General Microbiology - Schlegel, H. 1987

Microorganisms and the Environment
Evolution of Microorganisms

The biochemical unity, the idea of which seemed so incredible just a few decades ago, is nowadays a firmly established fact. The Cells of all living creatures—from the most primitive forms to the most highly developed animals and plants—consist of the same structural elements and utilize the same mechanisms for energy production and growth. Compared to this fundamental unity, existing differences and deviations appear negligible. It is believed that all currently living organisms have traversed a long evolutionary path together. From the simplest forms, more complex and specialized forms gradually evolved, and finally, those that inhabit our planet today. This process of the evolutionary development of organisms is one of the central problems of biology.

The Primary Atmosphere of Earth. Our Earth differs fundamentally from the other planets of the solar system. Compared to Jupiter and the Sun, it contains only negligible amounts of inert gases. It presumably formed through the coalescence of numerous meteorites; As a result of the heating and melting of Earth's inner core, Water and gases were driven to its surface. The primitive atmosphere likely contained a great deal of hydrogen, methane, nitrogen, and CO₂, but lacked oxygen. Photolysis of water vapor certainly released oxygen, but it repeatedly reverted to a bound state. Chemical evolution could only take place in an oxygen-free atmosphere.

Chemical Evolution. The hypothesis that life was brought to our planet from outside hardly deserves serious Structure/133.html">Discussion nowadays. Self-replicating biological entities must have originated on Earth itself during the early period of its existence. According to the concept proposed by Haldane and Oparin, large quantities of Organic compounds accumulated on Earth at that time, yet there were no organisms capable of utilizing and mineralizing them. Once Miller's initial attempts successfully and repeatedly confirmed in experiments that simple organic molecules can be synthesized from inorganic substances (H₂, CO₂, NH₃, H₂O) and methane under suitable conditions, doubts regarding the reality of chemical evolution completely vanished. It is assumed that in the reducing primary atmosphere (devoid of oxygen), organic substances were formed under METABOLISM/18.html">The Influence of solar radiation and electrical discharges, which subsequently washed into water and accumulated there. When they accumulated in large amounts, conditions presumably arose that allowed the transition from chemical evolution to The Emergence of the first self-replicating living creatures.

Biological Evolution. The transition from lifeless organic matter to a living Cell required a prolonged period of time (from 3.1 to 4.5 billion years). The emerging cellular organisms evidently gained such a massive selective advantage that all preceding organizational forms were displaced. Because pre-Cellular forms of life (if they existed) have not been preserved even in the fossil record, the transition from non-living to living appears to us exceptionally rapid.

Evolution of Prokaryotes. According to the widespread, albeit highly hypothetical view, The Development of prokaryotic organisms took place in the reducing primary atmosphere (Fig. 17.5). The first prokaryotes that could have appeared in bodies of water rich in organic compounds were organisms that survived through Fermentation and possessed the basic Functions of Anaerobic Metabolism (the fructose-bisphosphate and pentose-phosphate pathways). Assuming that sulfates were also present in those waters, the next milestone in organic evolution could have been efficient electron transport establishing a proton motive force as an energy source for ATP regeneration. At this evolutionary stage, iron- or nickel-containing tetrapyrrole derivatives likely emerged, along with the autotrophic mode of carbon assimilation (the acetyl-CoA pathway). Methanogenic and acetogenic Bacteria, as well as sulfate-reducing bacteria—which, with a few exceptions, can utilize H₂, CO₂, and certain fermentation products—can be regarded as relics of those times.

Following the "invention" of phosphorylation coupled with electron transport, Photosystem I—a "light-driven proton pump"—could also have emerged, making it possible to utilize light as an energy source. The reaction centers were magnesium Porphyrins (chlorophylls). The first phototrophic organisms likely assimilated carbon in the light, similar to the Rhodospirillaceae. With the acquisition of The ability to fix CO₂ via the ribulose bisphosphate cycle and utilize inorganic electron Donors (H₂, H₂S, S), the type of metabolism characteristic of purple sulfur bacteria (Chromatiaceae) developed. Greater independence from substances dissolved in water was subsequently brought about by the appearance of Photosystem II, which made non-Cyclic electron transport using water as an electron donor possible. This process was inevitably linked to the release of oxygen. Oxygenic Photosynthesis led to the atmosphere acquiring an oxidizing character. Representatives of the earliest microorganisms carrying out oxygen-evolving photosynthesis are the cyanobacteria.

The transition from the primary reducing atmosphere to an oxygen-containing atmosphere was undoubtedly the greatest event both in the evolution of living creatures and in the transformation of minerals. Through The conversion of Cytochromes into terminal oxidases and The Use of molecular oxygen as an electron acceptor, a new type of metabolism—aerobic Respiration—became possible in bacteria.

Class="center">

Fig. 17.5. Hypothetical stages in the evolution of metabolism in prokaryotes.

It is believed that 2.1 billion years ago, all phototrophic respiring prokaryotes known today were already in existence. According to geological data, small amounts of oxygen were already present 2.7 billion years ago. Over the past 1.2 billion years, all life on Earth has depended on biological photosynthesis and on the oxygen released by plants. By causing oxygen to accumulate in the atmosphere, the development of life thus influenced the inanimate world as well—via The oxidation of metals and minerals.

