presence with that of eukaryotic cells. For example, sterols
have been reported in a few species of cyanobacteria.
However, in all cases, the sterols were found at very low
concentrations (0.03 % of dry cell weight). To explain the
presence of sterols in low quantities in cyanobacteria, three
proposals were made:
1. Cyanobacteria do not synthesise sterols, and their
presence resulted from sample contaminations.
2. Cyanobacteria synthesise sterols, but the amount of
biomass analysed was insufficient in most experiments
for their detection.
3. Some cyanobacteria synthesise sterols in small quantities
and others do not.
This debate is not closed and has recently been revived by
the identification of genes homologous to eukaryotic genes,
coding for certain steps of sterol biosynthesis, in the genome
of several cyanobacteria (such as Anabaena) (Volkman
2005). For some authors, the presence of these homologues
in cyanobacterial genomes (in addition to the detection of
sterols in cyanobacteria) is sufficient to demonstrate the
ability of cyanobacteria to synthesise these molecules.
For others however, it is not, because the presence of these
homologous genes does not necessarily mean that they are
functional and they encode for proteins involved in the
biosynthesis of sterols (i.e. they may have completely different functions).
Sterols have also been discovered in several nonphotosynthetic prokaryotes, such as Methylococcus
capsulatus (Gammaproteobacteria), Gemmata obscuriglobus (Planctomycetales) and various myxobacteria
(deltaproteobacteria).
The evidence therefore seems to indicate that sterols are
present in some prokaryotes. Although the capacity to
synthesise sterols in small amounts and in a limited number
of bacteria is still unexplained, their existence requires caution in the use of these molecules as specific eukaryotic
biomarkers.
Using all the information obtained from the study of the
fossil record and of biogeochemical markers, it is possible to
propose a diagram depicting the main possible stages of the
evolution of microorganisms (Fig. 4.11).
4.3
The Passage from a Cellular
Organisation of the Prokaryotic
Type to a Cellular Organisation
of the Eukaryotic Type
Intermediate forms between the ‘prokaryotic’ and the
‘eukaryotic’ cellular organisations have not been found,
and there is almost no chance this will ever happen because
the differences are of the kind that cannot be preserved as
fossils. Many evolutionary scenarios have been proposed
in the literature and only the most ‘consensual’ ones will
be presented here. These scenarios are mainly based on
current knowledge of the cell biology of contemporary
microorganisms.
4.3.1 Eukaryotes Descend from Heterotrophic
Anaerobic Prokaryotes
This scenario was proposed by Christian De Duve (1995,
2002). In the long evolutionary history of life, it distinguishes an anaerobic phase and an aerobic phase.
Assumptions about events that have occurred during the
first phase – the transition from prokaryotes to anaerobic
eukaryotes – are speculative. In contrast, those that led to the
emergence of aerobic eukaryotes rely on solid arguments.
4.3.1.1 Anaerobic Phase
Membrane invaginations could have appeared in an anaerobic and heterotrophic organism of the prokaryotic type that
had lost its cell wall and increased in size, thus increasing
the exchange surface between the cell and its surrounding
environment (Fig. 4.17a–c). Some of these invaginations
gave rise to vesicles, sequestering food and digestive
enzymes (Fig. 4.17d). Digestion then became intracellular.
Subsequently, vesicles would have differentiated in
several kinds, creating a real membranous network with
the possible appearance of the smooth and rough endoplasmic reticulum, the Golgi apparatus and lysosomes
(Fig. 4.17e). Later, cytoskeletal and motor organs (not
shown in the figure) would have appeared. Finally, the
membranous network would have wrapped and isolated
DNA from the cytosol (Fig. 4.17f). The appearance of
these membrane structures would have allowed functional
specialisation of compartments within the cell. It is at this
stage that DNA transcription and mRNA translation became
decoupled.
4.3.1.2 Aerobic Phase
The emergence and development of cyanobacteria
performing oxygenic photosynthesis radically transformed
the chemical composition of the atmosphere, which gradually became aerobic. This transition had a great impact on
the biodiversity of the time, in particular by fostering the
emergence and expansion of organisms that had developed
the ability to use dioxygen via aerobic respiration. It is very
likely that many lineages of anaerobic organisms went
extinct. However not all did, in particular, the ancestor
of eukaryotes. Some may have found shelter in anoxic
habitats, while others managed to develop defence
mechanisms against dioxygen. Lynn Margulis, an American microbiologist who in the 1960s developed ‘the endosymbiotic theory’ (Margulis 1970), hypothesised that
96
J.-C. Bertrand et al.
have been reported in a few species of cyanobacteria.
