Geology Reference
In-Depth Information
many millions of years passed before the first bony animal felt fully at home breathing air.
The oldest known fossil bones of a four-legged land animal, a walking fish with finlike
feet, come from rocks about 375 million years old.
The gradual transition from fish to amphibian has increasingly come into focus over the
past two decades—a blossoming period of spectacular paleontological discoveries from
China to Pennsylvania. New fossil finds point to a 30-million-year interval of intermediate
forms, progressively more suited to land but still retaining distinctively fishlike anatomical
features. The first true amphibians appeared about 340 million years ago in the middle of
theso-calledCarboniferousPeriod,atimewhenswampyforestswerethrivinginlow-lying
areas around the world. These primitive land animals, characterized by broad, flat skulls
andequippedwithsplayedlegs,five-toedfeet,earssuitedforlistening inair,andotherter-
restrial adaptations, were clearly different from their fish ancestors. By the Carboniferous
Period, Earth's solid surface had for the first time evolved to a strikingly modern appear-
ance, with dense green forests of tall fernlike trees, dank swamps, and lush meadows pop-
ulated with an ever-widening cast of insects, amphibians, and other creatures. And thanks
totheprofoundinfluenceoflife,Earth'snear-surfacerocksandmineralshadalsoachieved
something akin to their modern state of diversity and distribution.
Not that Earth had achieved anything close to stasis, mind you. Climates waxed and
waned,droughtsandfloodsstressedtheland,andtheoddasteroidimpactandsupervolcano
eruption caused traumas to life the likes of which we may hope never to see. But Earth and
its biota have proven unfailingly resilient to such insults. Life always finds a way to adapt
to the reality of now—whenever now is.
The Third Great Oxidation Event
By 300 million years ago, Earth's forests were flourishing. Indeed, so much leafy biomass
wasbeingproducedandburiedthatanewrocktype,carbon-richblackcoal(hencetheCar-
boniferous Period), began to form by the pressure-cooking of thick masses of dead plants.
One consequence of this organic carbon sequestration was a new pulse of oxygen into the
atmosphere,justasinthepreviousNeoproterozoicoxidationevent.Theriseinoxygenwas
gradual, from roughly 18 percent of the atmosphere 380 million years ago, to 25 percent
about 350 million years ago, to a remarkable 30 percent or more 300 million years ago.
Indeed, by some estimates the atmosphere's oxygen content briefly soared to more than
35 percent, well above modern levels. And these extreme figures aren't simply guesses:
beautiful specimens of Carboniferous-age amber, fossilized tree sap, preserve ancient at-
mospheric bubbles that still hold 30 percent or more oxygen.
The rise in oxygen had beneficial consequences for animal life. More oxygen meant
more energy and increased rates of animal metabolism. Some creatures took advantage of
