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The Beginning

(13.7 billion–420 million years ago)

How a speck of infinite energy exploded our universe into existence, creating the conditions for the birth of life

TAKE A GOOD look around. Put everything you can see inside an imaginary but enormously powerful crushing machine. Plants, animals, trees, buildings, your entire house (including contents), your home town as well as the country where you live. See it all get pulverized into a tiny ball.

Now put the rest of the world in there too. Add the other planets in our solar system and the sun, which is about 1,000 times bigger than all the planets put together. Then put in our galaxy, the Milky Way, which includes about 200 billion other suns, and finally all the other galaxies in the universe. See all this stuff squeezed together, reduced to the size of a brick, then a tennis ball, now a pea – finally, see it crushed even smaller than the dot on top of this letter i.

Then it disappears. All those stars, moons and planets vanish into a single, invisible speck of nothing. That was it – the universe began as an invisible dot, a singularity, as scientists like to call it.

This invisible, heavy and very dense dot was so hot and under such enormous pressure from all the energy trapped inside it that about 13.7 billion years ago something monumental happened.

It burst.

This was no ordinary explosion. It was an almighty explosion, the biggest of all time – it was what we now call the Big Bang. What happened next is even more dazzling. It didn’t just make a bit of a mess; it made a huge mess, billions of miles wide. In a fraction of a second the universe expanded from being an invisible speck of nothing to something so enormous that it includes everything we can see, including all the matter needed to make the earth, the sun, the moon and the stars. And there’s also a whole lot more that we can’t see yet, because our telescopes can’t peer that far. In fact, the universe is so big that no one really knows how wide or deep it actually is.

Just after the Big Bang, more mysterious things started to happen. An enormous blast of energy was released. First it was transformed into the force of gravity, a kind of invisible glue that makes everything in the universe want to stick together. Then the massive surge of energy created countless billions of tiny building bricks – like microscopic pieces of Lego. Everything that exists today is made of billions of particles that originated a fraction of a second after the Big Bang.

About 300,000 years later things had cooled down enough so that these particles – the most common of which are electrons, protons and neutrons – could start to stick together into tiny blobs which we call atoms. With the help of that glue – gravity – and the passing of a little time, these atoms gathered together to make enormous clouds of very hot dust. Out of these clouds came the first stars, massive balls of fire supercharged with energy left over from the Big Bang. Gravity made the fiery stars gather into galaxies of many different shapes and sizes – some in spirals, some in the shape of spinning plates. Our galaxy, the Milky Way, was formed about 100 million years after the Big Bang – that’s 13.6 billion years ago. It’s the shape of a large disc – like two back-to-back fried eggs – and spins round at a dizzying speed of about 500,000 miles per hour.

About 4.6 billion years ago, the left-over gas and dust cloud from previous burned-out stars collapsed and ignited to form our sun. That means our sun is only about a third as old as the universe itself.

For a long time people believed that the earth was at the centre of the universe. But we now know that our solar system is located in one of the Milky Way’s outer spiral arms, called the Orion arm, and is currently travelling through a sparse and lonely part of the galaxy called the Local Bubble.

The early solar system was an extremely nasty place – and very unsuitable for life. An invisible rain of tiny highly charged particles streamed out of the hot, fiery furnace of the sun like a storm of razor-sharp daggers. These could cut through almost anything. They are still being fired out by the sun, about twenty billion tonnes every day. This is known as the solar wind and it can penetrate even the toughest space suits and helmets worn by astronauts.

It was hell on earth. A semi-molten crust of sticky volcanic lava burbled across the planet’s surface like burning-hot treacle. There was no solid ground and definitely no life. The unstable earth spun so fast on its axis that each day was only about four hours long.

What happened next was a freak. Experts believe two young planets happened to be on the same orbit around the sun, but moving at different speeds. One was the earth, the other another early planet called Theia. About fifty million years after the sun began to glow these two newborn planets ploughed into each other. With a massive jolt, the ailing earth fell on to its side, out of control, a crippled, hysterical, shaking wreck.

Thousands of volcanoes erupted following the impact. Huge volumes of gas, previously trapped inside the earth’s core, now spurted through the surface, giving birth to the earth’s early atmosphere.

