Geology Reference
In-Depth Information
contamination,bothancientandrecent.Thepaleontologicalcommunitygreetseveryreport
ofmolecules survivingforseveral billions ofyearswithcaution, ifnotoutrightskepticism.
Nevertheless, the chemical traces are there and may be the best window onto this tenuous
ancient biosphere. (More on how in
chapter 7
.)
Byourplanet'sone-billionthbirthday,lifehadestablishedafirm,ifrelativelyinconsequen-
tial, foothold on its surface. For another billion years, Earth's microbial life would gently
nudge near-surface environments, first byspeeding along redox reactions and then through
photosynthesis. As near as we can tell, even at two billion years old, Earth would not have
displayed any significant life-imposed mineralogical novelty at or near its surface. Cells
would simply make more iron oxides, more limestone, more sulfates and phosphates than
might otherwise have formed. They would build layered deposits of iron-rich minerals in
the deeper ocean and craft protective rocky mounds in coastal shallows—all phenomena
that had transpired on Earth before the dawn of life, and on other planets and moons in the
Solar System.
But Earth and its primitive microbial population were poised to make the most dramatic
transformation in the planet's history. Over the next 1.5 billion years, photosynthesizing
microbes would learn a new chemical trick—to exhale the highly reactive, dangerously
corrosive gas called oxygen.
