Geology Reference
In-Depth Information
practitioner of this twenty-first-century art is Australian paleontologist Roger Summons,
who has set up shop at MIT's department of Earth and planetary sciences. There he heads
theSummonsLab,acrackteam ofadozenmolecular fossilhunterswhostudyEarth'sold-
est rocks.
A dozen years ago, while working at Australian National University, Summons led a
group of scientists who made headlines when they studied promising sediments from the
2.7-billion-year-old Pilbara craton in Western Australia. Summons and his colleagues had
access to a unique drill core, a sequence almost half a mile long that included a tantaliz-
ing section of black, carbon-rich shale—the kind of sedimentary rock most likely to hold
molecular fossils. These Pilbara rocks were of special interest because they appeared to be
essentially unaltered by heat and uncontaminated by surface life or groundwater. If ever
there was a rock where old biomolecules might survive, that was it.
The Australian researchers focused on hopanes, the elegant class of hardy biomolecules
mentioned in
chapter 6
. Hopanes play important roles in stabilizing protective cell mem-
branes and, because of their rarity outside of living cells, are perhaps the most convincing
of all the molecular biomarkers. Each hopane has a distinctive backbone of five interlock-
ing rings—four little hexagons (each defined by six carbon atoms) and a fifth pentagon
(with five carbon atoms) onthe end.Each ringshares two carbon atoms with its neighbors,
for a total of twenty-one backbone carbon atoms in all.
Meticulous studies at Summons's Australian lab led to two high-profile papers, both
published in August 1999. The first one in
Science
, with Summons's Ph.D. student Jochen
Brocks as first author, described the discovery of hopanes in the 2.7-billion-year-old Pil-
bara rocks—what would have been the oldest known molecular fossils, breaking the pre-
vious record by a billion years. Discoveries of hopanes can reveal a lot about ancient eco-
systems, for different species use a number of different kinds of hopanes with extra carbon
atomsstuckonatvariousplacesaroundtherings.Brocksandhiscolleaguessuggestedthat
the Pilbara hopanes were diagnostic of rather advanced cells called eukaryotes—cells that
containanucleusthathousesDNA.Atthetimeofpublication, theoldestknowneukaryote
fossil cells were only about one billion years old, whereas primitive microbes of the type
thought to exist before about two billion years ago don't have a nucleus, so this interpreta-
tionwasmetwithsurprise,ifnotdownrightdisbelief.Ifthediscoveryistrue,thenonlytwo
conclusions are possible. Either eukaryotes appeared much, much earlier than anyone had
thought (and life's evolution was correspondingly accelerated), or else hopanes evolved
muchearlier thaneukaryotes. Ineither case, ourunderstanding oflife'shistorywouldhave
to be revised.
The second article, published in
Nature
with Summons as lead author, made the equally
startling claim that nearby 2.5-billion-year-old black shales from Mount McRae, a modest
3,300-foot peak in Western Australia, contain a variant of the hopane five-ring molecule
