Jason Bourne lives life on the edge. The protagonist of the Bourne spy fiction thriller series relentlessly pursues the truth, with danger lurking at every turn. Regardless of the peril, he must continue. In ways, Earth’s history demonstrates similar hazards. Starting from the most inhospitable circumstances, numerous physical transformations now enable Earth to teem with life. But many of those events brought the Earth—and its life––to the brink of extermination forever. One such change occurred roughly two-and-a-half billion years ago.
For the first two billion years, Earth’s atmosphere contained no free (uncombined with other elements) oxygen. Although oxygen was present, it was tied up in molecules like carbon dioxide and water vapor. Yet anything more sophisticated than single-celled organisms requires free oxygen because of the energy oxygen releases during chemical reactions.
Fortunately, photosynthetic organisms appeared on the scene at this time and began producing abundant quantities of oxygen. Over the course of a couple hundred million years, these organisms delivered a permanent oxygen component to Earth’s atmosphere—although at levels much lower than today. Scientists refer to this period as the Great Oxygenation Event (GOE). On the positive side, the permanent oxygen component generated an ozone layer in the stratosphere. Since the GOE, the ozone layer has protected life from the harmful ultraviolet radiation emitted by the Sun.
Negatively, this permanent oxygen reservoir also began wreaking havoc with Earth’s temperature. The dominant greenhouse gas before the GOE was methane (CH4). (While carbon dioxide receives a lot of press as a greenhouse gas, methane is over 60 times more effective at trapping heat from the Sun.) Because of oxygen’s high reactivity, during the GOE it reacted with the methane to produce carbon dioxide (CO2) and water (H2O). Like turning off a thermostat in the middle of winter, Earth’s temperature would have plummeted. In fact, an international team of geologists recently discovered evidence for extensive glaciations corresponding to this increase in atmospheric oxygen.1 This was the first widespread ice age on Earth.
The dramatic nature of this cooling likely resulted in glaciers covering the entire surface of Earth. Such a state, if it persisted, would drive life to near extermination. However, it appears the same process that initiated the covering of Earth with glaciers also helped remove the glaciers. Cooler oceans dissolve more oxygen. This dissolved oxygen then reacted with the carbon remains of previous life that rested on the ocean floor. The abundant carbon dioxide released further enhanced the greenhouse heating, leading to a warmer Earth.
By itself, either event (the cooling induced by the GOE or warming caused by the reaction of dissolved oxygen with carbon remains) had the potential to render Earth uninhabitable. The fact that both occurred concurrently suggests that a supernatural Designer––like a spy thriller novelist who knows where the plot’s going––orchestrated both events in order to prepare Earth for the arrival of human beings.
1 Qingjun Guo et al., “Reconstructing Earth’s Surface Oxidation across the Archean-Proterozoic Transition,” Geology 37 (May 2009): 399–402.