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Knowledge

Our Earth began its existence around 4.6 billion years ago as a glowing ball of gas. As it cooled, the crust, atmosphere and oceans gradually took shape.

Life first appeared in the oceans around 3.8 billion years ago. It consisted of simple cells without a nucleus: the prokaryotes. About 1.7 billion years ago, cells developed a nucleus and organelles, becoming eukaryotes.

About a billion years ago the first multicellular organisms appeared. In the oceans, algae produced so much oxygen that the atmosphere became rich in it. Gradually other organisms adapted to this toxic oxygen. Multicellular life grew increasingly complex and the earth became home to a wide variety of living forms. Some disappeared while others persisted. Algae, fish, plants, reptiles, dinosaurs, mammals… humans are one of the most recent results of this long process of adaptation.

The Proterozoic Era, 2.5 billion to 542 million years ago: the first signs of life

Because animals in this early phase of Earth’s history were still very small and had few hard body parts, very few fossils were formed. Most traces of life have also disappeared over time, as the Earth’s surface was still highly dynamic and constantly reshaping itself.

Only toward the end of this immense period, which lasted nearly two billion years, did the first fish begin to populate the seas around 540 million years ago. These early fish had no jaws, but they did have gills. Their mouths were small and better suited to feeding on tiny food sources, such as algae scraped from rocks.

Stromatolites: Living Fossils

The earliest life forms were so small that they left almost no trace behind. The first organisms to leave a clear mark are stromatolites. Although these life forms emerged around three billion years ago, they still exist today.
Along the west coast of Australia, in the tidal zone, you can see small domes and columns reaching about half a metre in height. They grow very slowly. A curious natural formation? It was only in the last century that scientists discovered they descend from the oldest recognisable life forms from the early history of our planet.

Foto: Collection Museon-Omniversum, Coll. nr. 82931

Cambrian Period, 542 to 488 million years ago: the Cambrian Explosion

At the beginning of the Cambrian Period, around 542 million years ago, nearly all the major animal groups we still know today were already present in the oceans, including worms, molluscs, arthropods and the earliest vertebrates. The first fish, which appeared around 540 million years ago had no jaws but did have gills. Their mouths were small and better suited to feeding on tiny food sources, such as algae scraped from rocks.

Some animals hunt actively, while others burrow into the seabed, creating complex tunnel systems. As the number of predators in the oceans increases, certain fish develop protection in the form of bony plates and scales. Predators in turn evolve more effective hunting tools, while other species respond with new ways to defend themselves. This marks the beginning of an evolutionary arms race in the animal kingdom, a process that continues to this day.

The land is still bare, with only algae growing along the edges of the seas.

A Jawless Fish as a precursor

Lancelets are soft bodied animals. Buried in the seabed, they filter plankton from the water using their gills. They have no hard skeleton.

Lancelets closely resemble Pikaia, a fish that lived over 500 million years ago. Both Pikaia and lancelets belong to a group of organisms with a notochord, a transparent fluid filled rod running along the back. From this group, all vertebrates eventually evolved, including us.

Foto: Hans Hillewaert / CC

Ordovician Period, 488 to 444 million years ago: life becomes more abundant

Fish are still jawless and covered with external armor. They hardly resemble the fish we know today. They are small, not even bigger than a herring.

Life continues to spread in the shallow coastal waters. Sea scorpions, such as Eurypterus, become common and grow to impressive sizes. They will survive all the way until the end of the Permian.

Silurian Period, 444 to 416 million years ago: the conquest of land

Before the Silurian, the land was empty and rocky, with bare stretches of sand, gravel, and clay. During the Silurian, algae mats begin to form in wet areas along seas, rivers and lakes. Providing a surface for spider-like animals to crawl on.

By the end of the Silurian, land plants begin to appear, such as Cooksonia. These are the first plants adapted to life on land. They must protect themselves against drying out, to transport water to all parts of the plant they develop vascular tissues. They also need enough structural support to remain upright. Reproduction takes place through specialized organs that produce spores, allowing them to spread beyond the water. In many ways, they still resemble their ancestors, the algae that dominate the waters.

