Geology Reference
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with an extra carbon atom sticking off the side of the first ring. These 2-methylhopanoid
molecules are known only from photosynthetic cyanobacteria, which are Earth's principal
oxygen producers. Summons concluded that photosynthesis was well under way on Earth
by 2.5 billion years ago. Such a chronology was consistent with the known rise of oxygen
at about that time, but the suggestion that the origin of photosynthesis could be sought in
preserved molecular fragments opened exciting new doors to paleontology.
Not everyone was persuaded. Like Bill Schopf's earlier claims of “Earth's oldest
fossils,”RogerSummons'sextraordinaryhopanefindingshavebeenmetwithsomeoppos-
ition, including grave doubts now raised by Jochen Brocks about his own doctoral work,
as well as all other studies of purported biomarkers more than two billion years old. Young
hopanes are everywhere, the skeptics say. The deep subsurface is teeming with microbes
living in the rocks, so contamination during more than two billion years of Earth history
is unavoidable. The hopanes and other biomolecules are no doubt there, but who can say
when or how they got there. Stay tuned: such debates are fun to watch, and they almost
always lead to new discoveries.
Sand Flats of Time
Where else is a paleontologist to look? Of the many clues in the fossil record related to the
history of photosynthesis, microbial mats may be at once the most obvious and the most
overlooked.Todaytheyform,theworldover,inshallowcoastalwatersandalongthebanks
of slow-moving rivers and streams, where algae can intertwine filaments in thick, tangled
layers. These tough, clothlike mats ensure that the algae have access to a wet, sunlit en-
vironment, while being protected from the inevitable eroding action of floods and waves.
In spite of their widespread distribution, the paleontological community all but overlooked
fossil microbial mats before Nora Noffke's discoveries.
For more than a decade, I've had the opportunity to assist Nora Noffke, a professor of
geobiology at Old Dominion University in Norfolk, Virginia, and the world's leading au-
thority on ancient microbial mats. Armed with a keen eye, a unique perspective, and steely
determination, she has chosen some of the most forbidding areas in the world to conduct
herfieldwork. Venturing into remote andhostile places inSouthAfrica, Western Australia,
Namibia, the scorching Middle East, and frigid Greenland, she has unearthed paleontolo-
gical wonders for which no one had previously thought to look. Over and over again Nora
has recognized evidence that microbial mats grew on many of Earth's most ancient sandy
shores.
The reason microbial mat fossils are so significant is that they must arise from some
kind of photosynthesis. The microbes that left their fragmentary remains in black cherts
andblackshalescouldhavecomefromdeepzones,farfromsunlight.Astrongcasecanbe
