Answering the question 'When did life first appear on Earth?' is extremely difficult, because all that we can say from the fossil and chemical evidence is that life was already in existence, when we find the first sedimentary rocks. The oldest sedimentary rocks, however, formed at least 600 million years after the formation of the planet. If there was photosynthesis at 3.7-3.9 billion years ago, then more primitive life forms must have existed well before this time.
An comparison between the history of the Early Earth and the history of the Moon shows that there was an important period of meteorite bombardment which ceased between 3.9 and 3.8 billion years ago. This is the same time interval as that of the Isua sediments, the Earth's earliest record of life. Meteorite bombardment of the young Earth would have generated sufficient energy to vaporise the surface layer of the oceans and thereby sterilise the Earth, killing all emergent life. It is possible therefore, that despite many attempts, life was unable to emerge on the Earth until after the 'late heavy bombardment' had ceased at about 3.8 Ga.