The emergence of the Earth and the early stages of its formation
One of the important tasks of modern natural science in the field of Earth sciences is the restoration of the history of its development. According to modern cosmogonic concepts, the Earth was formed from gas and dust scattered in the protosolar system. One of the most probable variants of the origin of the Earth is as follows. First, the Sun and a flattened rotating near-solar nebula were formed from an interstellar gas and dust cloud under the influence of, for example, the explosion of a nearby supernova. Next, the evolution of the Sun and the near-solar nebula took place with the transfer of the moment of momentum from the Sun to the planets by electromagnetic or turbulent-convective methods. Subsequently, the "dusty plasma" condensed into rings around the Sun, and the material of the rings formed the so-called planetesimals, which condensed to planets. After that, a similar process was repeated around the planets, which led to the formation of satellites. This process is believed to have taken about 100 million years.
It is assumed that further, as a result of differentiation of the Earth's substance under the influence of its gravitational field and radioactive heating, different in chemical composition, state of aggregation and physical properties of the shell - the Earth's geosphere - arose and developed. The heavier material formed a core, probably composed of iron mixed with nickel and sulfur. Somewhat lighter elements remained in the mantle. According to one of the hypotheses, the mantle is composed of simple oxides of aluminum, iron, titanium, silicon, etc. The composition of the earth's crust has already been discussed in sufficient detail in § 8.2. It is composed of lighter silicates. Even lighter gases and moisture formed the primary atmosphere.
As already mentioned, it is assumed that the Earth was born from a cluster of cold solid particles that fell out of a gas and dust nebula and stuck together under the influence of mutual attraction. As the planet grew, it warmed up due to the collision of these particles, which reached several hundred kilometers, like modern asteroids, and the release of heat not only by naturally radioactive elements now known to us in the crust, but also by more than 10 radioactive isotopes Al, Be, which have since died out. Cl, etc. As a result, complete (in the core) or partial (in the mantle) melting of the substance could occur. In the initial period of its existence, up to about 3.8 billion years, the Earth and other terrestrial planets, as well as the Moon, were subjected to increased bombardment by small and large meteorites. The result of this bombardment and an earlier collision of planetesimals could be the release of volatiles and the beginning of the formation of a secondary atmosphere, since the primary, consisting of gases captured during the formation of the Earth, most likely quickly dissipated into outer space. A little later, the hydrosphere began to form. The atmosphere and hydrosphere formed in this way were replenished in the process of degassing of the mantle during volcanic activity.
The fall of large meteorites created vast and deep craters, similar to those currently observed on the Moon, Mars, Mercury, where their traces have not been erased by subsequent changes. Cratering could provoke magma outpourings with the formation of basalt fields similar to those covering the lunar "seas". Thus, the primary crust of the Earth was probably formed, which, however, has not been preserved on its modern surface, with the exception of relatively small fragments in the “younger” crust of the continental type.
This crust, containing in its composition already granites and gneisses, however, with a lower content of silica and potassium than in "normal" granites, appeared at the turn of about 3.8 billion years and is known to us from outcrops within the crystalline shields of almost all continents. The method of formation of the oldest continental crust is still largely unclear. This crust, metamorphosed everywhere under conditions of high temperatures and pressures, contains rocks whose textural features indicate accumulation in the aquatic environment, i.e. in this distant epoch the hydrosphere already existed. The appearance of the first crust, similar to the modern one, required the supply of large amounts of silica, aluminum, and alkalis from the mantle, while now mantle magmatism creates a very limited volume of rocks enriched in these elements. It is believed that 3.5 billion years ago, gray-gneiss crust, named after the predominant type of its constituent rocks, was widespread on the area of modern continents. In our country, for example, it is known on the Kola Peninsula and in Siberia, in particular in the basin of the river. Aldan.
