Basics of Evolution - Korzh O.P. - 2006

Part III. PATHS OF LIFE EVOLUTION

Chapter 18. Evolution of Lower Organisms

18.3. Three Aromatoses of the Proterozoic

The Proterozoic era can be considered one of the most critical stages in the general evolution of organisms, as it witnessed three global aromorphic transformations that substantially influenced the subsequent development of life on Earth. These aromatoses occurred almost simultaneously, which is why they are discussed within a single chapter.

The appearance of The Introduction/5.html">Eukaryotic Cell can be considered the first critically important event.

Bacteria are the simplest, minute organisms capable of reproduction through simple binary fission. In nature, they occupy all ecological niches, which is explained by the remarkable diversity of their biochemical composition. It is believed that with the transition from a reducing to an oxidizing atmosphere (accumulation of O₂), primitive anaerobic forms ceased to play a prominent role in The Development of the biosphere (unlike aerobic ones). Free oxygen, which reacts with most cytoplasmic components, was toxic to anaerobes; nevertheless, due to its high reactivity, aerobes acquired a significantly more efficient source of chemical energy.

This challenge could have been overcome in two ways: by transitioning to anaerobic living conditions or by forming aerobic organisms. Today, Respiration serves as the primary oxidative pathway for the vast majority of organisms. One of the most important manifestations of adaptation to aerobic conditions is The formation of the eukaryotic cell, which gains greater stability compared to prokaryotes.

There are two fundamentally opposing theories regarding THE ORIGIN OF eukaryotic organisms: symbiogenesis and non-symbiotic generation through the segregation of cellular structures from the protoplasm of a Prokaryotic Cell. The Theory of symbiogenesis, proposed by K.S. Mereschkowsky in 1905, provides the most logical explanation for The Emergence of such a complex entity as the eukaryotic cell.

It is hypothesized that eukaryotes formed As a result of a Symbiosis between an anaerobic amoeboid-type prokaryotic cell and aerobic bacteria (the latter giving rise to Mitochondria). Without mitochondria, animal and fungal Cells would be obligate anaerobes and entirely dependent on the low-efficiency process of Glycolysis. Upon acquiring mitochondria, The Cell's Plasma Membrane transfers its energy-generating function to them while taking on new roles—such as controlling ion influx, which is absent in prokaryotes. It is precisely the advent of mitochondria that provides cells with an efficient energy source.

The possibility of a symbiotic origin of mitochondria is supported by the existence of certain modern anaerobic forms (such as the amoeba *Pelomyxa palustris*), which lack their own mitochondria and carry out oxidative METABOLISM through symbiosis with aerobic bacteria. The Nucleus, the most crucial organelle of the aerobic cell, also formed via a symbiotic pathway (having originated from a flagellated prokaryotic cell).

The emergence of Photosynthesis and the formation of plants as a distinct group of organisms should be considered the next aromorphic transformation.

Chloroplasts carry out photosynthesis on the same principle as prokaryotes (cyanobacteria) and bear a morphological resemblance to certain species of them. Like cyanobacteria, chloroplasts reproduce by fission, and their DNA nucleotide sequence is almost entirely homologous to specific regions of cyanobacterial Chromosomes. It is possible that chloroplasts shared a common ancestor with cyanobacteria and evolved from prokaryotes that were engulfed by Eukaryotic cells.

Mitochondria and chloroplasts share common features with modern aerobic bacteria and cyanobacteria, but There are also numerous differences between them. This is due to the fact that they have undergone a long path of substantial evolutionary transformations and have become quite dependent on their host cells.

According to the theory of the non-symbiotic Water/144.html">Origin of the eukaryotic cell, the formation of Organelles (nucleus, mitochondria, etc.) occurred through the invagination of The Cell wall, followed by its Separation and complexification.

Since the Genetic information of a eukaryotic cell is concentrated in the nucleus and enclosed by its membrane, direct division of the cell itself is rendered impossible. Therefore, a mechanism is required to eliminate the nuclear envelope and ensure the precise distribution of genetic information between daughter cells. Mitosis serves as such a mechanism.

