Question: how did the world start ?

Ciarán O'Brien answered on 13 Nov 2014:

Basically, it was left over stuff from the birth of the sun.

Our solar system used to be a gigantic cloud of mostly hydrogen gas that came together under its own gravity. This pulling together of the gas caused it to spin a little, and as the gas cloud got smaller it spun faster (It’s the same effect as when you spin in a swivel chair and then retract your arms and legs, you start spinning faster).

As it spun faster, the gas would throw off arcs of material in the same general direction (if you get someone to spin you faster and faster in a chair, the force of the spin will try to throw you off the chair), so the end effect was a flattned disc of gas with a really dense, hot bit at the centre.

This central blob of gas condensed under its own gravity and eventually became the sun. the rest of the gas in the flattened disc eventually gathered together into smaller blobs that became planets. The biggest clumps of gas became our gas giants Jupiter and Saturn, while the rest became relatively smaller planets like Venus, Mercury and our home, Earth. They started off with no atmosphere, but the molten rock and stuff they were made of also threw out loads of gases, which became trapped by the planet’s gravity (Except for Mercury, which is so close to the sun that the atmosphere kept getting burned away by the heat and radiation).

Starting off the Earth was really hot, and it took billions of years to cool down enough for the surface to even become solid.It took even more time to cool enough for rain to happen, and back then it would have been really really dangerous corrosive rain because the Earth was still spewing out dangerous hot gases from underneath its surface. There would have been almost no oxygen. Earth was not a nice place back then.

Somewhere in the steaming oceans and volcanoes spewing out chemicals and minerals into them and lightning striking everything constantly, life would eventually form, tiny bacteria or something very like them. They would have done the bulk of the work putting oxygen in the atmosphere, which made it possible for oxygen-fuelled life to develop, and from there on it was just a matter of evolution over about 4 billion years until we got to this point. 🙂