Theia’s outer layers vaporized into billions of tiny particles. Debris flew everywhere, surrounding the earth with an enormously thick blanket of hot dust, rock and granite. Trapped by the earth’s gravity, this fog of rubble swirled around in the sky making everything go dark. For months not even the brightest sunlight could penetrate the thick layers of dust which were once planet Theia. Her heavy, molten-iron core converged into the centre of the earth. It caused an almighty shock wave that fused the two planets’ cores into a single, tight, metallic ball, thousands of degrees hot, which plunged deep into the middle of the stricken globe, crushed by the force of the impact.

It’s just as well for life on earth that this almighty collision happened. The earth’s metallic core gave birth to a magnetic shield that deflects the most lethal effects of the solar wind away from the planet’s surface. The shield also prevents the solar wind from splitting water (H20) into separate hydrogen and oxygen atoms, preserving the earth’s vital supplies which would otherwise diffuse into space. Without this shield life on earth might never have evolved. Other planets that do not have an iron core, such as Mars and Venus, appear never to have developed life.

Today there is no physical evidence on earth of the impact of the collision with Theia – no crater – because such was its force that all the outer material vaporized and exploded into space. But visible evidence isn’t far away. The dust and granite that wrapped itself around the earth soon stuck together again, thanks to the glue of gravity, and turned into an enormous ball of dust. Only about a year after the giant impact the earth had a new companion – our huge, bright, crystal-like moon.

The first living things

One hot sunny afternoon in the autumn of 1951, Professor Harold Urey strolled into his lecture hall at the University of Chicago. The room was filled with students eager to hear this great scientist talk about his pet subject – the theory of the origins of life on earth.

For more than 150 years scientists had been struggling to come up with credible theories of how life began. The problem, as Urey knew, was that so far it had proved impossible to actually demonstrate how life could have started from a ragbag of primitive or ‘primeval’ substances such as those found on the early, hostile earth. As a result, no one could agree on the origins of life.

Urey dreamed of concocting a laboratory experiment that would simulate conditions on the early earth, and show how life had been created out of a lifeless jumble of nothing. One of the students in the audience that day was utterly gripped by what Urey was saying. Stanley Miller had stopped off in Chicago on his way across America. He was in the process of trying to decide on a research project that would complete his training as a scientist.

The more Urey spoke, the more excited the twenty-one-year-old Miller became. At the end of the lecture he went up to the professor and – after a great deal of talking – persuaded Urey to work with him on a project to try to create life in a laboratory out of nothing more than a cocktail of chemical junk.

The two men set to work by designing an elaborate glass apparatus, in the middle of which was a large jar that would contain all the substances which they believed existed at the time of the early earth, such as hydrogen, methane and ammonia. Steam was fed into the glass jar through a tube connected to a flask of boiling water. Inside the jar were two metal rods, or electrodes. A powerful electric current would surge through these to make sparks – re-creating smaller versions of the violent lightning strikes that were common on early earth. The whole apparatus was designed to reproduce the earth’s early atmosphere, complete with thunder and lightning.

Miller started off the experiment by boiling the water in the flask. Steam climbed up through a tube connected to the large glass chamber, where it mixed with the primeval gases. Next, he flicked the electric switch. Some 60,000 volts of electric current surged into the electrodes, beginning a constant stream of mini lightning strikes.

To his bitter disappointment nothing happened. A despondent Miller left the lab that night with nothing to show for his efforts.

But when Miller arrived the next morning he found that the water in the flask had turned pink, indicating that some kind of chemical reaction had taken place. After running the experiment for a week the results he had hoped for were unmistakable: the clear water had turned a definite shade of red. The water now contained amino acids – the vital ingredients for life, used by all plants and animals (including you and me) to construct their living cells. Surely this was the demonstrable proof that Urey had so strongly believed in! Life, Urey and Miller concluded, began by chance on the hell that was the earth some 3.7 billion years ago because the Goldilocks-like conditions for it to do so happened to be just right.

Yet for all the science and technology, and the brilliant academic minds that have tried to come up with an explanation, no one has actually solved the riddle of how those chemical building blocks re-created in the laboratory by Miller and Urey turned into living cells, the stuff of you and me.

