Geology Reference
In-Depth Information
mented the fossil photos with artistic line drawings (to “aid the eye”), which were placed
side by side with photos of cyanobacteria, similar-looking modern photosynthetic mi-
crobes.Schopfevenclaimedthathisfossilswereprobablyoxygenproducers.Withinafew
years,hismostconvincingphotoshadbecomeamongthemostreproducedpaleontological
images of all time, adorning numerous textbooks with captions that repeated the “earliest
fossil” claims, often with the suggestion that the microbes were photosynthetic.
It's a rule in science that extraordinary claims require extraordinary evidence. It's also
true that extraordinary claims usually receive extraordinary scrutiny. All of Schopf's fossil
specimens reside at the British Museum in London, preserved as carefully cataloged, thin
transparent sectionsofrockmountedonglassslides.In2000Oxfordpaleontologist Martin
Brasier began a detailed reexamination of the Apex chert material and came to a very dif-
ferent conclusion.
Schopf's “thin sections” of Apex chert actually turned out to be rather thick, at least
compared with the size of a microbe. Brasier and his colleagues were eventually able to
locate most of the tiny objects that Schopf had photographed and published, but they were
surprisedtorealizethatmanyofthephotographswereatbestmisleading.EachofSchopf's
now-classic photos represents a single microscopic focal plane—a thin two-dimensional
slice through his smudgy three-dimensional black objects. Brasier and his team employed
a newer photographic technique that created a three-dimensional montage of images and
thus revealed a much more complex story. Only if they set the microscope focus to the ex-
act depth of Schopf's photographs could they reproduce the now-classic images of Apex
“fossils.” But raise or lower the focus slightly, and what appeared at first to be a convin-
cing,elongatedstringofmicrobialcellsmorphedintoawavysheetorirregularblob,some-
timeswithfolds,branchings,orsquiggles.AccordingtoBrasier'sobservations,the“chains
of microbes” are misleadingly chosen cross sections through complex three-dimensional
structures that bear little resemblance to anything biological. The embarrassing challenge
byBrasierandcolleagues,“QuestioningtheEvidenceforEarth'sOldestFossils,”appeared
in the March 7, 2002, issue of the prominent periodical
Nature
.
Schopf fought back with his own article, “Laser-Raman Imagery of Earth's Earliest
Fossils,” published back-to-back with Brasier's in the same issue. Schopf and his col-
leagues presented new analyses of the Apex chert's carbon-rich black blobs and showed
them to have isotopic compositions and atomic structures consistent with biology. He
boldly repeated the “oldest fossil” rhetoric, though he seemed to back off his interpretation
that the microbes were photosynthetic. Nevertheless, the seeds of doubt had been sown on
Schopf's claims, and the bar had been raised in the hunt for the earliest signs of life.
(In a late-breaking sequel, Martin Brasier and his coworkers from Australia now claim
that they have found the “oldest fossils”—microbial remnants from the 3.4-billion-year-
oldStrelleyPoolformation,discoveredonlytwentymilesfromSchopf'sslightlyolder,but