Principles of periodization of the geological history of the Earth
Further events in geologic time are often determined according to relative geochronology, categories "old", "younger". For example, some era is older than some other. Separate segments of geological history are called (in decreasing order of their duration) zones, eras, periods, epochs, centuries. Their identification is based on the fact that geological events are imprinted in rocks, and sedimentary and volcanogenic rocks are located in layers in the earth's crust. In 1669, N. Stenoy established the law of stratification sequence, according to which the underlying layers of sedimentary rocks are older than the overlying ones, i.e. formed before them. Thanks to this, it became possible to determine the relative sequence of the formation of layers, and hence the geological events associated with them.
The main method in relative geochronology is the biostratigraphic, or paleontological, method of establishing the relative age and sequence of the occurrence of rocks. This method was proposed by W. Smith at the beginning of the 19th century, and then developed by J. Cuvier and A. Brongniard. The fact is that in most sedimentary rocks one can find the remains of animal or plant organisms. J.B. Lamarck and C. Darwin established that animals and plant organisms in the course of geological history gradually improved in the struggle for existence, adapting to changing living conditions. Some animal and plant organisms died out at certain stages of the development of the Earth, they were replaced by others, more perfect ones. Thus, according to the remains of earlier living more primitive ancestors found in some layer, one can judge the relatively older age of this layer.
Another method of geochronological separation of rocks, especially important for the separation of igneous formations of the ocean floor, is based on the property of the magnetic susceptibility of rocks and minerals formed in the Earth's magnetic field. With a change in the orientation of the rock relative to the magnetic field or the field itself, part of the "inherent" magnetization is retained, and the change in polarity is imprinted in a change in the orientation of the remanent magnetization of the rocks. Currently, a scale for the change of such epochs has been established.
Absolute geochronology - the doctrine of the measurement of geological time, expressed in ordinary absolute astronomical units(years), - determines the time of occurrence, completion and duration of all geological events, primarily the time of formation or transformation (metamorphism) of rocks and minerals, since the age of geological events is determined by their age. The main method here is the analysis of the ratio of radioactive substances and their decay products in rocks formed in different eras.
The oldest rocks are currently established in West Greenland (3.8 billion years). The oldest age (4.1 - 4.2 Ga) was obtained from zircons from Western Australia, but the zircon here occurs in a redeposited state in Mesozoic sandstones. Taking into account the concept of the simultaneity of the formation of all the planets of the solar system and the moon and the age of the most ancient meteorites (4.5-4.6 billion years) and ancient lunar rocks (4.0-4.5 billion years), the age of the Earth is assumed to be 4.6 billion years.
In 1881, at the II International Geological Congress in Bologna (Italy), the main divisions of the combined stratigraphic (for separating layered sedimentary rocks) and geochronological scales were approved. According to this scale, the history of the Earth was divided into four eras in accordance with the stages of development of the organic world: 1) Archean, or Archeozoic - the era of ancient life; 2) Paleozoic - the era of ancient life; 3) Mesozoic - the era of middle life; 4) Cenozoic - the era of new life. In 1887, the Proterozoic, the era of primary life, was singled out from the Archean era. Later the scale was improved. One of the variants of the modern geochronological scale is presented in Table. 8.1. The Archean era is divided into two parts: early (older than 3500 Ma) and late Archean; Proterozoic - also into two: early and late Proterozoic; in the latter, the Riphean (the name comes from the ancient name of the Ural Mountains) and Vendian periods are distinguished. The Phanerozoic zone is subdivided into the Paleozoic, Mesozoic and Cenozoic eras and consists of 12 periods.
Table 8.1. Geological scale
|
Age (beginning) |
|||
|
Phanerozoic |
Cenozoic |
Quaternary | |
|
Neogene | |||
|
Paleogene | |||
|
Mesozoic | |||
|
Triassic | |||
|
Paleozoic |
Permian | ||
|
Coal | |||
|
Devonian | |||
|
Silurian | |||
|
Ordovician | |||
|
Cambrian | |||
|
Cryptozoic |
Proterozoic |
Vendian | |
|
Riphean | |||
|
Karelian | |||
|
Archean | |||
|
Catharhean |
The main stages of the evolution of the earth's crust
Let us briefly consider the main stages in the evolution of the earth's crust as an inert substrate, on which the diversity of the surrounding nature has developed.
