While it might seem odd to think about, fish are the largest and most diverse group of animals on the planet. They are found across the globe from freshwater creeks to the darkest, coldest depths of the ocean. But this begs the question, where did all of these different forms of fish come from?
Fish are one of the oldest forms of life on Earth and have existed for hundreds of millions of years. They were the first animals to develop spines and eventually evolved into the first animals to step onto dry land. This means modern fish and modern people share a common ancient ancestor.
What is Evolution?
To understand how fish evolved it is important to understand what evolution is. The Theory of Evolution is designed to explain how diverse animal life is today by explaining how animals become different from each other. The process of evolution is called natural selection.
Animal populations have different traits that help them survive in their environment. Animals with more helpful traits or who develop mutations that make surviving easier go on to have more offspring. These offspring then have the same traits and mutations as their parents, making it easier for them to survive. For this reason evolution is sometimes called survival of the fittest, since the animal that best fits its environment survives.
If the environment around a group of animals changes then what traits or mutations help them survive also change. This causes them to develop new traits and evolve over time into new species as groups are separated and exposed to new environments.
Since most living beings have certain traits in common the Theory of Evolution suggests that all organisms have a common ancestor or group of ancestors. The theory says that this ancestor or group of ancestors evolved over millions of years to become the millions of different species that exist today.
The First Fish and the First Spines
The first fish developed during a period called the Cambrian Explosion, which took place 542 million years ago and took millions of years to complete. During the Cambrian Explosion many forms of new life emerged on Earth, including the ancestors of modern fish. In fact, these first fish were also the first vertebrate animals, meaning animals with internal backbone, on Earth.
Examples of these early fish, like the Pikaia and the Haikouichthys, looked more like worms than modern fish. While they did have distinct heads, eyes, and tails, they had no jaws and the nerve chord running down their backs had little to no bone around it. Later fish would develop full spines and skeletal systems.
Another early fish from this era, the Myllokunmingia, was one of the first animals with gill pouches. Before gill pouches fish absorbed oxygen directly through their skin, but as they grew they needed a more efficient system to breathe with. Gill pouches and later gills had more surface area than a fish’s skin letting them take in more oxygen and release more waste.
Between 490 and 410 million years ago during the Ordovician period jawless fish dominated the oceans, lakes and rivers of the Earth. These fish could not bite or catch prey instead they would filter feed by sucking up small marine animals and waste of the sea floor. Lampreys are considered a modern equivalent of what these early jawless fish might have looked like.
These jawless fish also had thick shells of bony plates instead of scales. This made it harder for them to maneuver in water but protected them from predators like the giant sea scorpion. These animals, called eurypterids, are the earliest ancestors of arthropods such as insects, spiders, and crustaceans.
The Age of Fish
During the Silurian period, 443 million to 419 million years ago, the second largest extinction event in the Earth’s history took place and wiped out 70% of all species on the planet. This was due to the oceans rising in level and temperature. During this time the first fish with forked tails, moveable jaws, and bones began to appear.
After this the Devonian period from 419 million to 358 million years ago became known as the Age of Fish. Here three different forms of fish developed, placoderms and the two forms of bony fish, ray-finned and lobe-finned. Cartilaginous ray-finned fish also started to appear around this time.
Placoderms were larger, jawed versions of the jawless fish from older periods. They had plates of bony armor instead of scales which did protect them from danger but also made them much slower. This made it harder for them to hunt and compete with the other new forms of fish such as sharks and ray. By the end of the period placoderm fish were extinct.
Bony fish had a full internal unlike placoderms, which made them faster and more maneuverable in the water. Ray-finned fish, sometimes called catchers, became the ancestors of all modern fish, the largest and most diverse group of vertebrates on the planet. Among these fish were some of the first cartilaginous fish, meaning their skeletons were made entirely of rubbery cartilage. These were the ancestors of modern sharks and rays.
Lobe-finned fish, also known as anglers, would evolve into the first tetrapods, four-legged animals, and eventually crawl out of the water onto dry land around 450 to 350 million years ago. This makes lobe-finned fish the ancestors of all land vertebrates today. One of the earliest tetrapods was the Tiktaalik which had primitive wrists and a neck which let it move on dry land for a short time.
Mass Extinctions and Recoveries
Around 252 million years ago the Permian-Triassic Extinction Event occurred, the largest mass extinction in world history. 96% of all marine life on the planet died during this event. Scientists are still debating what caused this event including volcanoes blocking out global sunlight and the forming of Pangea destroying shallow oceans. It is likely a combination of many factors and events led to this extinction event.
While most of the marine life on the planet disappeared this also created room for other animals to evolve. Fish with cartilage-based skeletons like sharks were almost wiped out during this event but ray-finned bony fish were able to recover quickly and thrive. For example, large fish such as sturgeons began to appear in the fossil record around this time.
The Permian-Triassic Extinction Event ended with the Mesozoic era, which lasted from 251 to 65 million years. The Mesozoic saw the rise of large bony fish as well as large reptiles, amphibians and aquatic dinosaurs. One famous fish from this era was the Leedsichthys, which measured 70 feet long and fed mainly on plankton and krill, similar to modern baleen whales and the whale shark.
The Mesozoic Era ended with the Cretaceous Extinction event, in which a meteor landed in the Yucatan Peninsula in Mexico. The impact created a dust cloud that blocked out sunlight and caused the mass death of plants and prey animals large predators needed to feed on. During this event many species, including the dinosaurs, became extinct across the world.
The death of dinosaurs in the Cretaceous Extinction event created an opportunity for mammals to dominate the land. However, researchers now also believe that the death of marine dinosaurs such as the Mosasaurs and Ichthyosaurs paved the way for ray-finned fish to thrive. This created a New Age of Fish as ray-finned fish could grow, reproduce and diversify since their biggest predators and competition were now extinct.
The Modern Fish
The last of the massive fish, the Megalodon, first appeared 20 million years ago and is believed to measure up to 67 feet long. Due to its size and evidence found on other fossils it is believed that the Megalodon fed on whales and other large marine mammals. The Megalodon went extinct around 2.6 million years ago along with many large aquatic predators when lower global temperatures killed off much of their prey.
Despite what happened to the Megalodon, many species exist that act as living records ancient species. One example this type of fish, sometimes called a living fossil, is the Coelacanth. The earliest fossil example of the Coelacanth dates back 400 million years but they were believed to be extinct. This changed when South African fishermen caught a live Coelacanth in 1938, with more being caught along the Indian Ocean.
Another modern lobe-finned fish that is teaching biologists about fish evolution is the African Lungfish. Lungfish have two unique traits that separate them from almost all other fish alive today, they have lungs and they can move on land. Studying lungfish has let researchers see what early lobe-finned fish might have looked like. These studies even suggest that the first animals on land might not have been tetrapods, but instead early lungfish.
Today over half of all vertebrate species are fish, around 32,000 separate species and, according to estimates, at least 3.5 trillion individual fish. They can be found all over the world from Challenger Deep, the deepest point on Earth, to lakes in the Himalayan Mountains.
Fish are, in a way, one of the most important animals in evolutionary history. The first primitive fish to grow spines and skeletons are not only the common ancestor of all modern fish but all vertebrate animals as well, both on land and underwater.
Without the evolution of fish, from jawless fish to placoderms to bony fish, life would not exist the way it does today. So the next time you pass an aquarium or fish tank, spare a thought for all the millions of years of history those fish represent.