Geology Reference
In-Depth Information
entistsdiscoverawayotherthanozonetoblockultravioletrays,thesulfurisotopedatapeg
the beginning of the Great Oxidation Event at about 2.4 billion years ago.
Making Oxygen
So where did all the oxygen come from? These days one of the first topics in any intro-
ductorybiologyclassisphotosynthesis—theremarkableabilityofplantstocombinewater,
carbondioxide,andsunlighttomaketheirtissues,whileproducingoxygenasaby-product.
Wenowtakeitforgrantedthatplantsplaythiscentralroleinmakingourworldahabitable
place,butthediscoveryofphotosynthesiswasoneofthegreatestadvancesinscience.And
like so many of science's pivotal discoveries, it came in piecemeal fashion.
The discovery of water's role came first. Detailed mechanisms of plant growth were
a mystery to scientists of the seventeenth century, but a common assumption held that a
plant's tissues must come from the mineral-rich soil, which must therefore be consumed as
plantsgrow.FlemishphysicianJanBaptistaVanHelmont(1579-1644)putthisassumption
to the test by conducting a simple experiment in the 1640s. In his own words:
I took an Earthen Vessel, in which I put 200 pounds of Earth that had been dried in
a Furnace, which I moystened with Rain-water, and I implanted therein the Trunk or
StemofaWillowTree,weighingfivepounds;andatlength,fiveyearsbeingfinished,
the Tree sprung from thence, did weigh 169 pounds, and about three ounces: But I
moystened the Earthen Vessel with Rain-water or distilled water (alwayes when there
was need)… At length, I again dried the Earth of the Vessel, and there were found the
same 200 pounds, wanting about two ounces. Therefore 164 pounds of Wood, Barks,
and Roots, arose out of water onely.
Van Helmont's discovery was a major advance, though water (as we now know) was only
part of the story.
A century later the English clergyman and naturalist Stephen Hales first suggested that
plants rely on some component of the air as well as water—trace atmospheric carbon diox-
ide.Wenowrecognizethatbothwaterinthesoilandcarbondioxideintheairareprincipal
ingredients forphotosynthetic organisms. (Ironically,itwasVanHelmont himself whodis-
covered carbon dioxide gas, but he did not realize its central role in plant growth.)
Even so, the role of sunlight was enigmatic, and it took another three hundred years for
the details to emerge. Advances in nuclear physics paved the way, as a new generation
of particle accelerators called cyclotrons provided the first steady supply of the highly ra-
dioactive isotope carbon-11—a sensitive probe of biological reactions. In the late 1930s,
Samuel Ruben and Martin Kamen at the University of California at Berkeley exposed
