In the section of this module on 'So When Did the First Life Appear on Earth?' the link between a late impacting event and the emergence of life at about 3.8 billion years ago was discussed. The logic of the argument was based upon the volatilisation of the upper levels of the ocean. However, if life was evolving in the deeper parts of the oceans it is possible that it was unscathed by all but the hugest asteroid impact, of the type which led to the formation of the moon at about 4.4 billion years ago. Thus during the late heavy bombardment, between 3.8-3.9 billion years ago, life at the bottom of the oceans would have been protected from the impacting.
In addition, the evidence for photosynthesis at about 3.8 billion years ago implies a long prior evolutionary history. This requires an environment protected from impacting and is consistent with a deep oceanic environment.