What is life?

The magic of a living cell is that it can reproduce. It can have buds, offspring, make replicas of itself. Usually single cells make exact copies of themselves – as viruses and bacteria do today, although sometimes a copying error creeps into the system to form a mutant cell. The ability to multiply is what makes life so completely different from anything else in the known universe. Nothing dead can do it.

Some experts believe the magical leap from life-giving amino acids to single-celled living organisms may have taken place deep down in the early oceans. Methanogens – one of two basic types of single-celled bacteria – evolved deep within the oceans so they could hide away from the lethal effects of the sun’s solar wind. They thrived next to volcanic vents called black smokers, which belched out thick, black, acrid fumes from the ocean floor, providing chemicals for food and warmth.

The other type of single-celled life form probably evolved from a copying error when food was scarce. It adapted to live off a completely new energy source – sunlight – which it used to split carbon dioxide (CO2) and water (H2O) into food. This simple but ingenious feeding process is what we now call photosynthesis.

Unlike the methanogens, these cyanobacteria needed to live close enough to the surface of the seas to feed off the light of the sun shining through the water. Their photosynthesis transformed the planet’s atmosphere, because its waste product is oxygen. Over billions of years cyanobacteria caused surplus supplies of oxygen to build up in the air.

How oxygen changed life on earth

To begin with, the oxygen bonded with iron on the sea floor left over from the giant impact with Theia. The iron became iron oxide, a rusty-red mineral ore. When all the available substances that oxygen could bond with ran out, it was simply left to hang around in the air – which is where it has remained ever since. Oxygen now accounts for about 21 per cent of all the air we breathe. The rest consists mostly of nitrogen (78 per cent), with a little water vapour and a number of other trace gases in small measures under 1 per cent, including carbon dioxide.

Without oxygen, life on earth would have carried on, but it might never have developed beyond columns of sticky, microscopic bacteria. Humans could never have evolved, because oxygen is an energy-rich gas that sustains all forms of advanced animal life. Also, high up in the atmosphere, oxygen provides an essential protective blanket in the form of the ozone layer, which protects land life from the sun’s powerful ultraviolet rays.

About two billion years ago, thanks to another mutation, a single-celled bacteria began to feed off the oxygen-rich atmosphere. Oxygen’s enormous energy-giving properties meant that this process – called respiration – could produce up to ten times more energy than other ways of life. Soon the oceans were filled with highly energetic microscopic cells that fed off oxygen dissolved in the oceans.

So energetic were these microscopic cells that some found they could drill their way inside other, larger cells and strike a mutually beneficial bargain. While the smaller cells fed off the larger cells’ waste products, the larger cells used up surplus energy created by the smaller cells’ respiration. Through such a collaboration, called endosymbiosis, these new combined cells were now better equipped for survival in the increasingly oxygenated world.

Some found that the best survival strategy was to link up. Gangs of cells joined together to form the world’s first multicellular creatures. Some of them became the forefathers of all animal life while others turned into the ancestors of today’s plants and trees.

Already we have travelled more than three billion years since the earth was first formed. If we think of the earth’s history as a twenty-four-hour clock, the first signs of life emerged at about 05.19. So far, we have already travelled to about 16.00, leaving just eight hours to go for all the rest of life to evolve. Although miraculous signs of life have already appeared in the form of complex microscopic bacteria, hundreds of millions of years have yet to pass before fish, animals, plants and trees make their first appearance.

They only ever made it thanks to another piece of extraordinary teamwork – one that prepared the planet for yet more dramatic changes to life on earth.

 

Life’s increasing complexity

Doctors in a medical emergency always have the same top priority: to protect the body’s vital life-support systems. If a patient’s internal transport system – the blood – cannot carry oxygen from the lungs and nutrients from the stomach to the cells in the body, then the victim quickly starves. And if waste products such as carbon dioxide and toxic acids are left to fester because they cannot be removed, the body is almost as quickly poisoned.

During this period of earth’s history a global life-support mechanism emerged that works in a strikingly similar way to the human respiratory system. Without this mechanism microscopic bacteria from two billion years ago could never have evolved into plants, animals and people.

