The association of early life and shallow ocean sediments seems to have been
very plausible, as further evidence in the form of 3.2-Ga-old spheroidal, organicwalled microfossils, for which cellular morphology and ultrastructure can be
discerned, has been described from South African siliciclastic tidal deposits (Javaux
et al. 2010) (Fig. 3.1). Without an aqueous environment to buffer the effects of shortwavelength solar UV radiation, the existence of life on land was not only improbable
but most likely impossible. Nucleic acids and proteins strongly absorb UV light,
which ruptures chemical bonds, causes cellular damage, and eventually leads to cell
death (Margulis and Dolan 2002).
With the progressive depletion of organic compounds by heterotrophs (a topic
that is still under debate whether it happened relatively fast or later among early life
forms), and despite the invention of simple metabolic pathways such as fermentation
and glycolysis, as forms of obtaining energy from simple organic molecules
(Margulis and Dolan 2002), the evolution of autotrophy may seem like the next
necessary evolutionary step. Some scientists argue that it might have been possible
for microbes to explore the chemoautotrophic pathway, as it occurs to this day
among hydrothermal vent archaea and bacteria (Nisbet and Sleep 2001; Rollinson
2007); some others even hypothesize that photosynthesis arose from chemotrophy
(Nisbet et al. 1995). On the other hand, the first photosynthesizing bacteria likely
were anaerobic and putatively devised a form of photosynthesis that did not generate
O 2 , nor did they consume it in any way (Margulis and Dolan 2002). But what has
been firmly established is that by 3.5 Ga ago, a certain clade of bacteria
(cyanobacteria) began producing a lithologic record attesting to their metabolic
activity (Brasier et al. 2004).
Cyanobacteria are photoautotrophs that use the sun’s radiation to fix inorganic
CO 2 into organic compounds, producing O 2 as a by-product of photosynthesis,
hence termed oxygenic photosynthesis (Nisbet and Fowler 1999). The gradual
accumulation of photosynthetic residual O 2 in the atmosphere over time led to
what geoscientists refer to as the Paleoproterozoic Great Oxygenation Event
(GOE) when some ~2.45–2.20 Ga, atmospheric oxygen levels rose to >1% of
modern levels (Hazen et al. 2008; Schopf 2012; Farmer and Cook 2013). The
GOE coincides with a peak in cyanobacterial diversity (Margulis and Dolan 2002;
Noffke and Awramik 2013), providing an unequivocal clue as to which microbes
were responsible for the O 2 accumulation in the atmosphere. The transition from a
reducing atmosphere to an oxidizing one not only produced a shift in chemical
reactions that impacted the lithosphere and the biosphere, but also the newly formed
stratospheric ozone layer resulted in a significant cutoff of most UV radiation.
Furthermore, no further synthesis of prebiotic compounds was possible in an
oxidizing atmosphere, indirectly benefiting photoautotrophs over the more primitive
heterotrophs (Margulis and Dolan 2002).
The transition from a non-oxidizing atmosphere to one characterized by free
oxygen has been recorded in sedimentary sequences from South Africa (Eriksson
and Cheney 1992), which document the disappearance of banded-iron formations
(BIFs, Fig. 3.1) which had presented a continuous record dating back to at least
3.9–3.8 Ga (Koehler et al. 2010). The BIFs are controlled by the availability of
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
61
very plausible, as further evidence in the form of 3.2-Ga-old spheroidal, organicwalled microfossils, for which cellular morphology and ultrastructure can be
discerned, has been described from South African siliciclastic tidal deposits (Javaux
et al. 2010) (Fig. 3.1). Without an aqueous environment to buffer the effects of shortwavelength solar UV radiation, the existence of life on land was not only improbable
but most likely impossible. Nucleic acids and proteins strongly absorb UV light,
which ruptures chemical bonds, causes cellular damage, and eventually leads to cell
death (Margulis and Dolan 2002).
With the progressive depletion of organic compounds by heterotrophs (a topic
that is still under debate whether it happened relatively fast or later among early life
forms), and despite the invention of simple metabolic pathways such as fermentation
and glycolysis, as forms of obtaining energy from simple organic molecules
(Margulis and Dolan 2002), the evolution of autotrophy may seem like the next
necessary evolutionary step. Some scientists argue that it might have been possible
for microbes to explore the chemoautotrophic pathway, as it occurs to this day
among hydrothermal vent archaea and bacteria (Nisbet and Sleep 2001; Rollinson
2007); some others even hypothesize that photosynthesis arose from chemotrophy
(Nisbet et al. 1995). On the other hand, the first photosynthesizing bacteria likely
were anaerobic and putatively devised a form of photosynthesis that did not generate
O 2 , nor did they consume it in any way (Margulis and Dolan 2002). But what has
been firmly established is that by 3.5 Ga ago, a certain clade of bacteria
(cyanobacteria) began producing a lithologic record attesting to their metabolic
activity (Brasier et al. 2004).
Cyanobacteria are photoautotrophs that use the sun’s radiation to fix inorganic
CO 2 into organic compounds, producing O 2 as a by-product of photosynthesis,
hence termed oxygenic photosynthesis (Nisbet and Fowler 1999). The gradual
accumulation of photosynthetic residual O 2 in the atmosphere over time led to
what geoscientists refer to as the Paleoproterozoic Great Oxygenation Event
(GOE) when some ~2.45–2.20 Ga, atmospheric oxygen levels rose to >1% of
modern levels (Hazen et al. 2008; Schopf 2012; Farmer and Cook 2013). The
GOE coincides with a peak in cyanobacterial diversity (Margulis and Dolan 2002;
Noffke and Awramik 2013), providing an unequivocal clue as to which microbes
were responsible for the O 2 accumulation in the atmosphere. The transition from a
reducing atmosphere to an oxidizing one not only produced a shift in chemical
reactions that impacted the lithosphere and the biosphere, but also the newly formed
stratospheric ozone layer resulted in a significant cutoff of most UV radiation.
Furthermore, no further synthesis of prebiotic compounds was possible in an
oxidizing atmosphere, indirectly benefiting photoautotrophs over the more primitive
heterotrophs (Margulis and Dolan 2002).
The transition from a non-oxidizing atmosphere to one characterized by free
oxygen has been recorded in sedimentary sequences from South Africa (Eriksson
and Cheney 1992), which document the disappearance of banded-iron formations
(BIFs, Fig. 3.1) which had presented a continuous record dating back to at least
3.9–3.8 Ga (Koehler et al. 2010). The BIFs are controlled by the availability of
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
61