Geology Reference
In-Depth Information
madethattheshallow-waterstromatolitesfrom3.5billionyearsagosupportedaphotosyn-
thetic lifestyle, though these mineralized mounds could also simply have been protective
high-rises in an otherwise harsh, wave-swept environment. But microbial mats must have
been photosynthetic. Why would a colony of microbes go to all the trouble of fixing itself
in a rough, shallow tidal zone if it wasn't after the sunlight?
To place Nora Noffke's contributions in context, consider other really old fossils. For
much of the past half century, paleontologists looking for Earth's oldest life have focused
onthreekindsofrockformations.Firstaretheblackcherts,likeBillSchopf'scontroversial
3.5-billion-year-old Apex chert. Black cherts first made paleontological headlines in the
early 1960s, when Harvard paleobotanist Elso Barghoorn recognized ancient microbial
fossils in the 1.9-billion-year-old Gunflint chert from northern Minnesota and western
Ontario. Barghoorn scrutinized thin, transparent sections of the fine-grained, silica-rich
rock and realized he was seeing ancient microbial body fossils in exquisite detail. With
geologistStanleyTyler,whohadfirstobservedenigmaticspherelikeobjectsintheGunflint
a decade earlier, Barghoorn described astonishing suites of unambiguous cells—a micro-
scopic ecosystem of spheres, rods, and filaments, some in the process of dividing. Indeed,
inspiteofdecadesofsubsequentclaimsforolderfossils,somepaleontologistsstillpointto
the Gunflint chert as holding the oldest absolutely unambiguous fossils of photosynthetic
cells on Earth.
Carbon-rich black shales, a second rock type, of the kind studied by Roger Summons
and his colleagues, are perhaps the best source of ancient molecular fossils. Black shales
are deep-water accumulations of mud and organic debris, so we can be confident that
they entomb the remains of ancient microbes. As a consequence, thick sequences of black
shales from Australia, South Africa, and other localities billions of years old are receiving
painstakingchemicalscrutiny,layerbymicroscopiclayer.Asnewer,moresensitiveanalyt-
ical tools come online, some capable of detecting single molecules, important discoveries
are sure to follow.
Thethirdintensivelystudiedtypeofancientfossil-bearingformationisthestromatolites,
thoselayereddomelikestructuresofmineralsdepositedbyearlylife.Paleontologistsmight
have been stumped by the origin of these mounds, usually preserved in limestone, were it
not for modern living stromatolite reefs in shallow seas, most famously in the scenic and
remote World Heritage Site of Western Australia's Shark Bay. These odd sedimentary fea-
turesarisewhenaslimysurfacecoatingofmicrobes—photosyntheticmicrobes,inthecase
of today's living reefs—produces layer upon layer of minerals. Hundreds of fossil stro-
matolite localities have been identified around the world, some in rocks older than three
billion years.
Black chert, black shale, and stromatolites. To this short list of Earth's oldest fossilifer-
ous formations, Nora Noffke has added a fourth rock type: sandstone. It's understandable