The first and most simple part of the earth’s life-support mechanism is very well known. It is rain. As the sun beats down on the planet’s surface, the seas get warmer, and some of the water evaporates into vapour. Once in the air, this vapour cools to form clouds, which get blown about by the wind across the planet, eventually to fall elsewhere as rain. Without this automatic fresh-water supply, most living things on land and sea would almost certainly perish. No pipes, no pumps, no need for power stations, no people to watch over the machinery – it just happens, every day, the most precious of all free gifts.

Beneath the surface of what seems a very simple process, an important partnership between the earth and its living things developed some time between 3.7 and two billion years ago.

For rain to fall, clouds need to form. Steam molecules can condense back into water only if there is some kind of surface or ‘seed’ around which they can cluster. Luckily, waste gas produced by early bacteria provided a perfect surface around which water vapour could turn back into water to form rain. In this way bacteria help nature operate one of her most important life-support systems by seeding clouds. Cloud cover also creates a reflective blanket that sends many of the sun’s scorching-hot rays shooting back into space. And so clouds help to cool the planet, greatly improving conditions for life on earth.

This is just one of a number of partnerships between the earth and living things that help control the climate and prevent excessively hot temperatures from harming life. Another of the earth’s life-support processes helped reduce the levels of salt in the sea, preventing the poisoning of early life. It is called plate tectonics.

As you read this page, you are sitting on a crust which is floating like a giant raft on an underground sea of boiling-hot lava. The earth’s crust is split into a number of floating plates that are in constant motion, like enormous, slow-moving bumper-cars (see plate 4). Each plate is either drifting apart from, or bashing into, another one. When they collide they form massive mountain ranges that soar high into the sky. When they drift apart, huge ocean ridges form in their wake. So much pressure builds up in the rocks of the earth that the movement of the earth’s plates causes massive earthquakes and volcanoes, hot geysers and tsunamis. This process has helped secure life on earth by removing excessive amounts of poisonous salt from the seas. Because the earth’s crust has split into separate plates that move around, like giant pieces of a puzzle, evaporated sea salt is safely stored deep beneath mountain ranges – millions of tonnes of salt are today buried beneath the European Alps and the Himalayas. As long as the plates continue to move, mountains of salt will always be safely buried under the rocks, leaving the levels of salt in the sea low enough for life to continue to thrive.

For billions of years this tectonic cycle has been churning up the surface of the earth in ultra-slow motion, drastically changing weather patterns, burying dangerous salts and minerals, making and destroying super-continents and crumpling crusts as if they were pieces of tin foil. Such are the earth’s life-support processes that seem to have kept everything – from the composition of atmosphere, global temperatures and the saltiness of the sea – sufficient for life to thrive. Without these systems, the evolution of complex life as we know it would have been impossible.

The coming of sex

Life on earth consisted of only two kinds of living things until about a billion years ago: the original, simple single-celled bacteria that produced methane and oxygen as waste products, and the newer, more complex multicellular organisms that fed off the increasingly abundant supplies of oxygen in the air.

For billions of years these tiny organisms were all the life there was. Then something triggered a spectacular and dramatic increase in the pace of the evolution. About one billion years ago living things developed a radical new form of duplication called sexual reproduction. It revolutionized the way life evolved.

Sexual reproduction helped life shoot forward in complexity, equipping it to survive the challenging conditions on the planet in far fewer generations than it would have taken without it. While it took 2.5 billion years for life to evolve into types of microscopic organism, it took less than half that time for life to transform completely into everything we know today – from fish, amphibians and reptiles to plants, trees, birds, mammals and man.

One of the first men who worked all this out was Gregor Mendel, a monk, born in 1822 in what is now the Czech Republic. He spent most of his life absorbed in the natural world, especially in his favourite place for study, the monastic vegetable garden. Brother Mendel became so interested in cultivation that between 1856 and 1863 he studied more than 28,000 different pea plants. What intrigued him was that when these slightly different plants had seedlings, their differences (or characteristics) would often be carried forward to the next generation. The seedlings had inherited features from their parent plants.

