Geology Reference
In-Depth Information
Green Earth
The Rise of the Terrestrial Biosphere
Earth's Age: 4.0 to 4.5 billion years (the last 542 million years)
Plate tectonics saved Earth from itself. Slowly, relentlessly, Earth's convecting interior
propelled the breakup of the sprawling equatorial Rodinian supercontinent into more man-
ageable chunks. Continental masses shifted poleward, liberating the Equator from ice-ac-
cumulating lands, moderating the extreme snowball-hothouse cycle. Abundant new photo-
synthetic algal life also helped buffer the wild fluctuations of carbon dioxide, while raising
oxygen concentrations close to modern levels. Earth has not since endured such excesses of
global temperature as those that preceded the Phanerozoic Eon.
At least five kinds of changes have been at work on Earth during these last 542 million
years. Continents have continued to shift, first closing one ocean to form yet another great
supercontinent, then breaking up to form the still-widening Atlantic Ocean. Climate has
fluctuatedfromhottocoldandbackagainmanytimes,thoughnottothesnowball-hothouse
extremes ofthe Neoproterozoic. Oxygen has enjoyed a third great enrichment event, only to
seeatmosphericconcentrationsdropinhalfandreboundagain.Sealevelshavealsochanged
repeatedly, dramatically reshaping Earth's coastlines; the rock record reveals countless rises
and falls, often by several hundred feet. But most spectacular of all, life has changed and
evolved radically and irreversibly. And throughout all these transformations, life and rocks
have coevolved.
Earth has always been a planet of change, but the story of the Phanerozoic Eon is much
more sharply in focus and seems correspondingly more elaborate and nuanced in its vari-
ations, thanks to a more extensive and less altered rock record. The key to this rich story
is a wealth of exquisitely preserved fossils, the consequence of life's newfound ability to
make durable hard parts: teeth, shells, bones, and wood. Animals and plants turn out to be
particularly sensitive to changes in Earth's near-surface environment, and so their fossilized
remainsrecordepisodeafterepisodeofadaptation.Microbescanweatheralmostanystorm;
that resilience, coupled with their unhelpful simple shapes and sparseness in the fossil re-
cord, means that no obvious mass extinction can be recognized in rocks of the Precambrian,
when microbes ruled. But the Phanerozoic Eon is an altogether different story.
Thus, over the last 542 million years, we see Earth in a new light. Not a planet changing
leisurely over tens or hundreds of millions of years, but a rapidly evolving world—every
