The length of the transition period from the reducing to the neutral biosphere.

The development of the complicated mechanisms for N2 fixation, which in nature is an endergonic process and requires a great deal of ATP, must have taken a long time. During that time primeval NH3 must still, albeit to a decreasing extent, have been available as a source of nitrogen. This is true, whether N2 fixation originally arose in the primitive anaerobes, or, according to Postgate, in more advanced bacteria. As NH3 resists UV radiation only in the presence of excess H2 it follows that the disappearance of H2 and the transition from the reducing to the neutral biosphere also took a long time, probably of the order of 10(9) degrees yr. According to previous evidence, the transition from the neutral to the oxidizing biosphere likewise took long; this length enabled the organisms to adapt the N2 fixing machinery to aerobic conditions.