Geology Reference
In-Depth Information
But continued doubling every week would have quickly produced a remarkable trans-
formation. Twenty more weeks would have seen miles of RNA-infested waters, perhaps
along the coast or in an inland lake or a deep-sea environment. And within two years, as-
suming that a single initial RNA strand doubled every week, Earth could boast as much as
a million cubic kilometers of living stuff—enough to clog the entire Mediterranean Sea.
Primitivesingle-celled organismsthatfedonthechemical energyofrockscouldn'thave
had much effect on Earth's geology—its near-surface distribution of rocks, for example,
or the diversity of minerals. Living or no, four billion years ago the ancient land remained
barren black and gray, surface weathering was slow, and the earliest life would have con-
tributed almost nothing to alter the globe-spanning blue oceans.
Because the first scrappy microbes would have made little mark, we can't tell for sure
whenlifebegan.AfewofEarth'smostancientsedimentary rocks,thoselaiddowninshal-
low ocean environments about 3.5 billion years ago, hold unmistakable microbial fossils.
Domelike rocky stromatolites, a few inches to a few feet across, formed where colon-
ies of cells precipitated thin layer upon layer of minerals in shallow settings. Microbial
mats covered wide swaths of shoreline, consolidating and patterning sand in tidal zones.
Even a few carbon-rich spheres with distinctive cell-like walls—possible microbial body
fossils—have survived the aeons. But no older incontrovertible fossils have been found.
Geochemical traces of carbon and other bioelements from intensely altered rocks 3.85 bil-
lion years oldare tantalizing, butbynomeans have they convinced the geological commu-
nity.
So when did life arise? If your hunch is that life emerges early and often on any
viable planet or moon, then perhaps you'd advocate a stable biosphere by 4.4 billion years
ago—within Earth's first 150 million years. All the ingredients were there: oceans and air,
mineralsandenergy.Giantimpactsofasteroidsandcometswouldhavechallengedthesur-
vival of this Hadean life, perhaps favoring those hardy cells that learned to live hot and
deep in protected rocky homes beneath the ocean floor. Perhaps life came into being more
than once, perhaps many times, before Earth settled into its calmer postadolescence. If so,
then the 3.5-billion-year-old fossils would represent an ecosystem almost a billion years in
the making.
If, on the other hand, you suspect that life's origins are difficult and rare in the cosmos,
then a date closer to 3.5 billion years ago may seem more likely. Perhaps life is so improb-
able that a billion years of mineral-molecule interactions throughout hundreds of millions
of cubic miles of ocean crust were necessary. Perhaps those precious, sparse fossil remains
of the so-called Archean Eon mark the true beginnings of the biosphere.
Living Earth
