actually be a eukaryotic fossil that cannot be assigned to any
currently known lineage. Although some authors consider
Grypania as a composite of filamentous prokaryotes, it is
more likely that this fossil is eukaryotic in nature (Knoll
et al. 2006).
The attribution of acritarchs, encountered from 1.9 to
1.8 Ga, to the eukaryotic domain is less controversial.
They are unicellular organisms with very resistant cell
walls, which may resemble cysts of extant dinoflagellates
(see Sect. 7.8.3). The term ‘acritarch’ means a cellular,
resistant organic structure. These cells are probably polyphyletic (i.e. consisting of various lineages that are not
particularly related to each other) and may represent
ancestors of extant unicellular eukaryotic lineages and/or
extinct lineages (see Chap. 7).
Tappania plana, Valeria lophostriata, Dictyosphaeria sp.
and Satka favosa were discovered in the formation of Roper,
Northern Australia, and dated at 1.5 Ga (Fig. 4.14). These
unicellular organisms were larger than 200 μm and lived in
estuaries or deltas. The presence of protrusions in the cell
wall for budding is difficult to explain without assuming the
presence of a cytoskeleton in the cytoplasm, a distinctive
eukaryotic feature (Javaux et al. 2001).
4.2.5 Fossils of the First Multicellular
Organisms
The oldest clearly multicellular eukaryotic fossil,
Bangiomorpha pubescens, was discovered by Butterfield
(2000) in carbonates dated at 1.2 Ga in Somerset Island,
Canada. Moreover, this is the earliest taxon that could be
attached to an extant lineage, bangiophytes (a class of red
algae). Indeed, B. pubescens has similar characters to those
involved in the sexual reproduction of extant bangiophytes.
It is, to date, the oldest fossil demonstrating sexuality, a
hallmark of eukaryotes. Between 1.2 and 0.6 Ga, other
fossils more or less credibly attributable to multicellular
eukaryotes are known. Then suddenly, between 600 and
540 Ma, i.e. just before the beginning of the Palaeozoic era
(its beginning is dated to 542 Ma, with the Cambrian), and in
sites now very far apart from each other (Ediacara in South
Australia, Doushantuo in South China, Spaniard’s Bay in
Newfoundland), most extant eukaryotic lineages (including
metazoa) appear in the fossil record. The beginning of the
Cambrian (542–500 Ma) is marked by an explosion in biodiversity, as documented in particular in the Burgess (British
Columbia) and Chenjang (China) deposits.
The discovery in 2010 of macrofossils dated 2.1 Ga,
interpreted as forms of multicellular life, would push the
emergence of multicellular life back by a billion years (El
Albani et al. 2010).
Was the emergence and diversification of eukaryotic
lineages, initiated between 2.7 Ga (see Sect. 5.4.1) and 2.1
to 1.9 Ga, progressive or did it experience a sudden acceleration at 600 Ma? There is indeed a change in the rate of the
diversification of life at the end of the Proterozoic but as yet
no consensus as to the cause – rising dioxygen and nutrient
levels and/or climatic forcing through a series of apparently
global glaciations that occurred at the end of the Proterozoic
between about 780 and 640 Ma (Lenton and Watson 2011).
4.2.6 The Analysis of Fossil Molecules:
Molecular Palaeontology
4.2.6.1 The First Traces of Oxygenic
Photosynthesis
After cell death, a very small portion of the organic matter
escapes the mineralisation process and is buried in the sediment. Once buried, the organic material will sink gradually
beneath layers of increasingly thick sediment, up to several
hundred metres. Under the combined effects of temperature
and pressure, the various constituents of the organic matter
will undergo chemical transformations whose importance
will depend, inter alia, of their molecular structure. Some
of these molecules are sufficiently resistant to the processes
of diagenesis and catagenesis to keep the carbon skeleton of
their precursors and can be used as biomarkers*. Thus, 2-α
methylhopanes are considered as specific biomarkers of
extant cyanobacteria that perform oxygenic photosynthesis.
What is their origin? The plasma membranes of prokaryotes
contain amphipathic lipid molecules called biohopanoids
that are functionally equivalent to sterols in eukaryotes.
Biohopanoids derive from a family of molecules, hopanes
and pentacyclic triterpenoid compounds with 30 carbon
atoms (Fig. 4.15a). The analysis of cyanobacteria, either in
culture or in microbial mats, reveals the presence of particular hopanes called bacteriohopanepolyols with a methyl
group in position 2 of ring A (Fig. 4.15b). After cell death
and during burial in sediments, these molecules experience
defunctionalisation processes (loss of the OH groups) and
stereochemical changes, but their carbon backbone (5 rings)
is retained and they occur in sediments in the form of 2-α
methylhopanes (Fig. 4.15c). The discovery of 2-α
methylhopanes in significant quantities in sediments dating
back 2.7 Ga collected in Western Australia has been a strong
argument for the existence of oxygenic photosynthesisperforming cyanobacteria at this period (Brocks et al.
1999). Since then, it has been shown that, although the
rocks analysed were dated correctly, the 2-α methylhopanes
came from younger (<2.2 Ga), contaminating carbon-rich
fluids, i.e. a period during which oxygenic photosynthesis
had already appeared (Rasmussen et al. 2008).