ATapxee The still rather thin and plastic crust, under the influence of extension, experienced numerous discontinuities, through which basaltic magma again rushed to the surface, filling troughs hundreds of kilometers long and many tens of kilometers wide, known as greenstone belts (they owe this name to the prevailing greenschist low-temperature metamorphism of basalt breeds). Along with basalts, among the lavas of the lower, most thick part of the section of these belts, there are high-magnesian lavas, indicating a very high degree of partial melting of the mantle substance, which indicates a high heat flow, much higher than the modern one. The development of greenstone belts consisted in a change in the type of volcanism towards an increase in the content of silicon dioxide (SiO 2 ) in it, in compressional deformations and metamorphism of sedimentary-volcanogenic fulfillment, and, finally, in the accumulation of clastic sediments, indicating the formation of a mountainous relief.
After the change of several generations of greenstone belts, the Archean stage of the evolution of the earth's crust ended 3.0 -2.5 billion years ago with the massive formation of normal granites with a predominance of K 2 O over Na 2 O. Granitization, as well as regional metamorphism, which in some places reached the highest stage, led to the formation of a mature continental crust over most of the area of modern continents. However, this crust turned out to be insufficiently stable: at the beginning of the Proterozoic era, it experienced crushing. At this time, a planetary network of faults and cracks arose, filled with dikes (plate-like geological bodies). One of them, the Great Dike in Zimbabwe, is over 500 km long and up to 10 km wide. In addition, rifting appeared for the first time, giving rise to zones of subsidence, powerful sedimentation and volcanism. Their evolution led to the creation at the end early Proterozoic(2.0-1.7 billion years ago) of folded systems that re-soldered the fragments of the Archean continental crust, which was facilitated by a new era of powerful granite formation.
As a result, by the end of the Early Proterozoic (by the turn of 1.7 billion years ago), a mature continental crust already existed on 60-80% of the area of its modern distribution. Moreover, some scientists believe that at this boundary the entire continental crust formed a single massif - the supercontinent Megagea (large land), which on the other side of the globe was opposed by the ocean - the predecessor of the modern Pacific Ocean - Megathalassa (large sea). This ocean was less deep than modern oceans, because the growth of the volume of the hydrosphere due to degassing of the mantle in the process of volcanic activity continues throughout the subsequent history of the Earth, although more slowly. It is possible that the prototype of Megathalassa appeared even earlier, at the end of the Archean.
In the Catarchean and the beginning of the Archean, the first traces of life appeared - bacteria and algae, and in the Late Archean, algal calcareous structures - stromatolites - spread. In the Late Archean, a radical change in the composition of the atmosphere began, and in the Early Proterozoic, a radical change in the composition of the atmosphere began: under the influence of plant life, free oxygen appeared in it, while the Catharchean and Early Archean atmosphere consisted of water vapor, CO 2 , CO, CH 4 , N, NH 3 and H 2 S with an admixture of HC1, HF and inert gases.
In the Late Proterozoic(1.7-0.6 billion years ago) Megagea began to gradually split, and this process sharply intensified at the end of the Proterozoic. Its traces are extended continental rift systems buried at the base of the sedimentary cover of ancient platforms. Its most important result was the formation of vast intercontinental mobile belts - the North Atlantic, Mediterranean, Ural-Okhotsk, which divided the continents of North America, Eastern Europe, East Asia and the largest fragment of Megagea - the southern supercontinent Gondwana. The central parts of these belts developed on the oceanic crust newly formed during rifting, i.e. the belts were ocean basins. Their depth gradually increased as the hydrosphere grew. At the same time, mobile belts developed along the periphery of the Pacific Ocean, the depth of which also increased. Climatic conditions became more contrasting, as evidenced by the appearance, especially at the end of the Proterozoic, of glacial deposits (tillites, ancient moraines and water-glacial sediments).
