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1. Evolution
The earliest known life appeared on Earth between 3.5 and 3.9 billion years ago. It is not known for certain where or when the building blocks for life first formed. There are a number of theories and debates over where the line between 'life' and 'not life' is, and what the origin, or origins, of life were. 'Extra-terrestrial' theories suggest that life formed from organic molecules transported to Earth by meteorites. 'Terrestrial' theories suggest that life started here on Earth, possibly in deep sea hydrothermal vents, on primordial beaches, or in a variety of other possible ways.
Charles Darwin suggested in a letter to a friend that "in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present ... a protein compound formed, ready to undergo still more complex change." This demonstrates one of the more popular theories of the origin of life: that at some time during the first billion years of the Earth's history a confluence of fortuitous physical and chemical conditions allowed for the creation of one of the building blocks of life: amino acids.
If conditions had been different it is conceivable that a different sort of life form may have emerged. All known life on Earth is based on carbon, but a common hypothesis is that under different conditions or on different planets, life could be based on other elements such as silicon.