A new stage in the evolution of eukaryotic organisms is the appearance of sexual reproduction, which is associated with the emergence of Meiosis. During this process, genomes are shuffled and recombined (combinative Variability), resulting in the appearance of individuals with novel genotypes. These processes significantly impact the course of subsequent evolution, leading to the accelerated formation of new traits.

Higher plants (except bryophytes) and animals spend the majority of their life cycle in the diploid phase, whereas their haploid phase is very brief. Evolutionary Processes favored sexual reproduction because random genetic recombination increases the survival chances of organisms in a highly variable environment. The sexual process is also essential for maintaining diploidy and creating conditions for the rapid generation of new genes in plants and animals. This considerably accelerates the overall pace of evolution.

Unicellular organisms are relatively well adapted to their environment—some of them possess quite complex forms. However, they are vastly inferior to Multicellular Organisms in the richness of their life forms and The Structure of the ecosystems they form.

The emergence of multicellularity (the third Proterozoic aromorphic transformation) led to a dramatic diversification of organisms on the planet, which increased the "pressure of life" and, consequently, accelerated the general rate of evolution.

There are several theories regarding the origin of multicellular organisms, of which the two most well-known are the gastraea theory and the phagocytella theory.

The gastraea theory was proposed by the German scientist E. Haeckel in 1872 (the theory of the origin of Multicellular animals from a hypothetical ancestral Organism, the gastraea). According to his views, the gastraea resembled an embryonic developmental stage of multicellular organisms known as the gastrula, appearing as an elongated, sac-like body with a two-layered wall and a central cavity opening to the environment via the blastopore (primitive Mouth). The gastraea arose from the blastaea (a single-layered spherical organism) through invagination. This theory is unprovable because there are no sufficient grounds to consider invagination the primary mode of Gastrulation. Furthermore, this theory implies that protostomes such as Flatworms and Roundworms, which lack a coelom, lost it secondarily, but this does not correspond to reality.

The phagocytella theory (concerning the origin of multicellular organisms from flagellated colonial forms) was proposed by I.I. Mechnikov in 1877. According to his concepts, the Evolution of the ancestors of multicellular organisms—namely, blastula-like colonial flagellates—proceeded through the differentiation of cells into locomotory and digestive types, as well as an increase in their interdependence: digestive cells lost their flagella, transitioned to an amoeboid state, sank into the cavity, and formed an internal mass of cells known as the phagocytoblast, while flagellated cells retained their locomotory function. Thus, the common two-layered ancestor of multicellular animals, the phagocytella, was formed (Fig. 18.3).

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Fig. 18.3. Most important hypothesized stages in the early phylogeny of multicellular organisms (after Yu.I. Polyansky, 1987):

1 - flagellate colony; 2 - migration of some cells inward; 3 - phagocytella; 4 - sponge; 5 - Trichoplax; 6 - late phagocytella with a mouth opening; 7 - primary bilateral animal; 8 - common ancestor of Coelenterates; 9 - primary flatworm

According to modern views, multicellular organisms evolved from colonial forms. The probability of such an evolutionary pathway is particularly evident in The Study of green flagellates, which include unicellular, colonial, and multicellular organisms. Among the colonial forms, Volvocales are of greatest interest, as they already exhibit a division of labor among colony cells: reproductive Functions are restricted to only a small number of cells, while all others have lost their capacity for reproduction and independent existence. Thus, Volvox cells display, in a primitive form, two essential features of all multicellular organisms:

1) specialization and 2) cooperation. This leads to the formation of a unified, coordinated organism that possesses greater adaptive potential than any of its constituent parts.

A.V. Ivanov hypothesized that multicellular organisms may have originated from unpigmented colonial flagellates. The discovery of Trichoplax in the late 19th century helped bridge the evolutionary gap between unicellular and multicellular organisms. The structural similarity of this animal to the phagocytella Supports the validity of I.I. Mechnikov's views.