3.2 Early Life Evolution on Earth and Microbe-Sediment
Interactions
The Earth is 4.5 Ga old (Ga ¼ Giga-annum, 10
9 yr. ¼ billion years), and the most
firm hypotheses about early life signal the primitive oceans as having been the
environment where life first became established; yet, due to the constant recycling
of seafloor by the process of plate tectonics, the oldest sediments and rocks in
modern oceans are 180 Ma (Ma ¼ 10
6 yr. ¼ million years). This makes reconstruction attempts of ocean history a difficult task, and evidences of past biological
processes that took place on marine settings need to be looked for in continental
rocks. The detection and understanding of the early fossil record, the biogenicity of
structures, and the interpretation of paleoenvironmental processes and patterns is by
no means an easy task. The authentication of early Archaean fossils calls for rigorous
criteria (Walter and Allwood 2005) that not only rely upon morphological premises,
as these may sometimes be misleading (García Ruiz et al. 2002). In that sense, the
contribution of actualistic perspectives (Bartley 1996) and geomicrobiological
experiments carried out with modern analogues (e.g., Cuadrado and Pan 2018;
Hickman-Lewis et al. 2019) contributes a valuable alternative to other traditional
paleontological approaches. Accordingly, the disciplinary approach of GeoBiology
is predominantly actualistic.
To put things in perspective, let us start this essay with a brief history of some
major events that have had a tremendous impact on Earth’s physical and living
systems. How about delving into the primitive ocean for a start? There is consensus
among scientists that primitive oceans formed around 4.0 Ga ago (Fig. 3.1), through
a series of planetary-scale processes that implied general cooling and condensation
of atmospheric water vapor into torrential rains that accumulated into arcane ocean
basins. This was followed by the absorbance of large amounts of atmospheric CO 2
into the primitive ocean, the formation of limestone deposits on the seafloor, and an
increase in the alkalinity of seawater in the form of bicarbonate (HCO
À3 ) and
carbonate (CO 3
À2 ) ions. On the other hand, with a thinning atmosphere, the penetration of incoming solar radiation progressively increased.
How life came to be on this planet is a subject for discussion elsewhere, and
indeed much has been written in that respect since the hypotheses of Oparin (1924)
and Haldane (1932) and Miller and Urey’s 1950s’ experiments (Miller 1953; Miller
and Urey 1959) that demonstrated the synthesis of prebiotic organic compounds. Let
us just mention briefly that very early in Earth’s history, a variety of different
metabolic pathways would have evolved and permitted various ways of life
(Oschmann et al. 2002) (Fig. 3.1). The general consensus favors a heterotrophic
metabolism for early life forms over an autotrophic one on the basis that a heterotrophic organism is simpler than an autotrophic one and that the synthesis of
prebiotic (non-biologically produced) substances under reducing conditions has
been successfully demonstrated by controlled experiments (Lazcano and Miller
1996).
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
59
Interactions
The Earth is 4.5 Ga old (Ga ¼ Giga-annum, 10
9 yr. ¼ billion years), and the most
firm hypotheses about early life signal the primitive oceans as having been the
environment where life first became established; yet, due to the constant recycling
of seafloor by the process of plate tectonics, the oldest sediments and rocks in
modern oceans are 180 Ma (Ma ¼ 10
6 yr. ¼ million years). This makes reconstruction attempts of ocean history a difficult task, and evidences of past biological
processes that took place on marine settings need to be looked for in continental
rocks. The detection and understanding of the early fossil record, the biogenicity of
structures, and the interpretation of paleoenvironmental processes and patterns is by
no means an easy task. The authentication of early Archaean fossils calls for rigorous
criteria (Walter and Allwood 2005) that not only rely upon morphological premises,
as these may sometimes be misleading (García Ruiz et al. 2002). In that sense, the
contribution of actualistic perspectives (Bartley 1996) and geomicrobiological
experiments carried out with modern analogues (e.g., Cuadrado and Pan 2018;
Hickman-Lewis et al. 2019) contributes a valuable alternative to other traditional
paleontological approaches. Accordingly, the disciplinary approach of GeoBiology
is predominantly actualistic.
To put things in perspective, let us start this essay with a brief history of some
major events that have had a tremendous impact on Earth’s physical and living
systems. How about delving into the primitive ocean for a start? There is consensus
among scientists that primitive oceans formed around 4.0 Ga ago (Fig. 3.1), through
a series of planetary-scale processes that implied general cooling and condensation
of atmospheric water vapor into torrential rains that accumulated into arcane ocean
basins. This was followed by the absorbance of large amounts of atmospheric CO 2
into the primitive ocean, the formation of limestone deposits on the seafloor, and an
increase in the alkalinity of seawater in the form of bicarbonate (HCO
À3 ) and
carbonate (CO 3
À2 ) ions. On the other hand, with a thinning atmosphere, the penetration of incoming solar radiation progressively increased.
How life came to be on this planet is a subject for discussion elsewhere, and
indeed much has been written in that respect since the hypotheses of Oparin (1924)
and Haldane (1932) and Miller and Urey’s 1950s’ experiments (Miller 1953; Miller
and Urey 1959) that demonstrated the synthesis of prebiotic organic compounds. Let
us just mention briefly that very early in Earth’s history, a variety of different
metabolic pathways would have evolved and permitted various ways of life
(Oschmann et al. 2002) (Fig. 3.1). The general consensus favors a heterotrophic
metabolism for early life forms over an autotrophic one on the basis that a heterotrophic organism is simpler than an autotrophic one and that the synthesis of
prebiotic (non-biologically produced) substances under reducing conditions has
been successfully demonstrated by controlled experiments (Lazcano and Miller
1996).
3 Microbes and Marine Sediments: A Lifelong Relationship on Earth’s Biosphere
59