When the Earth was only about a third of its eventual size, it was probably already beginning to form an atmosphere, mostly of carbon dioxide, nitrogen, methane and sulphur. Hardly the sort of stuff that we would associate with life, and yet from this noxious stew life formed. Carbon dioxide is a powerful greenhouse gas. This was a good thing, because the Sun was significantly dimmer back then. Had we not had the benefit of a greenhouse effect, the Earth might well have frozen over permanently25, and life might never have got a toehold. But somehow life did. 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 wonder how it had all happened.
( Bill Bryson )
[ A Short History of Nearly ]
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