Geology Reference
In-Depth Information
of millions of years. And so began the astonishing coevolution of the geosphere and bio-
sphere.
Evolutionbynaturalselectioncontinuedtodrivealltheseprocesses.Amicrobialspecies
that learned to use its iron food source more efficiently, or to tolerate more extreme con-
ditions, or to exploit new redox reactions, had a distinct advantage and ensured its own
survival. Consequently, mutating populations of microbes invented new catalysts that pro-
moted these energy-producing reactions more efficiently than the nonliving environment.
The results, here and there, were little mounds of limestone and modest deposits of iron
oxides,alongwithagradualincreaseintheprocessingofnear-surfacecarbon,sulfur,nitro-
gen, and phosphorus. Nevertheless, these earliest life-forms did little more than mimic the
chemistry that had already begun (albeit more slowly) on the previously nonliving world.
Light
Mostoriginsresearcherssuspectthattheearliestlife-formsreliedexclusivelyonthechem-
ical energy of rocks—an abundant source, to be sure, but one that greatly restricted where
life could thrive. At some point, a few microbes moved beyond their role as mediators of
chemical reactions intrinsic to their environment. They learned to collect solar radiation,
whichwouldproveanabundant,cheapenergysourceforanysurfacedweller,anywhereon
the planet.
In its most basic form, photosynthesis uses sunlight to make biomolecules from such
ubiquitous raw materials as carbon dioxide, nitrogen, and water. Given the right chemical
scaffolding,alloflife'sessentialbuildingblocks—aminoacids,sugars,lipids,andthecom-
ponents of DNA and RNA—can be made from atmospheric gases and the Sun's radiation.
Unlike modern green algae, those first microbial photosynthesizers generated no oxygen.
Indeed,modernanaloguesoftheseprimitiveorganismstendtoformabrownishorpurplish
scum on stagnant ponds. Some biologists have even suggested that immense floating rafts
of photosynthetic microbes discolored the blue Archean oceans with unsightly brownish-
purple splotches.
How would we know? Such microbes have no hard parts to preserve as fossils; nor do
floating algal mats alter the rock record in any obvious way. However, there may be a way
to tease out evidence of the most ancient light-loving microbes. Photosynthetic cells rely
in part on hopanes—distinctive molecules with five interlocking carbon rings (a configur-
ation closely related to that of the steroids so much in the sporting news these days). After
microbes die and decay, their telltale multiring hopane backbones can survive for billions
of years as a molecular residue in fine-grained ocean sediments. Meticulous chemical pro-
cessing is required to extract and analyze these geohopanes from the bulk rock. Tentative
interpretations must incorporate a litany of tricky assumptions about possible sources of