However, in all cases, the sterols were found at very low
concentrations (0.03 % of dry cell weight). To explain the
presence of sterols in low quantities in cyanobacteria, three
proposals were made:
1. Cyanobacteria do not synthesise sterols, and their
presence resulted from sample contaminations.
2. Cyanobacteria synthesise sterols, but the amount of
biomass analysed was insufficient in most experiments
for their detection.
3. Some cyanobacteria synthesise sterols in small quantities
and others do not.
This debate is not closed and has recently been revived by
the identification of genes homologous to eukaryotic genes,
coding for certain steps of sterol biosynthesis, in the genome
of several cyanobacteria (such as Anabaena) (Volkman
2005). For some authors, the presence of these homologues
in cyanobacterial genomes (in addition to the detection of
sterols in cyanobacteria) is sufficient to demonstrate the
ability of cyanobacteria to synthesise these molecules.
For others however, it is not, because the presence of these
homologous genes does not necessarily mean that they are
functional and they encode for proteins involved in the
biosynthesis of sterols (i.e. they may have completely different functions).
Sterols have also been discovered in several nonphotosynthetic prokaryotes, such as Methylococcus
capsulatus (Gammaproteobacteria), Gemmata obscuriglobus (Planctomycetales) and various myxobacteria
(deltaproteobacteria).
The evidence therefore seems to indicate that sterols are
present in some prokaryotes. Although the capacity to
synthesise sterols in small amounts and in a limited number
of bacteria is still unexplained, their existence requires caution in the use of these molecules as specific eukaryotic
biomarkers.
Using all the information obtained from the study of the
fossil record and of biogeochemical markers, it is possible to
propose a diagram depicting the main possible stages of the
evolution of microorganisms (Fig. 4.11).
4.3
The Passage from a Cellular
Organisation of the Prokaryotic
Type to a Cellular Organisation
of the Eukaryotic Type
Intermediate forms between the ‘prokaryotic’ and the
‘eukaryotic’ cellular organisations have not been found,
and there is almost no chance this will ever happen because
the differences are of the kind that cannot be preserved as
fossils. Many evolutionary scenarios have been proposed
in the literature and only the most ‘consensual’ ones will
be presented here. These scenarios are mainly based on
current knowledge of the cell biology of contemporary
microorganisms.
4.3.1 Eukaryotes Descend from Heterotrophic
Anaerobic Prokaryotes
This scenario was proposed by Christian De Duve (1995,
2002). In the long evolutionary history of life, it distinguishes an anaerobic phase and an aerobic phase.
Assumptions about events that have occurred during the
first phase – the transition from prokaryotes to anaerobic
eukaryotes – are speculative. In contrast, those that led to the
emergence of aerobic eukaryotes rely on solid arguments.
4.3.1.1 Anaerobic Phase
Membrane invaginations could have appeared in an anaerobic and heterotrophic organism of the prokaryotic type that
had lost its cell wall and increased in size, thus increasing
the exchange surface between the cell and its surrounding
environment (Fig. 4.17a–c). Some of these invaginations
gave rise to vesicles, sequestering food and digestive
enzymes (Fig. 4.17d). Digestion then became intracellular.
Subsequently, vesicles would have differentiated in
several kinds, creating a real membranous network with
the possible appearance of the smooth and rough endoplasmic reticulum, the Golgi apparatus and lysosomes
(Fig. 4.17e). Later, cytoskeletal and motor organs (not
shown in the figure) would have appeared. Finally, the
membranous network would have wrapped and isolated
DNA from the cytosol (Fig. 4.17f). The appearance of
these membrane structures would have allowed functional
specialisation of compartments within the cell. It is at this
stage that DNA transcription and mRNA translation became
decoupled.
4.3.1.2 Aerobic Phase
The emergence and development of cyanobacteria
performing oxygenic photosynthesis radically transformed
the chemical composition of the atmosphere, which gradually became aerobic. This transition had a great impact on
the biodiversity of the time, in particular by fostering the
emergence and expansion of organisms that had developed
the ability to use dioxygen via aerobic respiration. It is very
likely that many lineages of anaerobic organisms went
extinct. However not all did, in particular, the ancestor
of eukaryotes. Some may have found shelter in anoxic
habitats, while others managed to develop defence
mechanisms against dioxygen. Lynn Margulis, an American microbiologist who in the 1960s developed ‘the endosymbiotic theory’ (Margulis 1970), hypothesised that
96
J.-C. Bertrand et al.