweight framework with the fact that chondricthyes commonly use fat-like substances called lipids to maintain a lower overall body density, one can easily see what has made fish within this class such a success. Evolving just before the Chondricthyes were the osteichthyes – the class that is most commonly kept in aquaria and that is arguably the most fascinating based purely on its massive range of diversity.

Modern Evolution

Osteichthyes generally have three pairs of gill arches, a swim bladder of some sort and a predominately bony skeleton. The massive diversity of this class is often attributed to the fact that osteichthyes underwent a slow evolutionary migration into freshwater, where a phenomenon known as freshwater speciation occurred. The unique diversity of freshwater environments allowed species to evolve at a much faster rate than their marine counterparts.

Freshwater speciation works in a way that if we divided two groups of fish from the same species, then put each group into a different environment, with different levels of predation and food sources, over time the fish would evolve to suit its habitat. After many years, the two populations might not be interested in breeding with each other, as they would have different priorities in terms of their selection of breeding partners. Eventually, small changes to their bodies, brought about by the small changes from evolving to suit their habit would create two differing anatomies, which would not be capable of breeding together at all. At this point, we would have two species where before there had been only one. These abilities to adapt to different environments and to take advantage of new resources and habitats is what has allowed fish to dominate the aquatic world. Leading to a massive range of species that occupy an array of environments, from the great rift lakes of Africa to the narrow canals of the United Kingdom.

Fish in the modern world

However, it is very important for any naturalist, biologist, ichthyologist or fishkeeper to remember our own perceptions of time. It is very easy to believe that because we are here, in the present, the species that exist today are final and that all their descendants simply evolved to become this particular species. Yet we must remember that this is far from the truth; evolution doesn’t have a goal beyond the individual’s need to survive. Also, we must recognise that evolution is a linear process that will continue long after we are gone, and that the species we see today will inevitably evolve into new ones. These future forms of evolution will continue in a manner similar to what occurred before. However, it is undeniable that man’s role in shaping and destroying the landscape will impact these species. That is why, as fishkeepers, conservationist and naturalist, it is our duty to consider how we impact the planet and how we conduct ourselves in regards to the world’s flora, fauna and atmosphere.

To learn more about how fish have evolved to fit their own specific environments and how they communicate within an aquatic atmosphere, click the button below: AQUATIC COMMUNICATION AND ENVIRONMENTAL ADAPTATIONS