As to the question of there being enough information to deduce whether there was no life on earth originally...I thought there was.
And the way the evidence was indicated was through geology, i believe. Right now my information is a little sketchy its been a couple million words reading since i last read about life on earth earlier.
talk origins had a few faqs in regards to it, one of them indicated that oxygen dependant life had not occured yet because earth was a heavily reducing atmosphere. and when photosynthesis started occuring is when we started moving toward a more oxidizing atmoshphere. And the evidence that photosynthesis wasn't present the entire time was because of thick bands of oxidized iron compounds IIRC. Any oxygen that was created through photosynthesis pretty quickly bound up with iron which started forming thick layers on the crust.
Err...so the heavy bands of iron kind of indicate that start of oxygen type life (i'm not so sure about the not so oxygen kind of life that exists near thermal vents though)
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