The concept of inheritance was first described by Mendel in 1865. He went on to devise a number of laws that could predict how living characteristics are passed from one generation to another through sexual reproduction, massively increasing the variety of living things.

Finding fossils

We are now on the edge of an enormous transformation. The time is just past 9 p.m. on our twenty-four-hour trek across earth time. The rest of history will be played out in the final three hours. There’s still no life on land, no plants, no trees, no flowers, no insects, no birds or animals, let alone humans. The earth is very old, but human history is not. Compared to the age of the earth, everything else we will discover is either young, very young or just hatching out. Mankind is among the youngest of all.

About 550 million years ago the first full and clear pictures of what life on earth was actually like begin to emerge. When the fossil record begins it is rather like a theatre curtain being pulled back to reveal a stage bursting with actors in the middle of a play.

Fossils are the impressions of long-lost creatures that had hard surfaces such as bones, shells or teeth, which sometimes leave impressions in rocks. They are wonderful for helping investigators identify what kinds of creatures have lived on the earth.

Charles Doolittle Walcott was born near New York in 1850. As a young boy he found school rather boring. It wasn’t that he had no interest in things, rather the opposite. He was so curious that he wanted to get outside and explore the world for himself – in particular he liked to look for minerals, rocks, birds’ eggs and fossils.

By 1909 Walcott had become a well-established fossil collector. One day a freak accident changed the rest of his life. While he was walking high up in the Canadian Rockies his mule slipped and lost a shoe. In the process it turned over a glistening rock of black shale, a type of rock made out of compacted mud and clay. When Walcott stooped to pick up the rock he saw a row of remarkable flattened silvery fossils. These were perfectly preserved creatures from the Cambrian Period.

It turned out that the mountainside had collapsed about 505 million years ago, smothering these creatures, killing them instantly and burying them like a time capsule for posterity. Walcott’s discovery became one of the richest hoards of fossils ever. It is known as the Burgess Shale, named after Mount Burgess, near the site where Walcott found the fossils. Walcott returned to the site many times and eventually wrote a library shelf of books about his finds. And what a bizarre range of creatures they were.

First, there is the strange-looking Anomalocaris. This was one of the biggest sea hunters of its day and could grow up to a metre long. It used a pair of grasping arms to capture and hold its prey. For a long time the fossils that make up this extraordinary creature were thought to be three separate living things. The body was identified as a sponge, the grasping arms as shrimps, and the circular mouth as a primitive jellyfish.

Another was the remarkable Hallucigenia. This curious worm-like beast also kept fossil hunters and scientists scratching their heads. It was thought to have walked on stilt-like legs and to have had a row of soft tentacles on its back which it used for trapping passing food. Thanks to the discovery of similar fossils in other parts of the world, particularly in China, fossil investigators now think they’ve been looking at it upside down. Instead, it walked on paired tentacle-like legs and used the spines on its back as a form of body armour to protect itself from being eaten.

But nothing in the wildest imagination of science-fiction writers could conceive of such a beast as Opabinia. This swimming gem had five stalky eyes, a fantail for swimming, and a long grasping arm for feeding. It was smaller than other predators, being only about four centimetres long, and there’s nothing remotely like it alive today.

One of the most common forms of animal life at that time, and the most common of all the fossils unearthed in the Burgess Shale, were trilobites. Their fossilized remains look like giant woodlice and have been found all over the world. Trilobites were probably the first creatures ever to be able to see. They had eyes, like those of today’s flies, which divide into hundreds of different cells, giving them a kind of mosaic view of the world under the seas.

To create a realistic picture of how life on earth actually evolved, it is important to know when each species lived and died so they can be pieced together in chronological sequence. The genius of one man helped work out how to do this. Charles Darwin (1809–82) worked out that all living things have evolved according to a sequence that is still unfolding today.

Natural selection

Darwin’s book On the Origin of Species (published 1859) explained, for the first time, the theory that all living things originally evolved from a single common ancestor. Darwin worked this out because the fossil record frequently shows new creatures emerging and others disappearing. His conclusion was that all living things are related to each other, but that only those species best suited to the environment of the day survived. His theory gave scientists the first ever way of arranging fossils in different groups and eventually into a rough chronological order. Darwin’s theory came to the inevitable conclusion that even humans must be descended from simpler forms of life, like apes, and before that from mice, reptiles, fish, and ultimately from those bacteria found at the beginning of life on earth.

