as photolysis of water vapour in the upper atmosphere.
Indeed, even if the amount of dioxygen produced by this
mechanism is very small compared to that produced by
oxygenic photosynthesis, high concentrations of dioxygen
are not always necessary for aerobic respiration, as in the
case of microaerophilic bacteria.
4.5
Conclusion
Data concerning very ancient life (fossils, isotopic ratios,
biomarkers) are rare and sometimes difficult to interpret.
Despite that, the existence of cellular life between 3.5 and
3.2 Ga is well established. The discovery of fossil
biomarkers (steranes methylhopanes) and the use of stable
isotopes (carbon, sulphur, nitrogen) have provided important
information on the possible dates of occurrence of many
metabolisms.
Since the appearance of the first cell, microorganisms
were the only inhabitants of our planet for almost 3 billion
years. They have survived the intense geological upheavals
that have marked the history of the Earth. They profoundly
modified their environment to such an extent that there was a
true co-evolution between the biosphere and the geosphere.
Through their activity, they also created favourable
conditions for the emergence of multicellular aerobic
organisms (particularly through oxygenation of the
atmosphere).
Among all microorganisms that populated the Earth,
LUCA occupies a central position in evolution and is the
subject of much research because it is the ancestor of all
extant living organisms.
It is possible to outline some of the major stages of the
evolution of microorganisms (especially the most recent
steps). The endosymbiotic origin of mitochondria and
chloroplasts is a virtual certainty. It is now established that
many bacterial genes have contributed, via mitochondrial
endosymbiosis (and chloroplast endosymbiosis for photosynthetic eukaryotes), to build the eukaryotic genome.
However, this chapter also highlights the large uncertainties
about the nature of LUCA and the sequence of events that
led to modern organisms. The main reason is due to the
extremely old age of these events. Recall, for example,
that the exact causes of the extinction of the dinosaurs that
occurred only 65 million years ago are still debated.
Compared to the several billion-year-old events discussed
in this chapter, it was yesterday. It is thus no surprise that
many questions remain about the history of microorganisms:
was LUCA hyperthermophilic, thermophilic or mesophilic?
Was it heterotrophic or autotrophic? How and when did
eukaryotes appear? Was it 2.2 billion years ago, as suggested
by the discovery of steranes, or 1.5 billion years ago, the age
of some eukaryotic microfossils? What role did viruses
play in the evolution of cellular organisms? Did oxygenic
photosynthesis predate aerobic respiration or vice versa?
Despite huge gaps, our knowledge has progressed considerably on all these issues. Some of today’s speculative
hypotheses may become consensual tomorrow. Peter
Mitchell’s chemiosmotic hypothesis proposed in 1960 is a
good example. It is currently considered a mainstream theory after having been initially judged unacceptable by
specialists of bioenergetics. However, in 1975, Peter
Mitchell received the Nobel Prize. He had explained
mechanisms that most likely emerged during very early
phases of cellular life.
While many questions remain unanswered, they offer
many opportunities for research: improvement in the analytical techniques of ancient rocks samples, intensification of
genome sequencing (especially protists), deepening of our
understanding of the physiology and biochemistry of contemporary microorganisms, and improvement of techniques
for the isolation and cultivation of microorganisms. Indeed,
microbial biodiversity has not revealed all its secrets because
only a very small part of the microorganisms that inhabit
our planet is cultivable with current techniques (0.01–1 %).
Some answers might be brought in by exobiology
(especially, the research of traces of extraterrestrial life)
(see Chap. 10) or by a better understanding of incompletely
or partially explored terrestrial habitats including subglacial
Antarctic lakes, biota of the subsurface biosphere such as
petroleum reservoirs, underground aquifers and deep rocks.
References
Allwood AC, Walter MR, Kamber BS, Marshall CP, Burch IW (2006)
Stromatolite reef from the Early Archaean era of Australia. Nature
441:714–718
Beatty JT et al (2005) An obligately photosynthetic bacterial anaerobe
from a deep-sea hydrothermal vent. Proc Natl Acad Sci U S A
102:9306–9310
Boussau B, Blanquart S, Necsulea A, Lartillot N, Gouy M (2008)
Parallel adaptations to high temperature in the Archaean eon.
Nature 456:942–945
Brasier MD et al (2002) Questioning the evidence for Earth’s oldest
fossils. Nature 416:76–81
Brasier MD, Mc Loughlin N, Green O, Wacey D (2006) A fresh look at
the fossil evidence for early Archaean cellular life. Philos Trans R
Soc Lond B Biol Sci 361:887–902
Brinkmann H, Philippe H (2005) The universal tree of life: from simple
to complex or from complex to simple. In: Gargaud M, Barbier B,
Martin H, Reisse J (eds) Lectures in astrobiology: vol I. Advances in
astrobiology and biogeophysics. Springer, pp 617–656
Brochier C, Philippe H (2002) A non-hyperthermophilic ancestor for
bacteria. Nature 417:244
Brocks JJ, Summons RE (2003) Sedimentary hydrocarbons,
biomarkers for early life. In: Holland HD, Turekian KK (eds)
Treatise on geochemistry. New-Haven, Elsevier, pp 63–115
Brocks JJ, Logan GA, Buick R, Summons RE (1999) Archean molecular fossils and the early rise of eukaryotes. Science 285:1033–1036
104
J.-C. Bertrand et al.
