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Beatriz Eugenia Saldana Farias
BIO 2010-1: Microbiology
April 29, 2015
Dr. Alan Leonard
Evidence and Evolution of Precambrian Life
The history of our planet extends back 4.54 billion years, of which the first billion
years consisted of extreme geological activity incapable of sustaining life. The surface of the
planet was not entirely solid; it was consistently bombarded by nearby satellites and
extraterrestrial debris thus creating a molten surface consistency. These collisions increased
volcanic activity, leading to the creation of the primordial atmosphere through outgassing [10].
Over time, as the inner solar system stabilized, the Earth began to cool, allowing the
solidification of the crust thus permitting liquid water to exist on the surface of the planet.
Although the early atmosphere contained nearly no oxygen, it managed to stabilized global
temperatures through the greenhouse effect and aid the evolution of the first life forms [10].
With liquid water on the surface and stable temperatures, the early biosphere thrived and
eventually shaped the planet we currently inhabit.
The existing geological records state that the first life forms developed on the planet
between 3.5 and 3.8 billion years ago, this was detected through evidence of biogenic graphite
from 3.7 billion-year-old metasedimentary rocks
[6]
; this means that autotrophic carbon
fixation can be traced back 3.7 billion years. Fossil records of the morphology of such life
were observed in the form of microbial mats in 3.48 billion-year-old sandstone
[1]
. The
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microorganisms found in the microbial mats were cyanoacterium-like microbes; these
organisms are believed to be far too complex to be considered the earliest system of life
[8]
.
Unfortunately, the origin of life and its form is still unknown, but it is understood that the
current lineage of life originated at least 3.9 billion years ago, if not later, due to the fact that
at an earlier time, the Earth was still being bombarded by debris in the inner solar system with
enough force to vaporize the oceans and sterilize the planet
[7]
. The microbial life that
inhabited the Precambrian Earth closely resembled modern bacteria, and although the
environmental conditions exhibited prodigious discrepancies from those today, the same
mechanism of evolution prevailed.
It is still unclear as to which type of cell, prokaryotic or eukaryotic, first came into
existence, but the current scientifically accepted theory places the prokaryotic species first in
evolution. Due to the minuscule gaps in our understanding of evolution, such as abiogenesis
and the Cambrian explosion, many speculations about how certain forms of life came into
existence cannot be declared as completely reliable
[4]
. It is theorized that the earliest cells
evolved from protocells, a self assembled sphere of lipids encapsulating genetic material; this
theory is sometimes used as a stepping stone in the explanation of the origin of life [8]. There
are other theories as to how prokaryotic cells came into existence, but due to a lack of reliable
evidence, no hypothesis can be accepted as intransigent. Due to the primordial atmospheric
conditions, it is understood that the earliest life forms evolved under a highly reducing
anaerobic atmosphere.
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The evolution of early bacteria can be traced according to a cell’s tolerance of oxygen;
those cells that can function only under anaerobic conditions, called obligate anaerobes, are
believed to have first populated the planet. Then came the facultative anaerobes, cells that can
tolerate both anaerobic and aerobic conditions. Lastly, came the aerobes, cells that require the
presence of oxygen to survive
[11]
. An exceedingly significant event in the early evolution of
Precambrian life was the ability to photosynthesize light to produce oxygen; this process is
hypothesized to have radically changed the atmosphere and consequently the Earth, allowing
new variations of life to emerge. At first, the organisms inhabiting the planet could barely
tolerate the presence of oxygen, but with time, cells evolved to not only tolerate oxygen but
also incorporate it into their metabolic processes. Through natural selection, prehistoric
prokaryotic life developed an immense diversity in genetic traits allowing for metabolic and
biochemical variations that led to the oxygen revolution and eventually the stimulation of
biodiversity
[5]
. The evolution of oxygen-evolving photosynthesis was crucial for the
development of modern life due to the fact that it provided the oxidants required for cellular
respiration and produced the ozone layer that shields the Earth from ultraviolet radiation [3].
A specific group of facultative anaerobic microbes that can undergo photosynthesis is
called cyanobacteria. These bacteria tolerate the oxygen they produce and use it metabolically
for aerobic respiration and in synthetic processes, such as the synthesis of chlorophyll a,
which requires the presence of oxygen; this suggests that cyanobacteria evolved in times of
unstable oxygen concentrations. Although these bacteria need the presence of oxygen for
some biochemical processes, there is evidence suggesting the possibility that early
cyanobacteria were capable of undergoing some processes without the presence of oxygen by
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terminating the biosynthetic pathways at the last anaerobic step
[8]
. Diverse adaptations were
necessary for the survival of cyanobacteria in primitive Earth conditions. Around the time of
the cyanobacteria evolution, geological procedures had stabilized and it is possible that the
atmosphere was not completely anaerobic, thus allowing for cyanobacteria to properly
develop chlorophyll a; these bacteria require an oxygen concentration of about 10%, much
lower than modern atmosphere contains [8].
