Geology Reference
In-Depth Information
Earth more than two billion years ago. In their search for the earliest photosynthesis, the
fossil hunters naturally focus on Earth's oldest rocks.
Fossil evidence for ancient photosynthetic cells is spotty at best. Precious few microbial
remains make it through billions of years of burial, heating, squeezing, and chemical alter-
ation. What does survive is cooked and crushed, often in ways that necessitate a colorful
imagination to achieve any biological interpretation. Colonies of fossil microbes often ap-
pear as little more than scatterings of small black smudges, so it's not surprising that every
report of microbes more than two billion years old has been met with cautious skepticism,
if not outright ridicule.
For much of the past four decades, one of the most ardent defenders of paleontological
rigor has been J. William (Bill) Schopf, professor of paleontology at the University of
California's Los Angeles campus. Based on his studies of increasingly ancient microbial
fossils, Schopf has developed a checklist of traits necessary and sufficient to confirm the
claim of life. By first focusing on more recent, well-preserved, and unambiguous speci-
mens, Schopf is the scientist who most convincingly pushed the fossil record further and
further back, more than three billion years into the remote Archean Eon.
Schopf's criteria are straightforward and reasonable: fossil microbes must come from
properly dated sedimentary layers laid down in environments where microbes could have
once lived. Fossils must show uniformity of size and shape—consistent spheres or rods
or chains, unlike the kind of shapeless black blobs and streaks that are found in many old
rocks. Schopf and his students also employed statistics to remove some of the subjectivity
inherent in observations of Earth's oldest sedimentary rocks.
This quantitative catalog of essential traits for any suite of microbial fossils served
Schopf well. He was able to publish unassailable descriptions of new fossil finds, while
raising doubts about some of the more questionable claims of ancient life by competing
researchers. His most notable challenge came in 1996, when NASA scientists announced
thatmicrobial remainshadbeenfoundinaMartianmeteorite. Atadramatic NASA-hosted
press conference in August of that year, Schopf was the lone dissenting voice. With thinly
veiledcontempt,hepointedoutthattheMartian“fossils”weremuchtoosmall,lackedsup-
porting chemical and mineralogical evidence, and were in the wrong kind of rock to boot.
(In spite of Schopf's persuasive arguments, President Clinton lauded the discovery, which
mayhaveledtoasignificantbumpinNASAfundingforastrobiology—moneythatwound
up supporting many of us in the origins game, including Schopf.)
Ironically, Schopf would soon meet the same kind of withering criticism for an earlier
claim he'd made in 1993, when he announced his discovery of Earth's oldest microbial
fossils from the Apex chert, a rock formation almost 3.5 billion years old in northwestern
Australia.Photographsofsuggestiveelongatedblackstructureswithcell-likesegmentation
seemed compelling enough. The story, published in a high-profile
Science
paper, supple-
