GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

21. MICROORGANISMS AND THE ENVIRONMENT

21.4. EVOLUTION OF MICROORGANISMS

All living organisms on Earth have traveled a long developmental path together. Simpler forms gradually evolved into more complex and specialized ones, eventually leading to the diverse life forms that inhabit our planet today. This organic evolution represents one of the central problems in biology.

The primordial atmosphere of Earth most likely contained significant amounts of hydrogen, methane, nitrogen, and carbon dioxide, but lacked oxygen.

Chemical evolution. This evolutionary stage could only take place in an oxygen-free atmosphere. It is believed that under such conditions, solar radiation and electrical discharges drove the synthesis of Organic compounds, which subsequently washed into bodies of Water and accumulated there. When the concentration of organic matter became sufficiently high, conditions arose that enabled the transition from chemical evolution to The Emergence of the first self-replicating living entities.

Biological evolution. The transition from lifeless organic matter to a living Cell required a vast span of time (from 3 to 4.5 billion years). The emerging cellular organisms possessed such a profound selective advantage that all prior forms of Organization were outcompeted. Given that pre-cellular life forms (if they existed) did not survive, it can be assumed that the transition from non-living to living occurred relatively rapidly.

Prokaryotic evolution. The first prokaryotes to appear under anaerobic conditions in organic-rich aquatic environments were likely fermentative organisms. Assuming that sulfates were also present in these waters, the next milestone in organic evolution could have been the evolution of efficient electron transport coupled with the generation of a proton motive force as an energy source for ATP synthesis. This stage of evolution evidently gave rise to iron- or nickel-containing tetrapyrrole derivatives, as well as the autotrophic mode of carbon assimilation (the acetyl-CoA pathway). Methanogenic and acetogenic Bacteria, as well as sulfate-reducing bacteria—which, with few exceptions, can utilize H₂, CO₃, and certain Fermentation products—can be regarded as relics of those ancient times.

Following the "invention" of electron transport-coupled phosphorylation, Photosystem I may have evolved, making it possible to utilize light as an energy source. With the acquisition of the capacity for carbon dioxide fixation via The Calvin Cycle and the USE OF INORGANIC electron Donors, a metabolic type characteristic of purple sulfur bacteria emerged. The subsequent milestone was the advent of Photosystem II, which enabled non-Cyclic electron transport using water as an electron donor. This process was inherently tied to the release of oxygen. Cyanobacteria represent the earliest microorganisms capable of oxygenic Photosynthesis.

The transition from a reducing to an oxidizing atmosphere was a monumental event both in the evolution of living organisms and in the transformation of minerals. A new type of METABOLISM—aerobic Respiration—came into being. It is hypothesized that all currently known phototrophic respiring prokaryotes already existed 2.1 billion years ago. Over the subsequent epochs, all life on Earth became dependent on biological photosynthesis and the oxygen released by plants. The accumulation of oxygen in the atmosphere also impacted the abiotic world (through The oxidation of metals and minerals), leading to The formation of carbon deposits in the form of coal, petroleum, natural gas, and carbon-bearing sedimentary rocks.

Eukaryotic evolution. Eukaryotic Cells evidently arose only after oxygen appeared in the atmosphere. With few exceptions, all eukaryotes are aerobic organisms. The further evolution of prokaryotes faced insurmountable barriers primarily associated with their small Genome Size, haploid state, and limited cell volume. The new aerobic environment offered opportunities to harvest more energy; however, exploiting this required larger cells, broader capabilities for structural differentiation, and a significantly larger genome capable of storing a vast array of information. Continued evolution necessitated the creation of a novel cellular model. It is hypothesized that various transitional models and organizational forms emerged during the Cytology/cytology/16.html">Early stages of Introduction/5.html">Eukaryotic Cell evolution prior to the appearance of Multicellular Organisms.

It should be noted that eukaryotes specialized predominantly in photosynthesis and aerobic existence, whereas many other ecological Functions remained the domain of prokaryotes. These include MOLECULAR Nitrogen Fixation, nitrification, denitrification, sulfate and sulfur respiration, the oxidation of sulfur and metals, as well as methane production and consumption. The global nitrogen and sulfur cycles remain entirely under the "jurisdiction" of prokaryotes. Prokaryotes alone could sustain global biogeochemical cycles and preserve the biosphere, whereas eukaryotes would not be able to cope with such a task.

Throughout evolutionary history, eukaryotes have continuously interacted with prokaryotes. Multicellular organisms owe their highly developed defense mechanisms and other adaptations to the aggressiveness of prokaryotes. At the same time, eukaryotes acquired The ability to derive benefits from close associations with prokaryotes, even enlisting them as ecto- and endosymbionts.

The evolution of living organisms presents a multitude of challenges to be solved. Research into these processes is only just beginning.