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An organic geochemical perspective on terrestrialization
GERARD J. M.VERSTEEGH
1
* & ARMELLE RIBOULLEAU
2
1
MARUM, Universit ¨t Bremen, Leobenerstraße, D-28359 Bremen, Germany
2
Universit´ des Sciences et Technologies de Lille - Bˆ t. SN5,
UMR 8157 du CNRS G´osyst`mes, F-59655 Villeneuve d'Ascq Cedex, France
*Corresponding author (e-mail: versteegh@uni-bremen.de)
Abstract: The colonization of land required new strategies for safe gamete/diaspore dispersal,
and to cope with desiccation, harmful radiation, fire and gravity. Accordingly, the morphology, be-
haviour and physiology of the organisms changed. Here, we explore to what extent physiological
adaptations, reflected in the molecular content of the sediments, add to our understanding of
the terrestrialization. Many compounds considered characteristic of land organisms do not
provide valuable information from the fossil record since (1) they were not preserved; (2) they
occur or correspond to substances that evolved prior to the terrestrialization (e.g. cutan vs. algae-
nan, cellulose); or (3) they have been changed diagenetically and/or catagenetically. The latter
leads to geo(macro)molecules without a chemical fingerprint relating them to their original
bio(macro)molecules despite, sometimes, excellent morphological preservation of the organic
remains. Nevertheless, some molecular markers and their stable isotopes provide independent
information on the terrestrialization process. The odd predominance of n-alkane surface waxes
is a feature already apparent in early land plants and could, with caution, be used as such. Further-
more, fossil terpenoids and their derivatives are valuable for reconstructing the evolution of major
plant groups. The radiation of the phenylpropanoid pathway with for example, sporopollenin and
lignin seems to be closely related to the evolution of land plants.
As for other disciplines occupied with unravelling
past life and environment, organic geochemistry
relies heavily on the paradigm that the present
provides a key to the past. For organic geo-
chemistry, the biosynthetic pathways of living
organisms provide such a key. The biosynthetic
differences between organisms provide insight
into the evolution of biosynthetic pathways and
this can be applied to, and calibrated against,
the fossil record. Evolution also implies adapta-
tion and, by linking species ecology to their bio-
chemistry, the adaptive value of the biosynthetic
pathways and the biomolecules produced may
be resolved.
This biosynthetic link to environment can also
be applied to the past. For instance, the notion
that oxygen is required in steroid and non-hopanoid
triterpenoid synthesis implies that analysis of
steroids in ancient sediments may help to unravel
the early evolution of the atmosphere (Summons
et al. 2006). Analogous to the oxygenation of the
Earth's atmosphere, terrestrialization also required
major adaptations of the terrestrializing organisms
to the living conditions on land. The conquest of
land required the development or strengthening of
supportive structures such as skeletons, stems and
roots to withstand Earth's gravity without the
support of water and to resist wind. It also demanded
water-saving strategies such as arthropod and plant
cuticles, vertebrate skin and cork to survive low
humidity environments and sometimes fire, as well
as the development of water-conducting tissues
in plants (roots, tracheids). Finally, the higher
exposure to harmful radiation required sunscreens
such as pigments, aromatic and other substances
as UV filters. Whereas the endo- and exoskeletal
adaptations are intrinsically based on the formation
of large biopolymers, water saving and UV pro-
tection may be also realized by means of smaller
molecules.
Our working hypothesis is that the major
changes in biosynthetic pathways which accompa-
nied the terrestrialization of organisms are reflected
in the molecular composition of the organic matter
present in Palaeozoic sedimentary rocks. For inst-
ance, a minor change in the biosynthesis of sporo-
pollenin may have led to lignin. In return, we can
expect the changes in composition of the organic
matter (OM) in Palaeozoic rocks to help determine
following evolutionary trends. The question being
asked is to what extent is our working hypothesis
valid and what does this provide us with to under-
stand terrestrialization? We restrict ourselves to
primary producers and arthropods since they have
a rich fossil record, both organic geochemically
and as micro- and macroscopic remains. We
exclude other terrestrial (heterotrophic) life forms
(worms, snails, vertebrates, etc.) since they do not
usually leave a chemically characteristic signature
in the sediments.
