Geology Reference
In-Depth Information
Whenever first life emerged, whether before 4.4 billion years or after 3.8 billion years ago,
the fact remains: it little altered Earth's ancient surface. Those earliest microbes simply
learned chemical tricks that Earth already knew. From our planet's earliest days, chemical
reactions have taken place at or near its solid surface. The reason boils down to the distri-
bution of electrons: Earth's mantle has on average more electrons per atom than the crust.
The mantle is more “reduced” and the surface more “oxidized,” in the jargon of chem-
istry.Whenreducedandoxidizedchemicalsmeet—forexample,whenreducedmagmaand
gases from the mantle breach the more oxidized surface in a volcanic eruption—they often
undergo an energy-liberating chemical reaction. In the process, electrons transfer from the
former to the latter.
Rusting, in which iron reacts with oxygen, is a familiar example of such a reaction. Iron
metal is packed full of electrons—so many electrons, you'll recall, that some of them are
free to wander through the shiny metal and conduct electricity. Iron is thus an electron
donor.Oxygengas,ontheotherhand,issostarvedforelectronsthatpairsofoxygenatoms
mustpooltheirresourcestomakeanO 2 molecule,inwhichthemeagersupplyofelectrons
is shared by both atoms like rations on a desert island. Oxygen is the ideal electron accept-
or. So when iron metal meets molecules of oxygen, a rapid exchange of electrons occurs.
Each iron atom gives uptwo orthree electrons, while each oxygen atom takes uptwo elec-
trons. The result of this exchange is a new chemical compound, iron oxide, plus a little jolt
of energy.
In addition to iron, the common electron-sated metal elements nickel, manganese, and
copperweresubjecttooxidation.So,too,weremanyofthesimplecarbon-basedmolecules
that had been synthesized in prebiotic processes, including methane (natural gas), propane,
and butane. Oxygen gas was scarce in Earth's earliest atmosphere, but other electron-
hungry collections of atoms, including sulfate (SO 4 ), nitrate (NO 3 ), carbonate (CO 3 ), and
phosphate (PO 4 ), were readily available to fill that role.
Beforelife'semergence,redoxreactions proceededatarelatively leisurely pace.Butthe
firstmicrobeslearnedtoshuffleelectronsatanacceleratedrate.Inmanyplaces—primitive
coastlines, near-surface waters, ocean-floor sediments—living cells became the mediators
of these reactions. Communities of microbes made their livelihoods by speeding up reac-
tion rates of the rocks, using the resultant energy to live and grow and reproduce. Earth
had made iron oxides from the start, to be sure, but the first microbes made them faster.
In the process, life began, ever so slowly, to alter Earth's surface environment. Microbes
exploited the abundant energy available, in the form of the reduced iron dissolved in the
Hadean and Archean oceans; they oxidized iron to form the rusty red mineral hematite—a
chemical transformation that can release enough energy to support an entire ecosystem.
Massive Archean banded ironformations foundinAustralia, SouthAmerica, andother an-
cient terrains may thus represent the leavings of an epic microbial buffet that lasted tens
Search WWH ::




Custom Search