hydrothermal ferrous Fe
+2 in seawater, which may have precipitated following the
oxidation of Fe(II), either by biotic oxidation by chemolithotrophic bacteria or by
abiotic oxidation by cyanobacteria-produced O 2 (Koehler et al. 2010). After BIFs
became discontinuous, red beds and hematitic coatings on grains (forms of ferric
oxides, with oxidized Fe
+3 ) began to occur in 1.9-Ga-old sequences (Fig. 3.1).
Oxygenic photosynthesis is arguably the most significant bioenergetic process on
past and modern Earth, a biochemical pathway by virtue of which the radiative
energy from the sun is transformed into chemical energy within cells. The biochemical innovation that made oxygenic photosynthesis possible was the ability to split
the water molecule and use the produced H
+ as the electron acceptor in the electron
transport chain of the light reactions (Margulis and Dolan 2002). Evidences from
several lines point that this pathway evolved within the cyanobacteria clade and
paved the way for this group to be ecologically successful and almost ubiquitous to
this day, and having left their mark on the substrates they have used for living
(Noffke et al. 2008). Still, it has been challenging to find undisputable fossil
evidences of cyanobacteria oxygenic photosynthesis in the rock record. One of the
oldest is the record of a biosignature lipid that occurs in high proportion in modern
cyanobacterial mats and was found in 2.5-Ga-old bitumens (Summons et al. 1999).
One of the first undisputable life forms that left conspicuous fossils throughout
the Precambrian are stromatolites (Grotzinger and Knoll 1999). These lithified
biogenic structures have a fossil record dating back to 3.5 Ga (Walter and Allwood
2005) (Fig. 3.1) and showed a peak in diversity of forms in the Mesoproterozoic, 1.6
to 1.0 Ga ago (Noffke and Awramik 2013). The precursors of stromatolites are
microbial mats, as they are composed of microbial consortia dominated by photoautotrophic cyanobacteria and other types of bacteria (with aerobic and anaerobic
tolerances) that secrete copious amounts of exopolymers as part of their normal
metabolism, trapping sediment particles and secreting in situ sheets of carbonate.
These microbial mat laminae are sequentially stacked up in layers (stroma ¼ mattress) that grow in height by accretion, towards well-illuminated surface water,
creating a three-dimensional structure. Modern stromatolites occur in shallow seas
and have recently been found on high-altitude volcanic lakes (Farías et al. 2013).
The prevalence of well-illuminated, warm, alkaline (carbonatic), hypersaline waters
seems to be a condition for the occurrence of stromatolites, and in situ microbial
aragonite precipitation is a major feature.
With prokaryote clades well established and exploiting diverse ecological niches,
the course of microbial evolution then saw a series of successful associations
between prokaryotes or serial endosymbiosis, through which microbial cells
acquired genomes and metabolic pathways from one another and set up a novel
kind of cell with a distinct, compartmentalized cytoplasmic arrangement (Kutschera
and Niklas 2005; Keeling 2010). Biological lipids preserved in shales from Pilbara
Craton (Australia) suggest this happened around 2.7 Ga ago (Brocks et al. 1999),
although the earliest fossil eukaryotes are 2.1 Ga old (Han and Runnegar 1992). A
new kind of cellular division (meiosis) implying genetic recombination and the
invention of sex would follow, making genomes ever more complex and leading
to higher levels of biological complexity; by around 1.0 Ga ago, organisms evolved
62
J. Pan
+2 in seawater, which may have precipitated following the
oxidation of Fe(II), either by biotic oxidation by chemolithotrophic bacteria or by
abiotic oxidation by cyanobacteria-produced O 2 (Koehler et al. 2010). After BIFs
became discontinuous, red beds and hematitic coatings on grains (forms of ferric
oxides, with oxidized Fe
+3 ) began to occur in 1.9-Ga-old sequences (Fig. 3.1).
Oxygenic photosynthesis is arguably the most significant bioenergetic process on
past and modern Earth, a biochemical pathway by virtue of which the radiative
energy from the sun is transformed into chemical energy within cells. The biochemical innovation that made oxygenic photosynthesis possible was the ability to split
the water molecule and use the produced H
+ as the electron acceptor in the electron
transport chain of the light reactions (Margulis and Dolan 2002). Evidences from
several lines point that this pathway evolved within the cyanobacteria clade and
paved the way for this group to be ecologically successful and almost ubiquitous to
this day, and having left their mark on the substrates they have used for living
(Noffke et al. 2008). Still, it has been challenging to find undisputable fossil
evidences of cyanobacteria oxygenic photosynthesis in the rock record. One of the
oldest is the record of a biosignature lipid that occurs in high proportion in modern
cyanobacterial mats and was found in 2.5-Ga-old bitumens (Summons et al. 1999).
One of the first undisputable life forms that left conspicuous fossils throughout
the Precambrian are stromatolites (Grotzinger and Knoll 1999). These lithified
biogenic structures have a fossil record dating back to 3.5 Ga (Walter and Allwood
2005) (Fig. 3.1) and showed a peak in diversity of forms in the Mesoproterozoic, 1.6
to 1.0 Ga ago (Noffke and Awramik 2013). The precursors of stromatolites are
microbial mats, as they are composed of microbial consortia dominated by photoautotrophic cyanobacteria and other types of bacteria (with aerobic and anaerobic
tolerances) that secrete copious amounts of exopolymers as part of their normal
metabolism, trapping sediment particles and secreting in situ sheets of carbonate.
These microbial mat laminae are sequentially stacked up in layers (stroma ¼ mattress) that grow in height by accretion, towards well-illuminated surface water,
creating a three-dimensional structure. Modern stromatolites occur in shallow seas
and have recently been found on high-altitude volcanic lakes (Farías et al. 2013).
The prevalence of well-illuminated, warm, alkaline (carbonatic), hypersaline waters
seems to be a condition for the occurrence of stromatolites, and in situ microbial
aragonite precipitation is a major feature.
With prokaryote clades well established and exploiting diverse ecological niches,
the course of microbial evolution then saw a series of successful associations
between prokaryotes or serial endosymbiosis, through which microbial cells
acquired genomes and metabolic pathways from one another and set up a novel
kind of cell with a distinct, compartmentalized cytoplasmic arrangement (Kutschera
and Niklas 2005; Keeling 2010). Biological lipids preserved in shales from Pilbara
Craton (Australia) suggest this happened around 2.7 Ga ago (Brocks et al. 1999),
although the earliest fossil eukaryotes are 2.1 Ga old (Han and Runnegar 1992). A
new kind of cellular division (meiosis) implying genetic recombination and the
invention of sex would follow, making genomes ever more complex and leading
to higher levels of biological complexity; by around 1.0 Ga ago, organisms evolved
62
J. Pan