Fossil records of now-extinct species became the key for Darwin’s understanding of the evolutionary process. By studying fossils and comparing them with living things today, he could see that each species has adapted itself according to a principle that he called natural selection. Over successive generations those creatures best equipped to live life on earth at that time survived, flourished and become dominant, while those least well equipped died off, their species eventually becoming extinct.

Many people were outraged at the implications of Darwin’s theory when it was first published. Some still are. The suggestion that humans are descended from animals – more specifically apes – threatened the widely held view that mankind was somehow different, superior, to all other living things. Equally as implausible to many was the idea that humans are just another natural species, which, like all others, is destined one day to become extinct.

Today, scientists have discovered powerful new ways of dating rocks and fossils which back up Darwin’s theories of how creatures have evolved over time. They have been able to construct an accurate picture of how life on earth has changed since fossils began to appear in rocks about 550 million years ago. They have also made a map of the past called the geologic column (see plate 1), which is divided into a number of chronological eras and periods.

A snapshot of prehistoric life

The best way to get an idea of what life was like several hundred million years ago is to use our imaginations and dive down to the ocean floor. Before we start, here’s a quick time check on our twenty-four-hour clock. On our first stop we will see what life was like between about 9.05 p.m. and 10 p.m. Here are some of the key species that evolved in the prehistoric seas.

Sponges

These were among the simplest of all animals living in the ancient Cambrian seas. There are still many types alive today. About 5,000 different species of sponge have been discovered so far. They attach themselves to rocky surfaces at the bottom of the sea. The reason we use them for washing ourselves is that their bodies are full of absorbent holes. Sponges use tiny hairs called flagella to beat sea water through these holes to extract a diet of microscopic nutrients.

For a long time people thought sponges were plants, because they are rooted to the sea floor and they don’t seem to move, but actually sponges are in a distant way relatives of mankind. We are much more closely related to a sponge than to, say, a daffodil. Sponge fossils have been found dating back to the earliest part of the Cambrian Period (c.530 million years ago). A famous place for finding them is in the Sponge Gravels of Faringdon, Oxfordshire.

Corals

Most people have heard of coral reefs, but what many probably don’t realize is that these enormous constructions were built over hundreds of thousands of years by tiny marine organisms which secrete their homes on top of the skeletons of their ancestors.

When coral fish die their bones pile up to create vast underwater mountains that provide an ideal marine habitat for future generations of corals and other sea creatures. It is thought that up to 30 per cent of today’s marine species camp out in the earth’s biggest existing coral reef – the Great Barrier Reef off the coast of north-east Australia. This colossal structure, composed of more than 1,000 islands, stretches for more than 1,000 miles.

Coral fish need sunlight in order to live. As each generation of coral dies the underwater mountain gets taller and taller so the top of the reef is never far from the sunlit surface. Some reefs have broken through the surface, becoming small islands. These are now amongst the world’s most popular tourist destinations such as the Seychelles and the Maldives in the Indian Ocean.

Coral reef habitats seem to create an amazing degree of trust between different species. For example, small fish are often seen cleaning larger fish – even entering their mouths to wash their teeth. Communities of these small fish run cleaning stations, where larger fish come for rest and relaxation. Corals in the Cambrian seas would have been perfect examples of natural co-operation and community.

Jellyfish

These are part of the same family as corals, but nowhere near as friendly. The family, or phylum, is called the Cnidaria. Like sponges, they are primitive creatures, although they can swim using a pumping action of their bell-like heads. Jellyfish have a very simple nervous system, no sensory organs and only one opening – a combined mouth and anus. They were very common in the Cambrian seas and some could pack a punch worthy of a lion using a lethal arsenal of harpoons dangling from their tentacles.

Jellyfish hunt in packs. Great herds of them would have been seen in the Cambrian seas, rising to the surface at night to feed off green algae and falling to the depths by day to avoid being eaten by fish like squids.