Paleozoic stage The evolution of the earth's crust was characterized by the intensive development of mobile belts - intercontinental and marginal continental (the latter on the periphery of the Pacific Ocean). These belts were divided into marginal seas and island arcs, their sedimentary-volcanogenic strata experienced complex fold-thrust, and then normal-shear deformations, granites were introduced into them and on this basis folded mountain systems were formed. This process proceeded unevenly. It distinguishes a number of intense tectonic epochs and granitic magmatism: Baikal - at the very end of the Proterozoic, Salair (from the Salair ridge in Central Siberia) - at the end of the Cambrian, Takov (from the Takov mountains in the east of the USA) - at the end of the Ordovician, Caledonian ( from the ancient Roman name of Scotland) - at the end of the Silurian, Acadian (Acadia - the ancient name of the northeastern states of the USA) - in the middle of the Devonian, Sudeten - at the end of the Early Carboniferous, Saal (from the Saale River in Germany) - in the middle of the early Permian. The first three tectonic epochs of the Paleozoic are often combined into the Caledonian era of tectogenesis, the last three into the Hercynian or Varisian. In each of the listed tectonic epochs, certain parts of the mobile belts turned into folded mountain structures, and after destruction (denudation) they were part of the foundation of young platforms. But some of them partially experienced activation in subsequent epochs of mountain building.
By the end of the Paleozoic, the intercontinental mobile belts were completely closed and filled with folded systems. As a result of the withering away of the North Atlantic belt, the North American continent closed with the East European, and the latter (after the completion of the development of the Ural-Okhotsk belt) - with the Siberian, Siberian - with the Chinese-Korean. As a result, the supercontinent Laurasia was formed, and the dying off of the western part of the Mediterranean belt led to its unification with the southern supercontinent - Gondwana - into one continental block - Pangea. The eastern part of the Mediterranean belt at the end of the Paleozoic - the beginning of the Mesozoic turned into a huge bay of the Pacific Ocean, along the periphery of which folded mountain structures also rose.
Against the background of these changes in the structure and relief of the Earth, the development of life continued. The first animals appeared as early as the late Proterozoic, and at the very dawn of the Phanerozoic, almost all types of invertebrates existed, but they still lacked the shells or shells that have been known since the Cambrian. In the Silurian (or already in the Ordovician), vegetation began to land on land, and at the end of the Devonian there were forests that became most widespread in the Carboniferous period. Fish appeared in the Silurian, amphibians in the Carboniferous.
Mesozoic and Cenozoic eras - the last major stage in the development of the structure of the earth's crust, which is marked by the formation of modern oceans and the isolation of modern continents. At the beginning of the stage, in the Triassic, Pangea still existed, but already in the early Jurassic, it again split into Laurasia and Gondwana due to the emergence of the latitudinal Tethys ocean, stretching from Central America to Indochina and Indonesia, and in the west and east it merged with the Pacific Ocean (Fig. 8.6); this ocean also included the Central Atlantic. From here, at the end of the Jurassic, the process of moving apart the continents spread to the north, creating the North Atlantic during the Cretaceous period and the early Paleogene, and starting from the Paleogene, the Eurasian basin of the Arctic Ocean (the Amerasian basin arose earlier as part of the Pacific Ocean). As a result, North America separated from Eurasia. In the Late Jurassic, the formation of the Indian Ocean began, and from the beginning of the Cretaceous, the South Atlantic began to open up from the south. This meant the beginning of the disintegration of Gondwana, which existed as a whole throughout the Paleozoic. At the end of the Cretaceous, the North Atlantic joined the South, separating Africa from South America. At the same time, Australia separated from Antarctica, and at the end of the Paleogene, the latter separated from South America.
