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Our solar system: about 4.6 billion years ago
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In our own galaxy, the Milky Way, a star is formed about 4.6 billion years ago - about two thirds of the way through the story so far of the universe. It is the star which we know as the sun.
As its material contracts, many particles are left spinning freely round the central mass of the new star. It is these which coalesce to form the planets, including earth.
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In our emerging solar system many smaller lumps of matter are also within the sun's gravitational field. Some of them, the asteroids (varying from a few millimetres to a kilometre and more in diameter), settle into orbit round the sun.
Larger bodies, such as our moon or the satellites of Jupiter, begin orbiting individual planets - as do the particles, varying in size from pebbles to rocks, which form the rings of Saturn.
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Mercury, the nearest planet to the sun, has a daytime surface temperature of around 350° Centigrade - far too hot to support life. Pluto, the outermost planet, is believed to be covered in a blanket of ice some 150 miles thick.
By contrast the earth, third in distance from the sun, has the moderate temperature range with which we are all familiar. It is one, but only one, of the factors which make life on earth possible.
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The earth takes shape: about 4.5 billion years ago
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The particles which coalesce to form the earth are relatively cold. As they condense by gravity into a solid sphere, the physical conditions develop which survive today.
Theories on the earth's interior remain speculative, but it is believed that the central inner core - of extreme density and heat - is composed mainly of iron and nickel, under such pressure that it remains solid even at perhaps 7000° C. Outside this central region is the outer core, a thick layer of molten metal, again mainly iron and nickel. From there, almost to the surface of the earth, is the mantle - a shifting mass of molten rock, on which our own familiar landscape floats and drifts.
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The greater part of the earth's volume is the mantle, formed of rock and about 3000 km deep. It is molten in its lower parts and solid near the surface. The extreme outer surface - visible in mountains, plains and ravines - is conventionally defined as the crust of the earth, differing from deeper rocks through the effects of vegetation and weather. But in the context of the earth's long and unstable history, a more revealing distinction can be made between the underlying molten layer and the solid surface of the earth (the lithosphere).
It is in the relationship between these that the earth's own drama develops.
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Over the millions of years of the earth's existence, areas of land have emerged from the waters, drifted among them, crashed into each other, broken up and in part been submerged again - in an on-going dance of the continents of which we are still a part.
Ocean beds and mountains alike are fixed to large rigid plates (each of them a part of the lithosphere), which float on the molten layer of the earth's mantle. The theory of Plate tectonics has revealed how the movement of the plates develops new areas of ocean floor, pushes up mountains and causes the geological shudder of earthquakes.
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Pangaea: 250 - 200 million years ago
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In the shifting story of the face of the earth, the land surface merges into one single continent about 250 million years ago. It is from this land mass that our own geography has gradually emerged.
This continent has been given the name Pangaea (Greek for 'all earth'). About 200 million years ago Pangaea splits into two parts, north and south, separated by water - the Tethys Sea. The area north of the Tethys Sea, named Laurasia, includes the future north America, Europe and most of Asia. South of the sea, a continent named Gondwanaland is made up of what will be South America, Antarctica, Africa, India and Australia.
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From one continent to six: 200 - 20 million years ago
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The reshaping of the surface of the earth, into the pattern now familiar to us, takes place between 200 and 20 million years ago.
First south America splits from Africa and drifts westwards (it is the snug fit between their coast lines which suggests the idea of continental drift to Alfred Wegener in 1912). Then Antarctica, India and Australia separate from Africa. Antarctica moves to the south, while India and Australia drift north and east.
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Africa and India move slowly but forcefully towards Europe and Asia, reducing the Tethys Sea to its present-day remnant (the Mediterranean) and throwing up the Alps and the Himalayas from the force of the collision.
Finally north America splits from Europe and Asia (though remaining almost linked at its northern tip), thus forming the Atlantic ocean and completing the disposition of the continents as we know them.
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Our place in the scheme of things
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Since the rest of the story will take place on earth, this is the moment to put our own solar system into perspective. Until the 16th century it is assumed by mankind that the earth is the centre of the universe.
The discovery that the earth goes round the sun comes as a shock, but it still leaves the sun at the centre of the universe - a universe which is assumed to consist of our galaxy.
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Then, as recently as 1918, it is proved that the sun is quite near the edge of the galaxy, and that the galaxy is larger than previously thought.
Finally, in 1929, comes the discovery that Andromeda, visible to the naked eye in the night sky, is itself another entire galaxy, slightly larger than our own. Everything else that we can see in the sky amounts to less than is contained in the faint blob of Andromeda.
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The present orthodoxy, discovered by the techniques of astrophysics, is breathtaking. The sun is now revealed to be one of about a hundred thousand million similar stars in our own galaxy; and it is now known that our galaxy is one of hundreds of thousands of millions of galaxies. (These facts always come in round numbers, because a nought more or less is only a factor of ten; and that is as near as anyone can speculate.)
But if the numbers of stars are impossible to imagine, so is the space between them. Light travels 186,000 miles in a second; yet it takes a ray of light 30,000 years just to cross from one side to the other of our own galaxy.
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Many of the countless million stars in those countless million galaxies must have planets, and it seems unlikely that ours is the only one with conditions suitable for life. But if there is life out there, the distances make friendship hard.
In 1974 a radio message is beamed from a telescope in Puerto Rico towards a tight cluster of about 300,000 stars elsewhere in our galaxy. The message, in binary code, gives a few basic scientific facts about planet earth. But even travelling at the speed of light it will take 24,000 years to reach its destination, and double that to get back an instant reply.
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To put the same point in another way, the light by which we now see Andromeda left that galaxy 2.3 million years ago.
When it left, there were as yet no human beings. But there was a great deal of other life on earth and had been for billions of years.
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