The best way to get an idea of what life was like between 530 million and 360 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 24-hour clock. On this first stop we will see what life was like between about 21:05 and 22:00. Here are some of the key species that evolved in the prehistoric seas.
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 them 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 in the bath 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're rooted to the sea floor and they don't seem to move. But actually sponges are distantly related to 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 (about 530 million years ago). A famous place for finding them is in the Sponge Gravels of Farringdon, Oxfordshire.
Most people have heard of coral reefs, but what many probably don't realise is that these enormous constructions were built over hundreds of thousands of years by tiny marine organisms which built 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 out 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 that the top of the reef is never far from the sunlit surface. Some reefs have broken through the surface, becoming small islands which are now amongst the most popular tourist destinations such as the Seychelles and the Maldives in the Indian Ocean. Coral reefs are habitats that seem to have harboured an amazing degree of trust between different species.
For example, small fish are often seen cleaning the larger fish – even entering their mouths to wash their teeth. Communities of these small fish run their own types of "cleaning station" where larger fish come for rest and relaxation. Corals in the Cambrian seas would have been perfect examples of natural co-operation and community.
These are part of the same family as corals, but nowhere near as friendly. The family, or phylum, is called 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 that dangle 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 algae and falling to the depths by day to avoid being eaten by fish like squids.
Any fossil-hunter would recognise these creatures. Fossils in this characteristic spiral shape crop up everywhere. Although they look like snails, their closest relatives are cephalopods –the class 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 attacks.
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 with 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 they must go down in prehistory as the first forefathers of human beings.
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 also protect themselves from predators by burrowing into the sand on the ocean floor.
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 10m long and weighed over 4 tons. 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 extinction phases of the early Earth.
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 was formidable. It had a long, spiked tail, equipped with a deadly venomous sting. The creature could grow to over 2m in length making it one of the largest arthropods ever to have lived.
Sea scorpions died out along with many other species in what's called the Permian Mass Extinction, 252 million years ago ( see Part Three). More than 200 fossils of these terrifying creatures have been discovered. In fact, some fossilised tracks made by a 1.6m-long sea scorpion were found recently off the coast of Scotland.
In the prehistoric seas, ancestors of today's pike fish developed two remarkable features that proved decisive in their success, and which they passed on to other creatures. The first is that they learned to hunt by stealth – that is to say, unlike sharks, they didn't just rely on speed and brute force. Instead, they would very quietly swim up behind their prey, staying totally motionless in the water, and then suddenly pounce, leaving little chance for their surprised victims to escape.
They were able to do this because, unlike some fish that have to keep swimming to stop themselves from sinking, the pike developed a new system enabling it to stay totally still in the water. It uses an internal bag of air called a swim bladder. When the fish wants to sink, it absorbs some of the air from this sack into its bloodstream. If it wants to rise, it does the reverse, releasing air back into the swim bladder. The result is that these fish can always stay at the same depth in the sea without having to move. It's a bit like the way a submarine works.
Ancestors of today's pike fish also developed a pocket of air to help them listen to what was going on in the surrounding water. Pike, and other related fish, called Teleosts, were the first animals to hear. Sound waves travel through the water, and cause the air in the swim bladder to vibrate. Tiny bones, very much like those in our ears, send these vibrations to the fish's brain, which interprets them as sound.
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 primitive air-breathing apparatus. There are only six species of lungfish alive today, but something closely related to them emerged 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. This process is 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.