Thus, by the end of the Paleogene, all modern oceans took shape, all modern continents became isolated, and the appearance of the Earth acquired a form that was basically close to the present. However, there were no modern mountain systems yet.
From the Late Paleogene (40 million years ago), intensive mountain building began, culminating in the last 5 million years. This stage of the formation of young fold-cover mountain structures, the formation of revived arch-block mountains is distinguished as neotectonic. In fact, the neotectonic stage is a sub-stage of the Mesozoic-Cenozoic stage of the Earth's development, since it was at this stage that the main features of the modern Earth relief took shape, starting with the distribution of oceans and continents.
At this stage, the formation of the main features of modern fauna and flora was completed. The Mesozoic era was the era of reptiles, mammals began to predominate in the Cenozoic, and man appeared in the late Pliocene. At the end of the Early Cretaceous, angiosperms appeared and the land acquired grass cover. At the end of the Neogene and Anthropogene, the high latitudes of both hemispheres were covered by a powerful continental glaciation, the relics of which are the ice caps of Antarctica and Greenland. This was the third major glaciation in the Phanerozoic: the first took place in the late Ordovician, the second - at the end of the Carboniferous - the beginning of the Permian; both were common within Gondwana.
QUESTIONS FOR SELF-CHECKING
What are spheroid, ellipsoid and geoid? What are the parameters of the ellipsoid adopted in our country? Why is it needed?
What is the internal structure of the Earth? On the basis of what is the conclusion about its structure made?
What are the main physical parameters of the Earth and how do they change with depth?
What is the chemical and mineralogical composition of the Earth? On what basis is a conclusion made about the chemical composition of the entire Earth and the earth's crust?
What are the main types of the earth's crust are currently distinguished?
What is the hydrosphere? What is the water cycle in nature? What are the main processes occurring in the hydrosphere and its elements?
What is atmosphere? What is its structure? What processes take place within it? What is weather and climate?
Define endogenous processes. What endogenous processes do you know? Briefly describe them.
What is the essence of lithospheric plate tectonics? What are its main provisions?
10. Define exogenous processes. What is the main essence of these processes? What endogenous processes do you know? Briefly describe them.
11. How do endogenous and exogenous processes interact? What are the results of the interaction of these processes? What is the essence of the theories of V. Davis and V. Penk?
What are the current ideas about the origin of the Earth? How was its early formation as a planet?
On the basis of what is the periodization of the geological history of the Earth?
14. How did the earth's crust develop in the geological past of the Earth? What are the main stages in the development of the earth's crust?
LITERATURE
Allison A, Palmer D. Geology. The science of the ever-changing Earth. M., 1984.
Budyko M.I. Climate past and future. L., 1980.
Vernadsky V.I. Scientific thought as a planetary phenomenon. M., 1991.
Gavrilov V.P. Journey into the past of the Earth. M., 1987.
Geological dictionary. T. 1, 2. M., 1978.
GorodnitskyA. M., Zonenshain L.P., Mirlin E.G. Reconstruction of the position of the continents in the Phanerozoic. M., 1978.
7. Davydov L.K., Dmitrieva A.A., Konkina N.G. General hydrology. L., 1973.
Dynamic Geomorphology / Ed. G.S. Anan'eva, Yu.G. Simonova, A.I. Spiridonov. M., 1992.
Davis W.M. Geomorphological essays. M., 1962.
10. Earth. Introduction to general geology. M., 1974.
11. Climatology / Ed. O.A. Drozdova, N.V. Kobysheva. L., 1989.
Koronovsky N.V., Yakusheva A.F. Fundamentals of Geology. M., 1991.
Leontiev O.K., Rychagov G.I. General geomorphology. M., 1988.
Lvovich M.I. Water and life. M., 1986.
Makkaveev N.I., Chalov R.C. channel processes. M., 1986.
Mikhailov V.N., Dobrovolsky A.D. General hydrology. M., 1991.
