Geoscience Reference
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(Veuger et al. 2006). Although sugars and amino
acids may severely crosslink and form degradation-
resistant polymers (Maillard 1912), they have not
been used for evolutionary or environmental recon-
struction. Probably, they are too omnipresent and
taxon-unspecific for these purposes.
Polysaccharide synthesis is also ancient. Cellu-
lose is produced by cyanobacteria and proteo-
bacteria. It has been suggested that the ability of
vastly unrelated eukaryotic species to produce cellu-
lose has been acquired via endosymbiosis with these
bacteria and lateral gene transfer (Niklas 2004).
Despite a large chemical diversity in peptidoglycans
and polysaccharides among organisms, most are
degraded. This explains why there is so little evi-
dence of fossil bacterial polymeric products,
despite the bacterial omnipresence. Only cellulose
and chitin appear to be relatively resistant to biode-
gradation and form a considerable fossil record.
The refractory character of these macromolecules
is clearly related to the exact composition of
the monomers and their stereoconfiguration in the
polymer. This is demonstrated by comparing the
extremely low fossilization potential of starch
(poly 1 ! 4 b-d-Glucose) with that of cellulose
(poly 1 ! 4 a-d-Glucose) (Fig. 3).
The polysaccharide chitin (N-acetyl-d-glucosa-
mine; Fig. 4) which is so abundant in arthropods
and oomycetes today is not known from prokaryotes
(Niklas 2004). The capability to synthesize chitin is
therefore onsidered to have arisen much later in
evolution compared to cellulose synthesis. Its pres-
ence in many marine organisms such as arthropods,
molluscs and annelids places its evolution well
before the evolution of land-adapted organisms,
however.
Studies on chitin preservation provide another
argument as to why this substance is not suitable
for unravelling the terrestrialization process. Lab-
oratory experiments show that chitin belongs to
the most degradation-resistant parts of arthropods
(Baas et al. 1995; Briggs et al. 1995). At first
sight, this is not surprising since arthropod cuticles
are also abundant in the fossil record. But are
these cuticles still made of chitin?
The oldest known traces of the chitin marker
D-glucosamine (XXXX) occur in extremely well-
preserved weevil cuticles (up to 0.6% of the
organic matter) present in the 25 Ma lacustrine
sediments from Enspel (Stankiewicz et al. 1997;
Flannery et al. 2001). Less well-preserved or older
arthropod cuticles show no such traces of chitin
(Stankiewicz et al. 1998; Gupta et al. 2007a). This
alone probably explains why chitin could not be
detected in chitinozoans (Voss-Foucart & Jeuniaux
1972; Jacob et al. 2007); the biological affinity of
the chitinozoa therefore remains unresolved. This
also implies that a reassessment of the presence of
chitin in Palaeocene dinoflagellate cysts (Belayouni
& Trichet 1980) is called for. Mostly, chitin-barren
(
a
)
CH
2
OH
CH
2
OH
CH
2
OH
CH
2
OH
CH
2
OH
O
O
O
O
O
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
O
O
O
O
O
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
(
b
)
CH
2
OH
CH
2
OH
O
O
OH
H
H
H
H
H
OH
H
H
H
CH
2
OH
H
HO
OH
HO
O
H
OH
OH
OH
CH
2
OH
H
α
-D-glucose
β
-D-glucose
OH
H
O
O
CH
2
OH
H
OH
H
H
OH
O
O
CH
2
OH
H
OH
H
H
OH
O
(
c
)
O
CH
2
OH
H
OH
H
H
OH
O
O
H
OH
H
H
OH
O
H
OH
Fig. 3. Structural formulas of (a) cellulose which is made of (b) glucose and (c) starch (which is also made of glucose).
