Part one: in the beginning...

13.7 billion years ago to 420 million years ago

Our story begins with the explosion of our universe from an invisible speck of infinite energy 13.7 billion years ago, an episode popularly known as the Big Bang.

Why are scientists so sure such a thing happened, when no one was there to witness it? What led to the creation of our Sun, our Solar System and the planet Earth about 4.5 billion years ago? What makes the Earth different from other, neighbouring planets? The pages that follow attempt to tell the story in simple, chronological terms.

This opening part also looks at the beginnings of life on Earth, about 3.7 billion years ago. We are, however, still some way off the beginnings of human life. For more than 2 billion years, invisible microscopic life-forms filled the seas, changing the Earth's environment, filling the air with oxygen, seeding clouds with rain. Aeons later, more exotic, multi-cellular creatures began to emerge – worms, sponges and jellyfish – as rising levels of oxygen meant cells could glue themselves together into larger, visible forms of life.

By about 530 million years ago, hard surfaces that easily fossilise – bones, shells and teeth – created new species that became successful in the increasingly competitive seas, many of them leaving a precious record as fossils in the rocks: the ultimate museum of early life on Earth.

THE BIG BANG: How an invisible speck of infinite energy exploded our universe into existence

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 pulverised 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 tennis ball, now a pea – and see it crushed even smaller than the dot on top of the 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 hot 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 ( pictured below), was formed about 100 million years after the Big Bang – that's 13.6 billion years ago. It's in the shape of a large disc – like two back-to-back fried eggs – that 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 that 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 20 billion tons every day. It is known as the solar wind and 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 bubbled 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 an early planet called Theia. About 50 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 vapourised 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 a huge shockwave 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 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.

Today there is no physical evidence on Earth of the impact of the collision with Theia – no crater – because, such was its force, all the outer material vapourised. 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.

of very hot dust. Out of these clouds came the first stars, massive balls of hot 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 ( pictured below), was formed about 100 million years after the Big Bang – that's 13.6 billion years ago. It's in the shape of a large disc – like two back-to-back fried eggs – that 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 that 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 20 billion tons every day. It is known as the solar wind and 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 bubbled 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 an early planet called Theia. About 50 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 vapourised 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 a huge shockwave 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 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.

Today there is no physical evidence on Earth of the impact of the collision with Theia – no crater – because, such was its force, all the outer material vapourised. 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.

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