Geology Reference
In-Depth Information
then used to make the cell's building blocks, and even the myriad structures of “antenna
systems.” (Remarkably, cells have evolved clusters of molecules that operate as tiny light-
collecting antennas.)
Blankenship finds that life has devised a bewildering diversity of photosynthetic
strategies. Life, it seems, exploits any accessible energy source. Over and over again mi-
crobeshavefiguredoutnewwaystocollectlightforgrowthandreproduction—atleastfive
separate pathways, extending deep into Earth's evolutionary history. Many details of that
history are obscure, but the most ancient and primitive of these energy-gathering chemical
reactions, possibly dating to more than 3.5 billion years ago, clearly produced no oxygen
atall.Ancestorsofthoseearlycellssurvivetodayandillustratethatthemostdeeplyrooted
biochemistries were anaerobic, neither requiring nor even tolerating oxygen.
The research of Blankenship and his coworkers not only reveals the wide range of these
diverse chemical strategies but also points to a tendency for microbes to shuffle and swap
their light-collecting genes, co-opting their rivals' photosynthetic pathways like industrial
trade secrets. Indeed, the modern scheme of photosynthesis used by virtually all plants ap-
pears to be a combination of two more primitive schemes (prosaically named Photosystem
I and Photosystem II). Contemporary organisms can thus piggyback complex biosynthesis
reactionsandcollectandusesunlightfarmoreefficientlythanthoseinearlierstagesoflife
on Earth.
More Oxygen
Even without photosynthesis, Earth's surface would have experienced a leisurely (and cor-
respondingly trivial) oxidation through the slow loss of hydrogen molecules into space.
High in the atmosphere, H
2
O molecules are vulnerable to the destructive powers of ultra-
violet radiation and cosmic rays, which can fragment water into hydrogen plus oxygen.
Water's atoms rearrange into other simple molecules, mostly H
2
and O
2
, as well as traces
of ozone, O
3
. The resulting swift-moving hydrogen H
2
molecules, unlike the much heavier
lumbering O
2
and O
3
molecules of oxygen, are able to escape the incessant pull of Earth's
gravityandflyoutintothevoidofspace.ThroughoutEarth'shistory,smallamountsofhy-
drogenhavebeenlostinthisway,leavingbehindagradualaccumulationofexcessoxygen.
Even today the process continues, as a quantity of hydrogen roughly equal to the atoms
in a few Olympic-size swimming pools escapes to space every year. By the same process,
smallerMars,withmuchlessgravitytoholditshydrogen,hasshedmuchofitswater.Over
4.5 billion years, most of Mars's near-surface hydrogen has thus escaped to space, while
iron minerals near the surface have rusted to give the planet its present red color. Even so,
thetotalamountofoxygeninMars'sthinatmosphereistrivial:wereitalltocondenseonto
the surface, the layer of liquid oxygen would be less than a thousandth of an inch thick.