4 For Three Billion Years, Microorganisms Were the Only Inhabitants of the Earth
93
currently known lineage. Although some authors consider
Grypania as a composite of filamentous prokaryotes, it is
more likely that this fossil is eukaryotic in nature (Knoll
et al. 2006).
The attribution of acritarchs, encountered from 1.9 to
1.8 Ga, to the eukaryotic domain is less controversial.
They are unicellular organisms with very resistant cell
walls, which may resemble cysts of extant dinoflagellates
(see Sect. 7.8.3). The term ‘acritarch’ means a cellular,
resistant organic structure. These cells are probably polyphyletic (i.e. consisting of various lineages that are not
particularly related to each other) and may represent
ancestors of extant unicellular eukaryotic lineages and/or
extinct lineages (see Chap. 7).
Tappania plana, Valeria lophostriata, Dictyosphaeria sp.
and Satka favosa were discovered in the formation of Roper,
Northern Australia, and dated at 1.5 Ga (Fig. 4.14). These
unicellular organisms were larger than 200 μm and lived in
estuaries or deltas. The presence of protrusions in the cell
wall for budding is difficult to explain without assuming the
presence of a cytoskeleton in the cytoplasm, a distinctive
eukaryotic feature (Javaux et al. 2001).
4.2.5 Fossils of the First Multicellular
Organisms
The oldest clearly multicellular eukaryotic fossil,
Bangiomorpha pubescens, was discovered by Butterfield
(2000) in carbonates dated at 1.2 Ga in Somerset Island,
Canada. Moreover, this is the earliest taxon that could be
attached to an extant lineage, bangiophytes (a class of red
algae). Indeed, B. pubescens has similar characters to those
involved in the sexual reproduction of extant bangiophytes.
It is, to date, the oldest fossil demonstrating sexuality, a
hallmark of eukaryotes. Between 1.2 and 0.6 Ga, other
fossils more or less credibly attributable to multicellular
eukaryotes are known. Then suddenly, between 600 and
540 Ma, i.e. just before the beginning of the Palaeozoic era
(its beginning is dated to 542 Ma, with the Cambrian), and in
sites now very far apart from each other (Ediacara in South
Australia, Doushantuo in South China, Spaniard’s Bay in
Newfoundland), most extant eukaryotic lineages (including
metazoa) appear in the fossil record. The beginning of the
Cambrian (542–500 Ma) is marked by an explosion in biodiversity, as documented in particular in the Burgess (British
Columbia) and Chenjang (China) deposits.
The discovery in 2010 of macrofossils dated 2.1 Ga,
interpreted as forms of multicellular life, would push the
emergence of multicellular life back by a billion years (El
Albani et al. 2010).
Was the emergence and diversification of eukaryotic
lineages, initiated between 2.7 Ga (see Sect. 5.4.1) and 2.1
to 1.9 Ga, progressive or did it experience a sudden acceleration at 600 Ma? There is indeed a change in the rate of the
diversification of life at the end of the Proterozoic but as yet
no consensus as to the cause – rising dioxygen and nutrient
levels and/or climatic forcing through a series of apparently
global glaciations that occurred at the end of the Proterozoic
between about 780 and 640 Ma (Lenton and Watson 2011).
4.2.6 The Analysis of Fossil Molecules:
Molecular Palaeontology
4.2.6.1 The First Traces of Oxygenic
Photosynthesis
After cell death, a very small portion of the organic matter
escapes the mineralisation process and is buried in the sediment. Once buried, the organic material will sink gradually
beneath layers of increasingly thick sediment, up to several
hundred metres. Under the combined effects of temperature
and pressure, the various constituents of the organic matter
will undergo chemical transformations whose importance
will depend, inter alia, of their molecular structure. Some
of these molecules are sufficiently resistant to the processes
of diagenesis and catagenesis to keep the carbon skeleton of
their precursors and can be used as biomarkers*. Thus, 2-α
methylhopanes are considered as specific biomarkers of
extant cyanobacteria that perform oxygenic photosynthesis.
What is their origin? The plasma membranes of prokaryotes
contain amphipathic lipid molecules called biohopanoids
that are functionally equivalent to sterols in eukaryotes.
Biohopanoids derive from a family of molecules, hopanes
and pentacyclic triterpenoid compounds with 30 carbon
atoms (Fig. 4.15a). The analysis of cyanobacteria, either in
culture or in microbial mats, reveals the presence of particular hopanes called bacteriohopanepolyols with a methyl
group in position 2 of ring A (Fig. 4.15b). After cell death
and during burial in sediments, these molecules experience
defunctionalisation processes (loss of the OH groups) and
stereochemical changes, but their carbon backbone (5 rings)
is retained and they occur in sediments in the form of 2-α
methylhopanes (Fig. 4.15c). The discovery of 2-α
methylhopanes in significant quantities in sediments dating
back 2.7 Ga collected in Western Australia has been a strong
argument for the existence of oxygenic photosynthesisperforming cyanobacteria at this period (Brocks et al.
1999). Since then, it has been shown that, although the
rocks analysed were dated correctly, the 2-α methylhopanes
came from younger (<2.2 Ga), contaminating carbon-rich
fluids, i.e. a period during which oxygenic photosynthesis
had already appeared (Rasmussen et al. 2008).
4 For Three Billion Years, Microorganisms Were the Only Inhabitants of the Earth
93