BBC Walking with monster 1

This is Earth. Four point four billion years ago. A toxic world with no hope of life. Then everything changed. Another planet, Thea, smashed into Earth and the two planets fused, creating a brand new world. Our world. Even today, Thea lies right beneath our feet. A smaller chunk of Thea became our moon. And slowly our oceans formed. Until life on Earth was ready to begin. But who would inherit this blue planet? This series tells the extraordinary story of life before the dinosaurs.

A time when strange and savage creatures fought a ruthless battle to rule the Earth. Amongst them were our own earliest ancestors, whose survival would decide whether we humans would exist at all. As they evolved, these bizarre creatures created the blueprint not only for our bodies, but for everything living today. This is life's forgotten story. An epic war for our world. A war between monsters. This is our planet, five hundred and thirty million years ago. Nothing yet lives on land, but in the ocean it's a different story.

Life has already been evolving for millions of years at a slow and steady pace. The seas are full of simple, soft bodied creatures, blindly drifting in the currents. Now, however, in the coastal shallows below, evolution has stepped on the accelerator. Predators have taken their first bite. This is Anomalocaris, Earth's first super predator. This two metre long monster owes his success to a monumental evolutionary landmark. Eyes. They may look bizarre but they're not unique. Many predators in the Cambrian seas have also evolved eyes. And so have their prey. The consequences have been explosive. Being able to see and react to enemies has triggered an arms race between hunter and hunted.

This battle continues today and is a major force behind the variety of life. To combat being visible and vulnerable, eighty percent of creates in these shallow seas have sturdy skeletons on the outside of their bodies. These armoured animals are called Arthropods. In the future, they'll give rise to insects and spiders. But in these crowded waters there's competition everywhere, and even the mighty Anomalocaris's defences are constantly put to the test. Rigid armour splits if bent too far, leaving the loser vulnerable. To a completely different threat. This is Haikouichthys. He's the size of your thumbnail, but he's an evolutionary giant. He's the first ever fish. Our earliest known ancestor. He's unique, because instead of having armour on the outside, he's tough inside.

He's evolved a primitive backbone. He's the very first vertebrate. Forerunner of all future backboned animals, from the dinosaur, to the elephant, to us. His flexible backbone makes him more manoeuvrable than spineless Anomalocaris. He can scavenge flesh, then dart away unharmed. Our tiny backboned ancestors have survived a sea of monsters, but there are still many more battles ahead. They must adapt or die. Evolution takes over. As millions of years pass, fish build on their basic design. The muscles around their backbone evolve into a powerful tail and fins appear.

They evolve a distinct head. He may not look like you or I, but this odd fish is becoming the blueprint for our own bodies. This is Cephalaspis. She's a peaceful grazer who sucks up algae through her jawless mouth. But she's also developed a tough protective head and thick scales. Our ancestor's arthropod enemies have also been evolving and they're ready for round two. A hundred million years have passed and the fight for survival has filled
the Silurian seas with variety. Some creatures here would be recognisable today.

Sponges filter food alongside sea urchins. The orthocone is a distant relative of squid and cuttlefish, but he's as long as a truck. This world is terrorised by a new improved generation of armoured arthropods. Meet Brontoscorpio. He's a metre long monster scorpion with gills and a stinger the size of a light bulb. He zeros in on his next meal. But Cephalaspis has evolved an early warning system. Special sensors on her skin detect the tiniest vibrations in the water. We've inherited similar senses. They make us sensitive to touch.

With her defensive headgear, Cephalaspis can't swim fast for long. She must rest frequently. Soon she'll tire completely. Cephalaspis suddenly changes her path. She's picking up bad vibrations. Something Brontoscorpio can't detect. Pterygotus is the Titan of sea scorpions. The biggest arthropod of all time. More than three metres long, she's the size of a crocodile. She has turned the tables on Brontoscorpio. He'll make a good meal for her young. In such dangerous seas there's nowhere to hide. When breeding seasons comes, the Cephalaspis congregate to head for the one place they might escape a scorpion's grasp. Fresh water, inland.

Land at this time is like an alien planet. It's a barren expanse of roasting rock hotter than the Sahara. The air would be toxic to us. It has much less oxygen and three hundred times more carbon dioxide than today. But some forms of life have gained a foothold in this furnace. The first pioneering plants. Cooksonia has a unique survival strategy. It's the first plant to send shoots upwards, trapping extra light to help it grow. This basic design will eventually lead to our tallest forests. The Cephalaspis convoy ploughs upriver, away from the sea.

