New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 58

spectacularly preserved fossils, and the application of highly accurate dating
techniques linking disparate environments and processes.
A mechanistic understanding of the origin of life remains a vexing challenge
and one of the great opportunities of this century. Recently, the exploration of
extreme modern environments, such as hydrothermal vents, coupled with
metagenomics (e.g., Grzymskia et al., 2008), phylogenomics (Delsuc et al., 2005),
and proteomics (Gaucher et al., 2003), geochemical proxies (biomarkers) of various
microbial groups, and analysis of the isotopic proxies of past environmental
conditions has resulted not only in a better chronology of the major biotically
mediated transformations of Earth but also provided a chronology of the evolutionary
and physiological steps in the evolution of early life. Two surprising results from this
work are (1) that our last universal common ancestor (LUCA) was plausibly a
thermophile but not a hyperthermophile (Gaucher et al., 2008; Gouy and Chaussidon,
2008) in hydrothermal vents (Martin and Russell, 2007) and (2) while photosynthesis
evolved very early, the early photosynthetic organisms did not produce oxygen (i.e.,
were anoxygenic).
Geochemists have made great progress in using the elemental and isotopic
properties of ancient sediments to reconstruct the evolving redox state of the ocean
and atmosphere. A decade ago Farquhar et al. (2000) established anomalous mass-
independent fractionation of sulfur isotopes as the smoking gun for the near absence
of O 2 in the atmosphere before the Great Oxidation Event 2.4 billion years ago. Now,
frontiers for sulfur isotope approaches lie with recognition of specific microbial
metabolisms in the very old record and their environmental implications (Johnston et
al., 2008). Iron geochemistry calibrated in modern settings has become our most
reliable inorganic fingerprint of local oxygen deficiency in the ancient ocean (Poulton
and Canfield, 2005), while organic biomarkers further trace the co-evolution of life
and the environment (Brocks et al., 2005). Other redox-sensitive elements, such as
molybdenum, can provide a global picture of ocean oxygenation when viewed for
their mass balance relationships (Scott et al., 2008) and even delineate times when
biologically critical trace metals may have limited the evolutionary advance of life.
At the same time, metal isotope systems, such as iron and molybdenum, are providing
global perspectives on past ocean-atmosphere oxygen conditions as a backdrop to the
early evolution of life (Johnson et al., 2008).
Complementary geochemical and genomic studies are informing our
understanding of other major biogeochemically important paleobiological milestones.
While there are too many to detail here, these milestones include the origin of animals
in the late Proterozoic, the spread of grasslands and co-evolved grazers during the
Neogene (Cerling, 1992; Bouchenak-Khelladi et al., 2009), and our own evolution
(Feakins et al., 2005; Steiper and Young, 2006; NRC, 2010b). These examples are
associated with major shifts in climate mode and variability and elemental cycling.
The major challenge and opportunity is linking the evolutionary events with the
environmental causes or consequences via tests of mechanistic hypotheses, and the
tools to do this now exist.