Geology Reference
In-Depth Information
YoucouldhavesurvivedquitecomfortablyonEarthfourhundredmillionyearsago.There
was plenty of oxygen and water. There was food in the form of plants and bugs. There
was shelter under the giant stands of
Prototaxites.
But the landscape would have seemed
so strange. There were green stems and green branches but absolutely no leaves.
Indeed, the invention of the first tiny energy-trapping leaves took tens of millions of
years more—a transformative development that raised the stakes in the plant kingdom's
evolutionary struggle for sunlight. The tallest plant with the biggest leaves enjoyed an ad-
vantage and so followed the evolution of fanlike ferns, branching limbs, and sturdy woody
trunks. By 360 million years ago, forests had emerged as an utterly new terrestrial ecosys-
tem. For the first time in history, Earth's land was emerald green.
In a theme that has been repeated over and over again, rocks coevolved with this new
verdant life. The rise of fast-spreading land plants, some of which achieved giant treelike
stature,hadprofoundmineralogicalconsequences.Weatheringratesofmanysurfacerocks,
including basalt, granite, and limestone, increased by an order of magnitude as a conse-
quence of roots and their rapid modes of biochemical breakdown. The resultant soils, rich
in clay minerals, organic matter, and a host of microorganisms, became deeper and more
widespread and provided an ever-expanding habitat for more and larger plants and fungi.
Root systems, though hidden from view, evolved in remarkable ways as well. Most im-
portant were new symbiotic relationships between plant roots and vast networks of fungal
filaments called mycorrhiza. This astonishing evolutionary strategy affects the great ma-
jority of plants you see today; indeed, many plants tend to grow poorly in soils lacking
fungalspores.Themycorrhizalfungiefficientlyextractphosphateandothernutrientsfrom
the soil and pass them to the plant, which in turn provides the fungi with a steady diet of
energy-rich glucose and other carbohydrates. It's hard to imagine that subterranean geo-
metry,buttheextensivenetworkofatree'srootsandfungalfilamentsbelowgroundisquite
often far larger than the tree we see aboveground.
Animals, too, experienced profound evolutionary advances as edible plants expanded
across the landscape. Invertebrates—insects, spiders, worms, and other small
creatures—were the first terrestrial explorers. Vertebrates, which initially appeared in the
guise of primitive jawless fish about 500 million years ago, underwent more than 100 mil-
lion years of evolution in the oceans before their first halting attempts to colonize dry land.
Vicious, alien-looking armored fish with bony-plated jaws arose 420 million years ago;
much more familiar cartilaginous sharks and bony fish appeared and diversified over the
next 20 million years. But dry land was utterly devoid of vertebrates.
The recent discovery of 395-million-year-old fossil fish bones in China provided the
earliest signs of the evolutionary transition from fins to four-footed land animals. For at
least 20 million years, fish flirted with the shallow, sometimes dry coastal environment. A
few fish developed primitive lungs and ventured onto land for longer and longer stays, but