In the period up to 0.6 billion years ago, the oxygen content in the atmosphere probably increased to no more than 2%. Only after plants colonized the land and blanketed it in lush greenery did the concentration of oxygen in the air rise sharply to reach the modern level (21%). The accumulation of O₂ was accompanied by the deposition of carbon in the form of coal, petroleum, natural gas, and carbon-bearing sedimentary rocks.

Fossils dating back to the Early Precambrian are extremely rare. Because of the tiny size of primitive organisms and their lack of any hard components, their remains could only be preserved under exceptional circumstances. In Minnesota (USA), in deposits estimated to be 2.7 billion years old, structures interpreted as bacterial remains (including cyanobacteria) were discovered. The age of South African deposits, in which structures resembling bacteria were also found, reaches 3.1 billion years. These are the oldest known traces of life.

Bacteria are the surviving living witnesses to the early evolution of life. Many bacteria that were once widespread and dominant now lead a very modest existence. Anaerobic bacteria have likewise survived in ecological niches that provide them with suitable living conditions.

Evolution of Eukaryotes. Eukaryotic cells apparently arose only after oxygen appeared in the atmosphere. With very few exceptions, all eukaryotes are aerobic organisms. Prokaryotes occupied many different ecological niches. The development of diverse metabolic types in prokaryotes was presumably driven by their simple Cell Structure, highly sophisticated regulatory systems, rapid growth, and multiple Gene transfer mechanisms. Further prokaryotic evolution faced insurmountable hurdles, primarily associated with their small Genome Size, haploid state, and minute cell volume. The new aerobic environment offered access to more energy, but utilizing it required larger cells, broad capabilities for structural differentiation, and consequently a manifoldly larger genome capable of storing a vast volume of information. A genome size of 5·10⁹ Da was likely the upper limit for the molecular mass of a bacterial chromosome consisting of a single double helix. Further evolution demanded the creation of a new model.

The differences between prokaryotic and eukaryotic cells (eucyte and protocyte) are immense. Let us reiterate the most important features of eukaryotic cells:

1. The carrier of Genetic information (DNA) is separated from the "metabolic spaces" by the nuclear envelope.

2. Consequently, Transcription (in The Nucleus) is separated from Translation (in the Cytoplasm).

3. The Genome is partitioned—there are several (often many) linear Chromosomes instead of a single circular one.

4. DNA Replication occurs exclusively during interphase; each chromosome possesses multiple replicons; daughter chromosomes are segregated via mitosis.

5. Intracellular mechanisms utilizing Actin and tubulin exist for chromosome movement during Mitosis and Meiosis, alongside vesicle-like structures (Lysosomes, Peroxisomes, and other "Microbodies").

6. The genes contain non-coding insertions—introns.

7. DNA forms a complex with Histones, structurally resembling a string of beads (a chain of nucleosomes).

8. The life cycle includes meiosis, in which haploid cells are formed from diploid cells. This enables sexual reproduction involving gene recombination and the alternation of haplo- and diplophases.

9. Exocytosis: extracellular Enzymes are not synthesized directly at The Plasma Membrane (with their simultaneous release from The Cell), but rather on internal membranes, after which they are delivered to the surface in cisternae.

10. Endocytosis (in the form of phagocytosis and pinocytosis), which enables the acquisition of intracellular symbionts.

11. The presence of Mitochondria and Chloroplasts, which serve for energy production (ATP resynthesis).

12. 9 + 2 type flagella (or cilia).

Thus, the eucyte differs from the protocyte in many functions and structures. Although certain eukaryotes are known to lack one or another of these features, there are no such primitive forms that would allow us to determine the sequence in which new traits emerged. Apparently, each stage of evolution brought with it only a very slight selective advantage, at least compared to the immediately preceding step. Consequently, intermediate forms were not preserved and were probably so unstable that there are no fossil remains today allowing us to judge their functional characteristics. Currently, there is

only a small number of organisms that can be considered as having evolved from intermediate forms. The prospect of ever establishing the exact sequence in which the aforementioned new traits appeared must be viewed pessimistically. Nevertheless, it can be hypothesized that various models of Introduction/5.html">Eukaryotic Cell Organization emerged in the Cytology/cytology/16.html">Early stages of evolution long before Multicellular Organisms appeared.

It should be noted that eukaryotes specialized primarily in photosynthesis and aerobic existence, while a wide range of other crucial ecological functions remained with prokaryotes. These include Nitrogen Fixation, nitrification, denitrification, sulfate and sulfur respiration, sulfur and metal oxidation, as well as methane production and utilization. The nitrogen and sulfur cycles are entirely or predominantly managed by prokaryotes. Thus, prokaryotes could sustain nutrient cycles and preserve the biosphere on their own, whereas eukaryotes alone would not be able to cope with this task.

While prokaryotes evolved independently for billions of years, eukaryotes were never left alone. They constantly had to contend with prokaryotes, providing the latter with new ecological niches and protection, while also serving as their prey. Multicellular organisms owe their highly developed defense mechanisms and other adaptations partly to the aggressiveness of prokaryotes. On the other hand, eukaryotes learned to benefit from close associations with prokaryotes, enlisting them as ectosymbionts (in the intestinal tract, on the Skin, in the rumen of ruminants) and endosymbionts (for nitrogen fixation, biomass production via photosynthesis, utilization of H₂S, and removal of H₂).

The evolution of living organisms offers a multitude of fascinating problems to solve. Their exploration is only just beginning.