The Story of Our Oceans Throughout Earth’s History

Many of us can only imagine the oceans as they are today, with their waves hugging our familiar shaped continents whilst iconic species such as crabs, seals and dolphins glide in this underwater world. Yet throughout Earth’s long life, the oceans have in fact gone through some incredible changes, from major mass extinctions to global glaciation.

The Earth formed 4.5 billion years ago, from dust. It’s hard to believe that our enormous planet is formed of dust particles alone but this dust was fallen out debris from the famous ‘Big Bang’. Over time, dust particles collided with each other, merging into larger particles and when these larger particles hit each other, pebbles were created. The process continued with larger and larger rocks forming until finally, Earth was produced. So much heat was created in this haphazard process of collision that early Earth was actually molten with no water at all. After a significant cooling down period, it is thought that it was asteroids which brought the first droplets of water to our planet. Asteroids can contain ice, sometimes enough to fill an entire ocean, and so it is thought that these large chunks of space rock hitting the Earth helped kick start our seas. Today, our planet is unique due to its large abundance of liquid water as other planets only have water in the form of vapour or ice due to their proximity to or distance from the sun.

An astonishing one billion years after our planet was formed, life in the form of simple bacteria began to populate our hidden corners. In fact, it is thought that for 75% of Earth’s history, its only living inhabitants were single-celled microbes. Only at 2.5 billion years ago did photosynthesis start to take place, with carbon dioxide being converted into life-giving oxygen. This revolutionary process was kickstarted by cyanobacteria, tiny organisms which create the blue/green film we sometimes see on lakes or in the ocean. However, it took hundreds of millions of years for enough oxygen to build up to support complex living beings which took in oxygen as a source of energy, allowing them to grow larger and be more active. The first animals evolved in the ocean, with many having the same soft bodies as our jellyfish and anemones have today. However, many others were unlike anything we could imagine, with scientists often shocked by the strange fossil shapes they have discovered hidden inside ancient rock. Just as life was starting to take a hold, the world was plunged into one of the greatest ice ages it has ever seen as glaciers and slushy sea ice covered the entire planet in a phenomenon known as ‘Snowball Earth’.

The next great event in Earth’s history happened 542 million years ago when the Earth’s climate shifted and oxygen levels in the ocean rose. It is thought that oxygen levels must have finally reached an invisible threshold as suddenly a huge burst of new species evolved and started terrorising our oceans. This event is known as the ‘Cambrian Explosion’. With a newfound abundance of oxygen allowing species to grow and behave like never before, predators suddenly started hunting in the seas, leading to an evolutionary arms race as individuals fought to survive by developing new ways of protecting themselves. Around this time, sea creatures started to live within shells, much like the oysters and hermit crabs we see today whilst some individuals even crawled out of the ocean and onto land. The creatures that were born from this explosion set the pattern for every animal that followed, right up to the ones we see today. Some 70 million years later, the seas were teeming with life as the first reefs began to emerge, filled with Earth’s first vertebrates, jawless fish and algae. If you went diving in the ocean today, you would expect reefs to be constructed from corals, yet over time, many different species have taken it in turns to be the dominant reef builders. Molluscs and clams have previously constructed reefs throughout history with their designs looking more like crater rims or skyscrapers than the underwater jungles we see today. Yet whilst the ocean was buzzing, the land remained almost devoid of flora and fauna.

Just as our planet was starting to look healthy, a mass extinction event occurred which wiped out half of the newly evolved marine life. It is thought that Earth has suffered five major extinction events which are classed as an event where at least 75% of all species on our planet are lost. These incidents can occur over thousands or millions of years which is a mere blink of an eye in geological terms. Amongst the chaos, sea sponges fared well and scientists now believe that these humble beings may help recover entire ecosystems as they help stabilise sediment, creating a favourable home for some of our sea’s smaller critters which then provide a food base for our larger animals.

400 million years ago, the Earth had recovered once more with our oceans filled with fish, sea scorpions and the sharks we see today. This period saw some of the largest and most diverse coral reefs our world has ever experienced and so hundreds of fish species could be found in tropical waters, leading to the Devonian period often being nicknamed ‘the age of fish’. One of the largest creatures haunting the ocean at this time was called the ‘Dunkleosteus’, a 10 metre long fish covered in thick armour with sharp bones sticking out of its jaw in place of teeth; even sharks feared these ferocious beings! Some of the fish at this time even evolved so that their sturdy fins transformed into limbs, allowing them to make their way onto land where the Earth’s first forests were sprouting. Surprisingly, it was actually these forests which were thought to cause Earth’s next major extinction event. The new abundance of roots in the ground broke down many of the minerals and nutrients that were in rocks to form a soil. However, these nutrients soon found their way into the oceans which kickstarted a major growth in algae which quickly blocked the sunlight from large patches of sea. In addition, the young trees suddenly drew a significant amount of carbon dioxide out of the air, causing the temperature to drop so much that glaciers formed, creating conditions which didn’t suit many of the creatures on Earth. Only 100 million years later, another large extinction occurred with corals disappearing from our planet and reefs taking millions of years to reform. In the recovery process however, large marine reptiles developed which later transformed into our famous dinosaurs.

In the age of the dinosaurs, the world was relatively warm with little ice, however this changed 65 million years ago when an asteroid hit the shallow ocean near the Yucatan Peninsula in Mexico. Whilst the asteroid did contribute to our most recent mass extinction, scientists also believe that significant volcanic activity added to the dust, smoke and toxic gases which filled the atmosphere, blocking sunlight and causing nuclear winter conditions. Not only were dinosaurs lost but so were 75% of ocean species as our seas became colder and increasingly acidic. Yet once the climate had rebalanced, the lack of enormous predators meant other smaller mammals had a chance to thrive, as did the last surviving branch of dinosaurs: birds. Since then, many of the animals we know and love today have evolved; whales appeared 50 million years ago, seals 23 million years ago and penguins 34 million years ago. As you can see, the world has changed dramatically in its lifetime, with only 1% of the four billion species which have ever evolved here remaining alive. The creatures we see today have fought to survive through bursts of evolution and mass extinction, making their place on the planet well deserved.

Animals aside, it is only in the last century that we have uncovered the hidden geological secrets of the ocean. Shortly after World War Two, ex-naval officers used sonar technology to conduct the first detailed surveys of the sea floor and in the process, they uncovered 40,000 miles of oceanic ridge system; a chain of underwater volcanoes and valleys which circumnavigate the globe, much as our terrestrial mountains and gorges do. This is the largest geological feature on the planet yet amazingly, we have only just discovered it! In some places, the mountains rise so high they break the surface of the sea, forming well known outposts like Iceland and the Azores. Over millions of years, magma is pushed up through the cracks in the ridges causing the existing seafloor to be pushed and spread outwards, therefore carrying our well-known continents around the world in a slow-moving carousel. Did you know that all the world’s land mass used to be in one great continent called Pangea? Or did you know that it was this movement of the sea floor which caused two pieces of land to collide, rock to be pushed up and the Himalayas to be formed?

Our world has changed dramatically throughout history, with many of our oceans’ ancient secrets still to be uncovered. Though in the grand scheme of things, we humans seem incredibly minute; it is thought that we may currently be living in Earth’s sixth great mass extinction. Except this time, it isn’t caused by asteroids, volcanoes or falling oxygen levels, it is caused by burning fossil fuels, over consumption and over population. Clearly, we have incredible power in our hands to, literally, change the entire world. With our current ocean species having fought so hard to survive our planet’s turbulent past, it seems we must do all we can to protect them from the looming human destruction of their home.

By Neve McCracken-Heywood