Geology Reference
In-Depth Information
still disputed, smudges. Few observers expect this development to be the last word in the
debate.)
The Smallest Fossils
Imagine what happens when a colony of microbes dies. Almost always the tiny bag of
chemicals that once was a living cell fragments and disperses; big biomolecules break
down into smaller molecular pieces, mostly water and CO
2
. Other microbes may eat the
tastiest bits, while indigestible molecules dissolve in the oceans, or evaporate into the air,
or become trapped in rock. Usually after a few years, nothing is left, for time is not kind to
such fragile molecular remains.
Under extraordinary circumstances—if dead cells are quickly buried, if there's no cor-
rosive oxygen around, if the host rock never gets too hot—a few of the hardiest biomolec-
ules can survive, albeit in a rather altered form. Most likely to persist are molecules with a
ruggedbackboneofuptoabouttwentycarbonatoms,sometimesarrangedinasimplelong
chain (perhaps with a few carbon atoms sticking off to the side here and there), sometimes
in a group of interlocking rings (not unlike the Olympic logo). These diagnostic biobits are
like ultrasmall skeletons. They represent what's left of much larger collections of function-
ing molecules that have been degraded and stripped of everything but the most resilient
core.
If you can find such a molecular skeleton in an ancient sedimentary rock, and if you can
be sure that it's not contamination from nearby younger strata or the ubiquitous leavings of
morerecentlydeceasedcells(frommodernsubsurfacemicrobes,forexample,orevendead
skinfromyourthumb),thenyoumightbeabletoclaimdiscoveryofachemicalfossil—the
actual atoms of a once-living microbe. Hence the fascination with Schopf's black blobs in
the Apex chert.
Many modern molecular paleontologists lead a fascinating double life. On the one hand,
they may choose to endure the rigors of the field geologist, hiking miles across arduous
terrain, schlepping hundreds of pounds of promising rock from remote outcrops in baked
deserts, frozen tundra, and high mountains. Every year small teams set out for Western
Australia,SouthAfrica,Greenland,andcentralCanadainsearchofnewspecimens.Others
labor at drilling rigs in the hopes of securing drill cores of pristine ancient rock, uncontam-
inated by weather and vegetation. Such expeditions can mean months of hardship, danger,
and deprivation.
These adventures contrast with months of tedious analysis conducted in ultraclean labs,
where the slightest breath or thumbprint can irrevocably contaminate a precious three-
billion-year-old rock sample. It takes time and patience, exquisite care, and an arsenal of
sophisticated analytical apparatus to extract individual molecules from a rock. One leading