Eventually cyanobacteria transformed the atmosphere into an oxidizing environment,
allowing aerobic bacteria to thrive. This created a new problem for microorganisms; oxygen
is a common inhibitor in for nitrogenase, a nitrogen fixation enzyme, causing primitive
bacteria to experience multiple evolutionary adaptations to protect the enzyme, thus
influencing niche selection in several microbial species. The evolutionary course required for
the protection nitrogenase from molecular and reactive oxygen species resulted in various
adaptive mechanisms that can be observed in physioecological patters in various microbial
traits and community structure along a gradient from anaerobic to fully aerobic environments
[2]
. Fossil records of such bacteria can be observed in stromatolites, which are rock-masses
found in shallow waters composed of layers of prehistoric microorganisms. Photosynthetic
microorganisms occupy the outermost layer of stromatolites, taking advantage of the
uninterrupted sunlight; inside, other types of bacteria can be found in a gradient from
facultative anaerobes closer to the outside, to obligate anaerobes in the innermost layers
[1]
.
With time, mineral debris accumulates on the outside of the structure, and the stromatolites
become lithified [9]. The available fossil records place the appearance of photosynthetic life at
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least 3.5 billion years ago, allowing for a maximum of 400 million years of anaerobic
evolution [3].
Another significant event in Precambrian history was the emergence of eukaryotic
cells. Although it is no yet proven that such cells evolved second, it is widely understood that
oxygen is required for eukaryotic cells to undergo mitosis, ergo it can be deduced that the
logical trajectory places the evolution of prokaryote cells first. Only nucleated cells are
capable of undergoing complex sexual reproduction, allowing the parent cells to transfer their
genetic code in multiple combinations, thus increasing the genetic variation on the planet
[8]
.
These cells continued to evolve through natural selection and led to the emergence of
multicellular life. Eventually the evolution of such organisms resulted in the Cambrian
explosion 543 million years ago. The Cambrian explosion is a time period in which most
major animal body plans began to appear in the fossil record; the preceding events to the
Cambrian explosion remain unknown and the fossil records continue to fail to explain the
seemingly random germination of animal life.
Precambrian life was made possible by the stabilization of the Earth and inner solar
system. The process by which organic matter sprouted life and the creation of cells remains
unknown, but the evolutionary trajectory of the earliest cells can be traced by observing the
microorganisms’ biochemical byproducts and its effect on the environment. The Earth
developed as a parallel to the evolution of Precambrian life from obligate anaerobe to aerobic
species; this process gravely influenced the development of multicellular organisms and
ultimately led to the evolution of our species.
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References
[1] Awramik, S. M., J. W. Schopf, and M. R. Walter. "Filamentous fossil bacteria from
the Archean of Western Australia." Developments in Precambrian Geology7 (1983):
249-266.
[2] Berman-Frank, Ilana, Pernilla Lundgren, and Paul Falkowski. "Nitrogen fixation and
photosynthetic oxygen evolution in cyanobacteria." Research in Microbiology 154.3
(2003): 157-164.
[3] Blankenship, Robert E. "Origin and early evolution of photosynthesis.
"Photosynthesis Research 33.2 (1992): 91-111.
[4] Levchenko, Vladimir F., et al. Early Biosphere: Origin and Evolution. INTECH
Open Access Publisher, 2012.
[5] Schopf, J. William. "Earth's earliest biosphere: its origin and evolution." (1983).
[6] Schopf, J. William. "Fossil evidence of Archaean life." Philosophical Transactions of
the Royal Society B: Biological Sciences 361.1470 (2006): 869-885.
[7] Schopf, J. William, ed. Major events in the history of life. Jones & Bartlett Learning,
1992.
[8] Schopf, J. William. "The evolution of the earliest cells." Scientific American239.3
(1978): 111-38.
[9] Walter, M. R., R. Buick, and J. S. R. Dunlop. "Stromatolites 3,400–3,500 Myr old
from the North Pole area, Western Australia." (1980): 443-445.
[10] Wetherill, George W. "Formation of the Earth." Annual Review of Earth and
Planetary Sciences 18 (1990): 205-256.
[11] Woese, Carl R. "Bacterial evolution." Microbiological reviews 51.2 (1987): 221.
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