Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits

Although the notion of an early origin and diversification of life on
Earth during the Archaean eon has received increasing support in
geochemical, sedimentological and palaeontological evidence,
ambiguities and controversies persist regarding the biogenicity
and syngeneity of the record older than Late Archaean1–3. Nonbiological
processes are known to produce morphologies similar
to some microfossils4,5, and hydrothermal fluids have the potential
to produce abiotic organic compounds with depleted carbon
isotope values6, making it difficult to establish unambiguous
traces of life. Here we report the discovery of a population of large
(up to about 300 mmin diameter) carbonaceous spheroidal microstructures
in Mesoarchaean shales and siltstones of the Moodies
Group, South Africa, the Earth’s oldest siliciclastic alluvial to tidalestuarine
deposits7. These microstructures are interpreted as
organic-walled microfossils on the basis of petrographic and geochemical
evidence for their endogenicity and syngeneity, their
carbonaceous composition, cellular morphology and ultrastructure,
occurrence in populations, taphonomic features of soft wall
deformation, and the geological context plausible for life, as well
as a lack of abiotic explanation falsifying a biological origin. These
are the oldest and largest Archaean organic-walled spheroidal
microfossils reported so far. Our observations suggest that relatively
large microorganisms cohabited with earlier reported
benthic microbial mats8 in the photic zone of marginal marine
siliciclastic environments 3.2 billion years ago.