Monin A.S. Introduction to the theory of climate. L., 1982.
Monin A.S. History of the Earth. M., 1977.
Neklyukova N.P., Dushina I.V., Rakovskaya E.M. and etc. Geography. M., 2001.
Nemkov G.I. and etc. Historical geology. M., 1974.
Restless landscape. M., 1981.
General and field geology / Ed. A.N. Pavlova. L., 1991.
Penk V. Morphological analysis. M., 1961.
Perelman A.I. Geochemistry. M., 1989.
Poltaraus B.V., Kisloe A.V. Climatology. M., 1986.
26. Problems of Theoretical Geomorphology / Ed. L.G. Nikiforova, Yu.G. Simonov. M., 1999.
Saukov A.A. Geochemistry. M., 1977.
Sorokhtin O.G., Ushakov S.A. Global evolution of the Earth. M., 1991.
Ushakov S.A., Yasamanov H.A. Continental drift and the Earth's climate. M., 1984.
Khain V.E., Lomte M.G. Geotectonics with the basics of geodynamics. M., 1995.
Khain V.E., Ryabukhin A.G. History and methodology of geological sciences. M., 1997.
Khromov S.P., Petrosyants M.A. Meteorology and climatology. M., 1994.
Schukin I.S. General geomorphology. T.I. M., 1960.
Ecological functions of the lithosphere / Ed. V.T. Trofimov. M., 2000.
Yakusheva A.F., Khain V.E., Slavin V.I. General geology. M., 1988.
Geological time and methods for its determination
In the study of the Earth as a unique cosmic object, the idea of its evolution occupies a central place, therefore an important quantitative evolutionary parameter is geological time. The study of this time is engaged in a special science called Geochronology- geological reckoning. Geochronology may be absolute and relative.
Remark 1
Absolute geochronology deals with the determination of the absolute age of rocks, which is expressed in units of time and, as a rule, in millions of years.
The determination of this age is based on the rate of decay of isotopes of radioactive elements. This speed is a constant value and does not depend on the intensity of physical and chemical processes. Age determination is based on nuclear physics methods. Minerals containing radioactive elements, during the formation of crystal lattices, form a closed system. In this system, the accumulation of radioactive decay products occurs. As a result, the age of the mineral can be determined if the rate of this process is known. The half-life of radium, for example, is $1590$ years, and the complete decay of the element will occur in $10$ times the half-life. Nuclear geochronology has its leading methods − lead, potassium-argon, rubidium-strontium and radiocarbon.
Methods of nuclear geochronology made it possible to determine the age of the planet, as well as the duration of eras and periods. Radiological time measurement proposed P. Curie and E. Rutherford at the beginning of the $XX$ century.
Relative geochronology operates with such concepts as "early age, middle, late". There are several developed methods for determining the relative age of rocks. They fall into two groups - paleontological and non-paleontological.
First play a major role due to their versatility and ubiquity. The exception is the absence of organic remains in the rocks. With the help of paleontological methods, the remains of ancient extinct organisms are studied. Each rock layer has its own complex of organic remains. In each young layer there will be more remains of highly organized plants and animals. The higher the layer lies, the younger it is. A similar pattern was established by the Englishman W. Smith. He owns the first geological map of England, on which the rocks were divided by age.
Non-paleontological methods determinations of the relative age of rocks are used in cases where there are no organic remains in them. More efficient then will be stratigraphic, lithological, tectonic, geophysical methods. With the help of the stratigraphic method, it is possible to determine the sequence of stratification of layers in their normal occurrence, i.e. the underlying layers will be older.
Remark 3
The sequence of formation of rocks determines relative geochronology, and their age in units of time determines already absolute geochronology. A task geological time is to determine the chronological sequence of geological events.
