The consensus view is that the 02 concentration of the Archean atmosphere was very low and that it rose to its present level of 21 in a series of steps, two of which dwarf the others in importance. The first, known as the Great Oxidation Event, occurred at similar to 2.4 Ga. It involved an increase in the relative abundance of O-2, which has been estimated at three orders of magnitude, and it is important because it led to the first surface weathering. The second, although less important in relative terms, involved the addition of 9 x 10(17) kg of O-2 to the atmosphere, at least ten times as much as that required to produce the Great Oxidation Event. Its importance lies in the fact that it correlates with the rise of animals in the Ediacaran and Early Cambrian periods. Although it is widely accepted that an increase in atmospheric O-2 facilitated the appearance of animals at similar to 575 Ma, followed by the Cambrian Explosion similar to 50 Myr later, the cause of this increase remains controversial.