Geology Reference
In-Depth Information
Red Earth
Photosynthesis and the Great Oxidation Event
Earth's Age: 1.0 to 2.7 billion years
Fast-forward to the present day, and it's clear that life has irrevocably transformed
Earth's near-surface environment—most conspicuously the oceans and atmosphere but the
rocks and minerals as well. It would take more than a billion years, after the innovation of
the first living cell, for such a transformation to begin. In that period, new varieties of mi-
crobes may have created a brownish or purplish scum in some coastal regions. There may
even have been patches of greenish slime decorating equatorial shores and populating shal-
low ponds, as a few clever cells experimented with new ways to harness the Sun's radiant
energy.Butthecontinentswerestillbarren:noplantsclungtotherockylandscape,norwere
any animals present to eat them. You still would have died quickly, in agony, if you were
stranded in this anoxic world.
Earth'ssurfacechangedfromdullgraytobrickred,inageologicalafternoon,withthein-
novation ofoxygen-producing photosynthesis andtheconsequent riseofanoxidizing atmo-
sphere. It's difficult to document exactly when and how quickly slimy green algae evolved
to trigger this transformation, called the Great Oxidation Event. Our best guess comes from
subtle changes in the rock record, which suggest a pulse of photosynthesis shortly after
Earth's two-billionth birthday—about 2.5 billion years ago. After that modest start, things
happened relatively quickly: by 2.2 billion years ago, atmospheric oxygen had risen from
zero to more than 1 percent of its modern level, forever changing Earth's surface.
The intriguing story of Earth's initial oxygenation is only now coming into focus, as un-
expected new clues have emerged and promising new lines of evidence have been pursued
in earnest. The past half century of paleo-atmosphere research has seen many competing,
sometimes diametrically opposed ideas, but the scientific method is a great winnower of the
untenable and the false. We don't yet have the whole narrative, but we are getting much
closer, and the picture that is emerging is (literally) breathtaking.
Testimony of the Rocks
Evidence for the Great Oxidation Event comes from a growing catalog of observations on
rocks and minerals that date from a vast chunk of Earth's history—roughly 3.5 to 2.0 bil-