Geological table
To determine the age of rocks and their study, scientists use various methods, and for this purpose a special scale has been compiled. Geological time on this scale is divided into time periods, each of which corresponds to a certain stage in the formation of the earth's crust and the development of living organisms. The scale is called geochronological table, which includes the following divisions: eon, era, period, epoch, century, time. Each geochronological unit is characterized by its own set of deposits, which is called stratigraphic: eonoteme, group, system, department, tier, zone. A group, for example, is a stratigraphic unit, and the corresponding temporal geochronological unit is era. Based on this, there are two scales - stratigraphic and geochronological. The first scale is used when it comes to deposits, because in any period of time some geological events took place on the Earth. The second scale is needed to determine relative time. Since the adoption of the scale, the content of the scale has been changed and refined.
The largest stratigraphic units at present are eonotemes - Archean, Proterozoic, Phanerozoic. In the geochronological scale, they correspond to zones of different duration. According to the time of existence on Earth, they are distinguished Archean and Proterozoic eonotemes covering nearly $80$% of the time. Phanerozoic eon in time is much less than the previous eon and covers only $ 570 $ million years. This ionoteme is divided into three main groups - Paleozoic, Mesozoic, Cenozoic.
The names of eonotems and groups are of Greek origin:
- Archeos means ancient;
- Proteros - primary;
- Paleos - ancient;
- Mezos - medium;
- Cainos is new.
From the word " zoiko s”, which means vital, the word “ zoi". Based on this, the eras of life on the planet are distinguished, for example, the Mesozoic era means the era of average life.
Eras and periods
According to the geochronological table, the history of the Earth is divided into five geological eras: Archean, Proterozoic, Paleozoic, Mesozoic, Cenozoic. The eras are further subdivided into periods. There are much more of them - $12$. The duration of the periods varies from $20$-$100$ million years. The last one points to its incompleteness. Quaternary period of the Cenozoic era, its duration is only $1.8 million years.
Archean era. This time began after the formation of the earth's crust on the planet. By this time there were mountains on the Earth and the processes of erosion and sedimentation had come into play. The Archean lasted for approximately $2 billion years. This era is the longest in duration, during which volcanic activity was widespread on Earth, there were deep uplifts, which resulted in the formation of mountains. Most of the fossils were destroyed under the influence of high temperature, pressure, mass movement, but little data about that time was preserved. In the rocks of the Archean era, pure carbon is found in dispersed form. Scientists believe that these are altered remains of animals and plants. If the amount of graphite reflects the amount of living matter, then there was a lot of it in the Archaean.
Proterozoic era. In terms of duration, this is the second era, spanning $1 billion years. During the era, there was the deposition of a large amount of precipitation and one significant glaciation. Ice sheets extended from the equator to $20$ degrees of latitude. Fossils found in the rocks of this time are evidence of the existence of life and its evolutionary development. Spicules of sponges, remains of jellyfish, fungi, algae, arthropods, etc. have been found in the Proterozoic deposits.
Palaeozoic. This era stands out six periods:
- Cambrian;
- Ordovician,
- Silur;
- Devonian;
- Carbon or coal;
- Perm or Perm.
The duration of the Paleozoic is $370$ million years. During this time, representatives of all types and classes of animals appeared. Only birds and mammals were missing.
Mesozoic era. The era is divided into three period:
- Triassic;
The era started about $230 million years ago and lasted $167 million years. During the first two periods Triassic and Jurassic- most of the continental regions rose above sea level. The climate of the Triassic is dry and warm, and in the Jurassic it became even warmer, but was already humid. In state Arizona there is a famous stone forest that has existed since Triassic period. True, only trunks, logs and stumps remained from the once mighty trees. At the end of the Mesozoic era, or rather in the Cretaceous period, a gradual advance of the sea takes place on the continents. The North American continent experienced a subsidence at the end of the Cretaceous and, as a result, the waters of the Gulf of Mexico joined with the waters of the Arctic basin. The mainland was divided into two parts. The end of the Cretaceous period is characterized by a large uplift, called Alpine orogeny. At this time, the Rocky Mountains, the Alps, the Himalayas, the Andes appeared. In the west of North America, intense volcanic activity began.
