Geology Reference
In-Depth Information
split off Gondwana and moved south, forming their own island continents. A rift between
South America and the west coast of Africa opened the South Atlantic, while India broke
off the east coast of Africa and began its 50-million-year journey northward, ultimately to
smash into Asia and crumple up the Himalayan Mountains.
Throughout this protracted history, each of the various continental players scurried here
and there, forming partnerships and then breaking up, not unlike a human drama. It helps
toseethisglobalplayunfold:justGoogle“Pangaea animations.” Asyouwatch, remember
that the shifting continents imposed other changes on Earth. Greater stretches of coastline
promoted more life in shallow waters. Polar landmasses promoted thick ice sheets, which
inturnloweredsealevel.Lifeevolvedinharshcompetitiononlargerlandmasses,butevol-
ution proceeded independently on isolated continents or in widely separated seas. The loc-
ationsofmountainrangesandoceansalteredclimate.Throughouthistory,astoday,eachof
Earth's great cycles has affected every other one.
The Animal Explosion!
For billions of years, the extent of Earth's microbial life had ebbed and flowed in response
toclimate,nutrients,sunlight,andmore.Newevidencefromshallow-watersedimentssug-
gests that the great algal blooms at the end of the Neoproterozoic were more than just a
few temporary blips. For the first time, green photosynthetic algae evolved new strategies
to achieve a firm footing on swampy land—the continents were finally looking green at
the edges, rather than Martian orange against a blue ocean. As atmospheric oxygen soared
in concentration, so too did the stratospheric ozone layer—the radiation barrier that effect-
ively shields Earth's solid surface from the Sun's lethal ultraviolet rays. Such a protective
blanket was an essential prelude to the rise ofa viable terrestrial biosphere offirmly rooted
plants and freely roaming animals.
Strangely, it took animal life another hundred million years to crawl fully onto land. For
averylongtime,mostbiological innovationtookplaceintheshallowsunlitseas.Forforty
million years, multicellular jellyfish and worms appear to have dominated the postglacial
oceans. Myriad soft-bodied animals, rarely preserved in the fossil record, fed on seafloor
detritus and hid in the recesses of minerals laid down by their microbial forebears. For tens
of millions of years, an ecological status quo seems to have prevailed.
That status quo was permanently disrupted roughly 530 million years ago by a striking
evolutionary trick: many types of animals learned to build their own protective shells out
ofhardminerals.Nooneisquitesurehowthisevolutionarydevelopmentoccurred,though
life had already been depositing mineral layers in reeflike stromatolites for billions of
years. Somehow, somewhere, following the Gaskiers glaciation 580 million years ago, an
unknownanimal evolvedtheexquisite trickofgrowingitsownprotective hardpartsoutof