Foto: Cooksonia, Collectie Museon-Omniversum Coll. nr. 234618

Cooksonia: One of the Earliest Land Plants

These plants are very small, only a few centimetres tall. From the sediments in which they are found, researchers conclude that they grew along rivers or seashores and were regularly flooded. How they anchored themselves to the ground is unknown. They probably had no roots, but instead spread across the soil with growing stems.

At the tips of the stems are small, button like or cup shaped structures containing spores, which allow the plant to reproduce. Ferns still reproduce in this way today.

The oldest Cooksonia specimens have been found in Ireland and date back 425 million years. Cooksonia grows taller than mosses and can thrive in drier areas, thanks to a vascular system that transports water to the upper parts of the plant.

Devoon, 416 to 359 Million Years Ago: Changes in the Air

In the oceans there are sharks that do not have a bony skeleton, instead their skeleton is made of cartilage.

Rhynia a land plant that resembles the long extinct Cooksonia grows along the water’s edge. Its red spore capsules are particularly noticeable. Further from the water on drier ground, the first clubmosses appear. Their history is one of extremes. The very first clubmosses grew no taller than 25 cm, but later in the Carboniferous period some species developed into trees reaching up to 40 meters, thriving in moist environments. By the end of the Carboniferous as the climate became drier, these large species disappeared. Today only low growing varieties remain, spreading across the ground and reaching no more than 5 cm in height.

Foto: Rhynia en wolfsklauw, collection Museon-Omniversum

The growing number of land plants increases the amount of oxygen in the atmosphere while reducing the levels of carbon dioxide. Their roots which penetrate the soil, also change its composition.

Amphibians and arthropods, such as spiders and insects, move further inland. For reproduction however, they still maintain a close connection to their original habitat, the water.

(Foto: Diorama van Ichtyostega, one of the first land animals. Collection Museon)

Long after invertebrates like millipedes left the water, the first vertebrates ventured onto land. They have lungs, the four limbs of a land animal and the tail fin of a fish. Ichthyostega is a striking example of a transitional form between aquatic and terrestrial animals. Fossils also show that its hind limbs had seven toes, while the front limbs are not completely known.

Carboniferous, 359 to 300 million years ago: exotic forests with tree ferns

Exotic forests of tree ferns spread across the landscape. Just inland from the coasts and at river mouths vast swamps form. Dead plants sink to the bottom and accumulate, creating the foundation of today’s coal deposits. Insects and amphibians inhabit the swamps, while the first reptiles appear in drier areas. The first plants with true roots also emerge, though most rely on supporting roots to stay upright.

In the water, placoderms are past their peak. The fish of both saltwater and freshwater increasingly resemble those we see today, but not entirely yet.

Permian, 300 to 250 million years ago: a massive catastrophe

In the seas, trilobites and placoderms disappear. On the supercontinent Pangea the climate is extremely dry as the oceans can no longer influence the land far inland. Amphibians are past their peak. The life forms that once began as fish are now evolving toward mammals, but the path is long. Fins become limbs and eggs break free from the water.

Reptiles resembling early mammals begin to appear. Due to the dry climate tree ferns decline, making way for conifers. A well known example is the ginkgo, which still exists today. At the end of the Permian, sudden and dramatic changes occur. A massive catastrophe causes 70% of all land species and 95% of all marine species to go extinct.

The first reptiles land

On the supercontinent Pangea, a new group of land animals emerges: the reptiles. These animals no longer depend on water for reproduction. A hard protective shell helps prevent them from drying out. Unlike earlier land animals, they can lift their heads and travel longer distances than amphibians.

