But a billion years of bacterial evolutionary progress was soon
stunted by a catastrophic global event. Geologists find no signs
of a great meteor impact nor a volcanic eruption, but they have
uncovered the unmistakable geologic scars of rapid worldwide
climate change. Average temperatures, which were previously
comparable to our present climate, plummeted to minus 50 degrees
Celsius and brought the planet into its first major ice age.
This environmental shift triggered a massive die-off which
threatened to extinguish all life on Earth, and
paleoclimatologists have good reason to believe that this world-
changing event was unwittingly caused by some of the planet's
own humble residents: bacteria. The period in history is known
as the Paleoproterozoic era, and prior to that time the Earth's
ecology and environment were significantly different. The iron-
rich waters of the oceans lent them a green tint, and the
atmosphere was comprised of gasses other than oxygen. Although
oxygen atoms were abundant, such as thos