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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 Beatriz E. Saldana 8797 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. 2 Beatriz E. Saldana 8797 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 3 Beatriz E. Saldana 8797 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 4 Beatriz E. Saldana 8797 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. 5 Beatriz E. Saldana 8797 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. 6