Back to the water

Evolution can be unpredictable. Not long after reptiles gave up their amphibian lifestyle and adapted to life on land; some return to the water. This means they must re adapt to aquatic life: webbed feet reappear and their tails become flattened for swimming. Yet there is a difference, they move into freshwater rather than the salty sea. These aquatic reptiles appear just before the supercontinent Pangea begins to break apart and Africa and South America start drifting apart. This explains why fossils of these freshwater reptiles are found on both sides of the ocean. They never reach the size of modern crocodiles, the largest known individuals were about one meter long.

Foto: Mesosaurus, Collection Museon-Omniversum, Coll. nr. 86804

Mesosaurus belongs to one of the oldest known groups of reptiles, the Anapsida. Fossils of Mesosaurus have been found in Permian sediments in South Africa and eastern South America. Mesosaurus was fully adapted to life in water; with a long snout lined with a row of small, sharp teeth. The presence of Mesosaurus on both sides of the Atlantic Ocean is considered one of the pieces of evidence for the theory of continental drift. According to this theory, proposed by Alfred Wegener, Africa and South America among others were once part of a single continent. Initially controversial, this theory is now widely accepted.

Triassic, 250 to 200 million years ago: the first dinosaurs

In the Triassic landscape, the first mammals and dinosaurs appear. The land is dominated by ferns such as cycads, while conifers continue to spread. Changes are also happening in the seas. Squids protected by their self made shells, inhabit the oceans from the seabed up to the surface.

In the seas, among the many ammonites, three groups of fish can be found. Jawless fish (Agnatha) are nearly extinct. Cartilaginous fish (Chondrichthyes) and bony fish (Osteichthyes) are on the rise. Bony fish already resemble modern fish in many ways, but their tails are still asymmetrical and their skeletons partly made of cartilage. Their scales lie either loosely or tightly against the skin. From the lungs of bony fish the swim bladder develops, allowing them to maintain a constant depth like a submarine. In some species the swim bladder also functions as a hearing organ.

The swim bladder is truly revolutionary. Without it fish sink to the bottom if they stop swimming. Most bony fish have one. By adjusting its contents -air or oil- a fish can rise, sink or hover in the water without any effort. In some species the swim bladder is connected to the ear, acting as an amplifier. In others it amplifies the sounds they produce.

Photo: Starfish Steriacites lumbricalis. Age: Triassic. Location: Verona, Italy. Collection: Museon.

Jurassic, 200 to 144 million years ago: the age of dinosaurs

The seas are highly diverse. After the first cartilaginous fish the sharks appeared in the Silurian, rays emerge 200 million years later. Many species of bony and cartilaginous fish swim in the oceans. Marine reptiles such as plesiosaurs and ichthyosaurs become common. Above the water flying reptiles soar and the first birds begin to appear.

Cretaceous, 144 to 65 million years ago: continents flooded by shallow seas

Large parts of the former continents are covered by shallow seas. Dead algae form thick layers of chalk, giving this period its name. Life in the seas is diverse and increasingly resembles that of today. On land reptiles, insects and flowering plants are abundant, while more species of birds and small mammals begin to appear.

At the end of the Cretaceous, many species go extinct, from the largest dinosaurs to the tiniest sea creatures. This was likely caused by the impact of a massive meteorite. With this event, the Mesozoic era comes to an end.

Tertiary, 65 to 1.8 million years ago: the age of giants

In what is now the United States, large areas of land were pushed upward around 50 million years ago. The regions between these mountain ranges filled with water, forming vast lakes that resembled inland seas. The fine grained limestone and clay deposits, known as the Green River Formation, are a treasure trove of fossils. On land plants and animals begin to resemble those of today. The landscape is home to giant flightless birds and various giant mammals, such as long necked rhinoceroses and elephants. The first humans appear in Africa at the end of the Tertiary.

Quaternary, 1.8 million years ago to the present: we are bony fish!

Photo: Gosiutichthys parvus. Age: Tertiary | Collection Museon

Since Darwin we have seen ourselves as part of the primates, mammals, tetrapods, vertebrates, or chordates certainly not as fish. We have no fins and would drown underwater. And yet while we do not look very fish like, if we go back far enough back in evolution the trail leads to bony fish.