They're making the marathon journey back to the spawning grounds where they hatched. Incredibly our fish ancestors already use memory. They use familiar landmarks to navigate. Their toughened heads protect a vital weapon. One of the first complex brains. It's much more developed than their scorpion rivals who have no memory at all. It's thanks to these primitive fish that we can think and solve problems today. But the fish have underestimated their enemy.

It is the arthropods and not our ancestors who have taken the first momentous steps out of the sea into dry land. Brontoscorpio has a huge advantage. As well as gills he has simple lungs made up of hundreds of thin layers of tissue. He can't breath in and out like we do. He just absorbs the oxygen into his blood. Equipped to maximise the little oxygen available and with their armour to protect them from the sun, the scorpions patrol the shoreline,
scavenging on whatever the sea washes up next. Finally the fish approach their destination.

They've navigated their way back to the spawning pool, where their lives began. Weak from their long journey, now they have to cross a ridge of rock to make it from the river, to the pool. The first fish make it through and start to lay their eggs. But the exhausted Cephalaspis have company. Passing scorpions have stumbled on this bounty. But the fish have numbers on their side. The clever Cephalaspis have navigated their way, while Brontoscorpio are only here by luck. They're soon stuffed to the gills while the fish keep jumping.

One scorpion is still hungry, but he can't feed. He's become a prisoner in his own skin. His rigid skeleton is now a handicap. It can't grow with his body. He needs to shed his hard skin and then grow another, expanding while the new one is still soft. For such a large creature, this is a long process. Next morning, there's no sign of life in the spawning pool. The scorpion has missed his chance. Our ancestors have survived. They've laid their eggs
and are returning to the sea. Brain has triumphed over brawn, and soon they won't be such soft targets.

Evolution starts to give them weapons to fight back. Over millions of years, the fish's gills adapt to form the first jaw with the very first teeth. Now they're equipped to go on the attack. Some develop tougher bones and muscles in their fins and shoulders, which become the first limbs. This is where our arms and legs began. With this four-limbed design, our ancestors finally haul themselves out of water on to land. This is the giant amphibian, Hynerpeton. The prototype land dweller for the next three hundred million years.

Hynerpeton are over a metre and a half in length, much larger than most amphibians today. They've carved out a home along the water's edge. Arthropod enemies still exist, but they've shrunk since their Brontoscorpio glory days. Still, life for this pioneer is far from easy. It's a whole new world. In the last fifty million years, plants have developed into trees. And with nothing around to eat them, they've grown into vast forests pumping oxygen into the air. Hynerpeton has evolved complex lungs to exploit this new oxygen. His lungs are sacks, just like ours, and he breaths like we do.

Forcing air in and out so his blood can absorb more oxygen. We still rely on the design developed in this strange amphibian. Hynerpeton can breath on land, but he's still water bound. His skin is much thinner than ours and it dries out in minutes, so he has to keep it wet. And water is a danger zone. The fish are now our ancestor's enemies. Primitive sharks are constantly on the hunt. But even sharks are small fry in comparison to some flesh eating fish. Hyneria weighs two tons. And is five metres long. She's an insatiable carnivore. The amphibian limbs are his saving grace.

For now. As the burning sun dips, Hynerpeton can spend more time on land. This stretch of shoreline is his territory and his trump card with the opposite sex. Hynerpeton females are choosy and will only go for males who can defend their turf. They also only mate during a short season. The male's future depends on passing on his genes and tonight could be his last chance. As night arrives, so does the competition. Another male with his eye on this prime patch. To avoid injury, the males demonstrate their strength in a strange push up contest. This rival is not up to the challenge, but now our male may be too late for love. Dawn. And all the females in the area have mated and moved on.

Hynerpeton seems to have missed his chance. The only attention he's attracting comes from the dark waters of the lake. A female finally answers his call and the male seizes the opportunity. Amphibian eggs are soft and their young have gills, not lungs, so they must be laid in water. Where amphibians are most vulnerable. Hyneria can attack like a killer whale after a seal. Only just missing her prey. But she has remarkably powerful fins. And she takes the male by surprise. The end for this Hynerpeton.

But the amphibians are about to find a way to leave the dangers of the water behind for good. The key to their future success lies in changing their eggs. They evolve a hard waterproof casing which protects the young inside from the drying sun, so they can be laid on land. The babies will hatch out, fully developed, air breathing and independent. They're the first ever true vertebrate landlubbers. The very first reptiles. But as they move inland,
they'll face an ancient enemy. More deadly than ever before.

The arthropods are back. Next time on Walking with Monsters, we enter the world of killer bugs. From huge flesh eating spiders to three metre millipede relatives. And we meet the first giant reptiles, our strange sail-back ancestors who face their toughest enemy yet. Each other.

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