At a certain period in Earth' s history, its atmosphere contained almost no oxygen, although plants were producing vast quantities of oxygen  As a way of reconciling these two facts, scientists have hypothesized that nearly all of the oxygen being produced was taken up by iron on Earth' s surface  Clearly, however, this explanation is inadequate  New studies show that the amount of iron on Earth' s surface was not sufficient to absorb anywhere near as much oxygen as was being produced
Ttherefore, something in addition to the iron on Earth' s surface must have absorbed much of the oxygen produced by plant life
The first expresses the argument's dismissal of an objection to the position it seeks to establish; the second is that position.