About 4.6 billion years ago, a great swirl of gas and dust some 24 billion kilometres across accumulated in space where we are now and began aggregate. Virtually all of it - 99.9 per cent of the mass of the solar system - went to make the Sun. Out of the floating material that was left over, two microscopic grains floated close enough together to be joined by electrostatic forces. This was the moment of conception for our planet. All over the inchoate solar system, the same thing was happening. Colliding dust grains formed larger and larger clumps. Eventually the clumps grew large enough to be called planetesimals. As these endlessly bumped and collided, they fractured or split or recombined in endless random permutations, but in every encounter there was a winner, and some of the winners grew big enough to dominate the orbit around which they travelled.
It all happened remarkably quickly. To grow from a tiny cluster of grains to a baby planet some hundreds of kilometres across is thought to have taken only a few tens of thousands of years. In just 200 million years, possibly less, the Earth was essentially formed, though still molten and subject to constant bombardment from all the debris that remained floating about.
As this point, about 4.4 billion years ago, an object the size of Mars crashed into the Earth, blowing out enough material to form a companion sphere, the Moon. Within weeks, it is thought, the flung material had reassembled itself into a single clump, and within a year it had formed into the spherical rock that companions us yet. Most of the lunar material, it is thought, came from the Earth's crust, not its core, which is why the moon has so little iron while have a lot. The theory, incidentally, is almost always presented as a recent one, but in fact it was proposed in the 1940's by Reginald Daly of Harvard. The only recent thing about it is people paying any attention to it.
For the next 500 million years the young Earth continued to be pelted relentlessly by comets, meteorites and other galactic debris, which brought water to fill the oceans and the components necessary for the successful formation of life. It was a singularly hostile environment, and yet somehow life got going. Some tiny bag of chemicals twitched and became animate. We were on our way.
Four billion years later, people began to winder how it had all happened."
- 'A Short History of Nearly Everything' by Bill Bryson