How Long Ago Did Earth Start Supporting Life and How Do We Date Its Formation?

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I guess it'd be hard to tell when the Earth became the earth, so I suppose a better question would be; 'How long ago was the Earth able to support life?' I've read that life has only had approximately 200 million years to get to where it is today because of the period of heavy bombardment which lasted for the first 400-600million years of our spinning rock's existence, but my anthropology professor pointed out that the first recorded fossilized microbial life is 3 billion years old. Naturally I'm puzzled, please help me gain clarity. (also, for curiosity's sake, I'm interested in how we date the earth)
 
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BuddyPal said:
I guess it'd be hard to tell when the Earth became the earth, so I suppose a better question would be; 'How long ago was the Earth able to support life?' I've read that life has only had approximately 200 million years to get to where it is today because of the period of heavy bombardment which lasted for the first 400-600million years of our spinning rock's existence, but my anthropology professor pointed out that the first recorded fossilized microbial life is 3 billion years old. Naturally I'm puzzled, please help me gain clarity. (also, for curiosity's sake, I'm interested in how we date the earth)

Hopefully the links above cleared up most of your questions! However, some simplified dates just in case:

4.55 Ga: Formation of the Earth and Moon
3.8 Ga: End date of the 'late heavy bombardment'
3.7 Ga: First evidence of bacterial life (geochemical indicators)
2.5-2.0 Ga: Oxygenation of the Earth's atmosphere via increased bacteria
630 Ma: Oldest geochemical indicators of animal life (early demosponge)
580 Ma: First macroscopic animal life observed in fossil record
~450 Ma: Land plants appear

Abbrevations above: Ga means 'billion years ago', and Ma means 'million years ago'.