Ammonites

Any fossil hunter would recognize these creatures, even though they have been extinct for many millions of years. Fossils in this characteristic spiral shape crop up everywhere. Although they look like snails, their closest relatives are actually cephalopods – the family that includes today’s octopus and squid.

Ammonites first appeared about 400 million years ago, in the Devonian Period. The animal’s living parts were contained in the last and largest of its shell chambers. Shells were ideal protection against sharp-toothed predators. Ammonite fossils have been discovered showing teeth marks, scars from unwelcome attacks.

Sea squirts

These look like giant sacks anchored to the sea floor. They filter massive volumes of water each day in order to extract particles of food. At first glance they seem similar to sponges, but actually they’re a lot more sophisticated. Not only were squirts a common feature of the prehistoric sea bed but the way they evolved was important for all kinds of creatures fortunate enough to live on earth in the future – and that includes humans.

Sea squirts have babies that swim about like tadpoles. They propel themselves with a special tail that contains a very primitive form of backbone called a notochord. Descendants of sea squirts developed these notochords into vertebrae – the bones that form our spinal column. Animals that have nerve cords or spines belong to this group, called the chordate, which includes all the fish, amphibians, reptiles, birds and mammals. Baby sea squirts are the most basic form of chordate that has ever lived and so must go down in prehistory as the first forefathers of human beings.

Lancelets

Our first fish-like creature may not be big, but it’s very old. Something like today’s lancelet emerged about 500 million years ago. It seems to have evolved from some copying mistakes in those baby squirts – perhaps one that never glued itself properly to the bottom of the sea.

Like all fish, the lancelet is a distant relative of ours because it has a spinal cord running the length of its body. But that’s just about where the similarities stop. Unlike us, it cannot be called a vertebrate, because its cord is not surrounded by bones. The lancelet has no brain but it does have small gills at the side that breathe sea water in and out. It uses these for feeding by filtering small food particles. These fish protect themselves from predators by burrowing into the sand on the ocean floor.

Placoderms

Among the most fearsome creatures of the prehistoric seas were the now extinct placoderms. These were among the first fish with jaws and teeth. Recent research has shown that some species of placoderm had one of the most powerful bites of any creature ever known. Their teeth could tear a shark in two with a single snap. A placoderm could grow up to ten metres long and weighed over four tonnes. It was built like a tank. Heavy, articulated armour plating covered its head and throat, and its body was thickly scaled. Even its fins were encased in armour-plated tubes.

Placoderms were some of the world’s first true vertebrates. Their spinal cords were protected, like ours, in a series of bony segments. Ugly as they were, they are our cousins nonetheless. They died out in the late Devonian Period, during one of the earth’s extinction phases.

 

Sea scorpions

Here’s a good reason why fish like the placoderms needed to protect themselves with such highly developed body armour. The now-extinct sea scorpion or eurypterid was formidable. It had a long spiked tail equipped with a deadly venomous sting. The creature could grow to more than two metres in length, making it one of the largest underwater creatures of its day.

Sea scorpions died out along with many other species in what’s called the Permian Mass Extinction, 252 million years ago (see page 41). More than 200 fossils of these terrifying creatures have been discovered. In fact, some fossilized tracks made by a 1.6-metre-long sea scorpion were found recently off the coast of Scotland.

 

Lungfish

Imagine being a medium-sized fish fighting for survival in the violent, dangerous prehistoric seas. Forefathers of today’s lungfish were among the first creatures to develop the equivalent of an escape hatch from the prehistoric seas by adapting one of their gills into a primitive air-breathing apparatus. There are only six species of lungfish alive today, but something closely related to them emerged from the oceans around 417 million years ago.

Lungfish look like powerful elongated eels. They burrow into the mud and use their lungs to survive dry periods when water is scarce, a process called aestivation. They lived in the estuaries of rivers, and learned to survive in dried-up river mouths by breathing oxygen from the air. They developed other features that helped them live on land, including four highly developed fins, well adapted for ‘walking’ across hard, dry surfaces. Such devices provided the key to surviving in a dramatically different habitat.

 

Now it’s time to explore the emergence of life on land.