Indeed, even if the amount of dioxygen produced by this
mechanism is very small compared to that produced by
oxygenic photosynthesis, high concentrations of dioxygen
are not always necessary for aerobic respiration, as in the
case of microaerophilic bacteria.
4.5
Conclusion
Data concerning very ancient life (fossils, isotopic ratios,
biomarkers) are rare and sometimes difficult to interpret.
Despite that, the existence of cellular life between 3.5 and
3.2 Ga is well established. The discovery of fossil
biomarkers (steranes methylhopanes) and the use of stable
isotopes (carbon, sulphur, nitrogen) have provided important
information on the possible dates of occurrence of many
metabolisms.
Since the appearance of the first cell, microorganisms
were the only inhabitants of our planet for almost 3 billion
years. They have survived the intense geological upheavals
that have marked the history of the Earth. They profoundly
modified their environment to such an extent that there was a
true co-evolution between the biosphere and the geosphere.
Through their activity, they also created favourable
conditions for the emergence of multicellular aerobic
organisms (particularly through oxygenation of the
atmosphere).
Among all microorganisms that populated the Earth,
LUCA occupies a central position in evolution and is the
subject of much research because it is the ancestor of all
extant living organisms.
It is possible to outline some of the major stages of the
evolution of microorganisms (especially the most recent
steps). The endosymbiotic origin of mitochondria and
chloroplasts is a virtual certainty. It is now established that
many bacterial genes have contributed, via mitochondrial
endosymbiosis (and chloroplast endosymbiosis for photosynthetic eukaryotes), to build the eukaryotic genome.
However, this chapter also highlights the large uncertainties
about the nature of LUCA and the sequence of events that
led to modern organisms. The main reason is due to the
extremely old age of these events. Recall, for example,
that the exact causes of the extinction of the dinosaurs that
occurred only 65 million years ago are still debated.
Compared to the several billion-year-old events discussed
in this chapter, it was yesterday. It is thus no surprise that
many questions remain about the history of microorganisms:
was LUCA hyperthermophilic, thermophilic or mesophilic?
Was it heterotrophic or autotrophic? How and when did
eukaryotes appear? Was it 2.2 billion years ago, as suggested
by the discovery of steranes, or 1.5 billion years ago, the age
of some eukaryotic microfossils? What role did viruses
play in the evolution of cellular organisms? Did oxygenic
photosynthesis predate aerobic respiration or vice versa?
Despite huge gaps, our knowledge has progressed considerably on all these issues. Some of today’s speculative
hypotheses may become consensual tomorrow. Peter
Mitchell’s chemiosmotic hypothesis proposed in 1960 is a
good example. It is currently considered a mainstream theory after having been initially judged unacceptable by
specialists of bioenergetics. However, in 1975, Peter
Mitchell received the Nobel Prize. He had explained
mechanisms that most likely emerged during very early
phases of cellular life.
While many questions remain unanswered, they offer
many opportunities for research: improvement in the analytical techniques of ancient rocks samples, intensification of
genome sequencing (especially protists), deepening of our
understanding of the physiology and biochemistry of contemporary microorganisms, and improvement of techniques
for the isolation and cultivation of microorganisms. Indeed,
microbial biodiversity has not revealed all its secrets because
only a very small part of the microorganisms that inhabit
our planet is cultivable with current techniques (0.01–1 %).
Some answers might be brought in by exobiology
(especially, the research of traces of extraterrestrial life)
(see Chap. 10) or by a better understanding of incompletely
or partially explored terrestrial habitats including subglacial
Antarctic lakes, biota of the subsurface biosphere such as
petroleum reservoirs, underground aquifers and deep rocks.
References
Allwood AC, Walter MR, Kamber BS, Marshall CP, Burch IW (2006)
Stromatolite reef from the Early Archaean era of Australia. Nature
441:714–718
Beatty JT et al (2005) An obligately photosynthetic bacterial anaerobe
from a deep-sea hydrothermal vent. Proc Natl Acad Sci U S A
102:9306–9310
Boussau B, Blanquart S, Necsulea A, Lartillot N, Gouy M (2008)
Parallel adaptations to high temperature in the Archaean eon.
Nature 456:942–945
Brasier MD et al (2002) Questioning the evidence for Earth’s oldest
fossils. Nature 416:76–81
Brasier MD, Mc Loughlin N, Green O, Wacey D (2006) A fresh look at
the fossil evidence for early Archaean cellular life. Philos Trans R
Soc Lond B Biol Sci 361:887–902
Brinkmann H, Philippe H (2005) The universal tree of life: from simple
to complex or from complex to simple. In: Gargaud M, Barbier B,
Martin H, Reisse J (eds) Lectures in astrobiology: vol I. Advances in
astrobiology and biogeophysics. Springer, pp 617–656
Brochier C, Philippe H (2002) A non-hyperthermophilic ancestor for
bacteria. Nature 417:244
Brocks JJ, Summons RE (2003) Sedimentary hydrocarbons,
biomarkers for early life. In: Holland HD, Turekian KK (eds)
Treatise on geochemistry. New-Haven, Elsevier, pp 63–115
Brocks JJ, Logan GA, Buick R, Summons RE (1999) Archean molecular fossils and the early rise of eukaryotes. Science 285:1033–1036
104
J.-C. Bertrand et al.