Cenozoic era. This is a new era that has not yet ended and continues at the present time.
The era was divided into three periods:
- Paleogene;
- Neogene;
- Quaternary.
Quaternary period has a number of unique features. This is the time of the final formation of the modern face of the Earth and ice ages. New Guinea and Australia became independent, moving closer to Asia. Antarctica has remained in its place. Two Americas united. Of the three periods of the era, the most interesting is quaternary period or anthropogenic. It continues today, and was allocated in $1829$ by a Belgian geologist J. Denoyer. Coolings are replaced by warmings, but its most important feature is appearance of man.
Modern man lives in the Quaternary period of the Cenozoic era.
Geological chronology, or geochronology, is based on elucidating the geological history of the most well-studied regions, for example, in Central and Eastern Europe. Based on broad generalizations, comparison of the geological history of various regions of the Earth, patterns of evolution of the organic world at the end of the last century, at the first International Geological Congresses, the International Geochronological Scale was developed and adopted, reflecting the sequence of time divisions during which certain sediment complexes were formed, and the evolution of the organic world . Thus, the international geochronological scale is a natural periodization of the history of the Earth.
Among the geochronological divisions are distinguished: eon, era, period, epoch, century, time. Each geochronological subdivision corresponds to a set of deposits, identified in accordance with the change in the organic world and called stratigraphic: eonoteme, group, system, department, stage, zone. Therefore, the group is a stratigraphic unit, and the corresponding temporal geochronological unit is represented by an era. Therefore, there are two scales: geochronological and stratigraphic. The first is used when talking about relative time in the history of the Earth, and the second when dealing with sediments, since some geological events occurred in every place on the globe in any period of time. Another thing is that the accumulation of precipitation was not ubiquitous.
- The Archean and Proterozoic eonotemes, covering almost 80% of the time of the Earth's existence, are distinguished in the Cryptozoic, since the skeletal fauna is completely absent in the Precambrian formations and the paleontological method is not applicable to their division. Therefore, the division of Precambrian formations is based primarily on general geological and radiometric data.
- The Phanerozoic eon covers only 570 million years, and the division of the corresponding eonoteme of deposits is based on a wide variety of numerous skeletal fauna. The Phanerozoic eonoteme is subdivided into three groups: Paleozoic, Mesozoic and Cenozoic, corresponding to major stages in the natural geological history of the Earth, the boundaries of which are marked by rather abrupt changes in the organic world.
The names of eonotems and groups come from Greek words:
- "archeos" - the most ancient, most ancient;
- "proteros" - primary;
- "paleos" - ancient;
- "mesos" - medium;
- "kainos" - new.
The word "cryptos" means hidden, and "phanerozoic" means explicit, transparent, since the skeletal fauna appeared.
The word "zoi" comes from "zoikos" - life. Therefore, "Cenozoic era" means the era of new life, and so on.
Groups are subdivided into systems, the deposits of which were formed during one period and are characterized only by families or genera of organisms characteristic of them, and if these are plants, then by genera and species. Systems have been identified in different regions and at different times since 1822. At present, 12 systems are distinguished, the names of most of which come from the places where they were first described. For example, the Jurassic system - from the Jura Mountains in Switzerland, the Permian - from the Perm province in Russia, the Cretaceous - according to the most characteristic rocks - white writing chalk, etc. The Quaternary system is often called Anthropogenic, since it is in this age interval that a person appears.
The systems are subdivided into two or three divisions, which correspond to the early, middle, and late eras. The departments, in turn, are divided into tiers, which are characterized by the presence of certain genera and species of fossil fauna. And, finally, the stages are subdivided into zones, which are the most fractional part of the international stratigraphic scale, which corresponds to time in the geochronological scale. The names of the stages are usually given according to the geographical names of the regions where this stage was distinguished; for example, the Aldanian, Bashkirian, