The initial stages of the development of life on Earth

Natural sciences are called upon to help mankind to know both themselves and the world around them and, of course, to find out how life on all our forms and manifestations arose on our planet. Without going into the religious dogmas about the creation of the world by the supreme spiritual force - God, we will study the hypotheses of the origin of living matter, which biology operates on. The main stages in the development of life on Earth will help us to solve the problem of its origin and manifestation in the surrounding world.

Representation of scientists about the process of evolution of living nature

If you combine all biological species of organisms, both modern and long extinct, you will get an astronomical number - up to one billion species. It is not surprising that scientists who lived at different times, sought to identify the main stages in the development of life on Earth, which led to the emergence of these species of living organisms, as well as the formation of a modern picture of nature. The founder of the systematics of Carl Linnaeus in the 18th century put the principle of "living from the living" in the basis of this science, in which he asserted that life can arise only from an already existing living substance. Linnaeus did not allow even a hint of the so-called self-generation of organisms. German biologist E. Haeckel first expressed the idea of a monophilia - the origin of all living organisms from one ancestor. Jean-Baptiste Lamarck also defended the idea of a unicellular ancestral form, the emergence of which occurred in the early stages of the development of life on Earth. Summarizing all of the above, it can be argued that the hypotheses about the origin of life that exist in science are classified into two groups. The first - abiogenic, it includes ideas about the formation of living matter from inanimate nature (A. Oparin, D. Haldane, S. Miller). The other - biogenic, contains ideas about the appearance of living beings only on their own kind (R. Virchow, K. Linney, C. Darwin).

Did primary organisms have a common ancestral form

The first stages of the development of life on Earth, namely, abiogenic (chemical), then the period of the appearance of biopolymers (proteins and nucleic acids), called prebiological and, finally, the stage of biological evolution (the formation of primary unicellular organisms). They were combined and called biopoiesis. Some researchers (for example, D. Bernal, S. Miller) proposed the idea of a progenitor, the progenitor, from which archebacteria, eubacteria, nuclear cells occurred. Other researchers believe that eukaryotes did not originate from the progenitor, but were the result of symbiosis, or were formed as a result of changes in the outer membrane of the protobiont. Consider these hypotheses in more detail.

The hypothesis of Oparin-Haldane

Among the many scientific versions that try to explain the phenomenon known in science as a prebiological stage in the development of life in the Earth, the hypothesis of coacervate drops is very popular. It was formulated by the Russian scientist AI Oparin. Similar ideas were expressed by British researcher D. Haldane. The ideas of scientists resonated with the long-known hypothesis in biology about the spontaneous generation of life.

The essence of the coacervate hypothesis

Academician A. Oparin suggested that groups of molecules of organic compounds that appeared as a result of the processes of the chemical stage could create a fairly dense shell. Thus, they were separated from the primary broth - the aquatic environment of the ancient ocean. These groups of molecules scientist called coacervates. They were already capable of independent existence, continuing to exchange with the primary solution. To very important, according to Oparin, the properties of coacervates were their ability to grow and crush (multiply). D. Haldane, relied on the experiments of Miller-Urey, in which the complex of organic substances from a mixture of methane, ammonia, hydrogen and carbon dioxide was experimentally obtained. It included molecules of amino acids and simple sugars. This led to the possibility of the emergence of isolated structures - probionts.

According to Oparin and Haldane, the initial stages in the development of life on Earth, leading to the formation of primary complexes - the precursors of cells, provided the basis for the further evolution of living matter. It should be noted that for the experiments, scientists successfully modeled possible conditions that could be in the atmosphere and in the primary world ocean, namely: high temperature and pressure, ionizing radiation and electrical discharges.

Probionts and their properties

The earliest stages in the development of life on Earth-the Katarchean and then the Archaean-were marked by the transition from self-organizing coacervates (probionts) to primary living cells. It became possible due to the unique properties of probionts: the ability to form isolated membranes, the ability to the simplest forms of reproduction, the elementary processes of exchange with the external environment. Self-organized molecules of nucleoproteins, which arose as a result of the chemical stage, provided the probionts with the manifestation of the most important property of living matter-the ability to transmit hereditary information.

Characteristics of the first living organisms

Long ago (about 3.5 billion years ago) sedimentary rocks formed in which traces of organic life were discovered. They had the appearance of calcareous shells of cyanobacteria and the remains of the murein walls of bacterial cells. The geochemical conditions of the lithosphere in the Archean era were constantly changing, so the primary ecosystems of prokaryotes could adapt to them by accumulating many phenotypic variants. Photosynthesis, carried out by blue-green algae (cyanobacteria), provided a fundamental change in the gaseous composition of the Earth's primary atmosphere, which allowed the further release of living creatures from the aquatic habitat to the land. The activity of the first prokaryotic organisms, and they were mainly gray and iron bacteria, led to the formation of not only sedimentary rocks, but also oil and natural gas.

Why it became possible the appearance of eukaryotic cells

As we mentioned earlier, the photosynthetic activity of green and purple sulfur bacteria, as well as iron bacteria, contributed to the formation of a shielding ozone layer and the appearance of aerobic eukaryotic cells. In other words, the first 3 stages of the development of life in the Earth led to the formation of biocenoses containing single-celled and multicellular eukaryotic organisms. Most scientists are inclined to believe that their emergence occurred about 600 million years ago. At first, nuclear organisms were represented by unicellular flagellar forms. As a result of divergence, the first plants - algae, as well as protozoa and fungi - originated from them. Interesting is the judgment of some researchers that prokaryotes are a dead-end branch of evolution, since they practically did not change in the first stages of the development of life on Earth. Biology puts forward two reasons explaining the lack of evolutionary development of non-nuclear organisms.

The first of these is the inability of prokaryotic cells to increase organization and to differentiate. The second reason is a rigid rigid hereditary prokaryotic material, represented by a single circular DNA molecule, called a plasmid.

Symbiosis, which led to revolutionary changes in nature

It is customary to explain the appearance of nuclear cells in scientific circles from the position of symbiogenesis, the theory proposed by A. Schimper. So the formation of the cell nucleus, the presence of which serves as the main sign of eukaryotes, as well as the formation of chloroplasts and mitochondria became possible as a result of the transformation of some aerobic bacteria. Having penetrated into the primary cell with a hereditary substance, a separate membrane, the bacteria synchronized their metabolism with the host cell. As a result, they lost the ability to independently exist outside the cell and became its obligatory organelles. Noteworthy is the hypothesis that explains the appearance of chloroplasts. After all, it should not be forgotten that it was the appearance of these organoids that provided the phenomenon of autotrophic nutrition and the process of photosynthesis. Following A. Shimper, such well-known Russian scientists as KS Merezhkovsky, BM Kozo-Polyansky and others drew attention to a group of photosynthetic bacteria capable of symbiosis with heterotrophic cells. Once in the cytoplasm, they apparently assimilated with cellular metabolism and began to function as organoids, later called chloroplasts. The heterotrophic cells themselves were transformed into unicellular green algae, becoming the first autotrophic eukaryotes.

Biogeocenoses of the Vendian period

So, the coexistence of several types of non-nuclear organisms - bacteria - could lead to the formation of a new living system - a eukaryotic cell. Continuing to study the first stages of the development of life on Earth, let us dwell on the Vendian period of the Proterozoic era, which replaced the Archaean. In the aquatic environment - the main cradle of life on the planet, the first biogeocenoses were formed. The producers were photosynthetic bacteria, as well as unicellular and colonial green algae.

They isolated oxygen, synthesized organic substances, which used heterotrophic organisms: single-celled protozoa, as well as multicellular forms: coelenterates and trilobites. The Vendian period ends the first stages of the development of life on Earth. Eram and the periods following it were to carry out further processes of evolution of living nature, based on hereditary variability and natural selection.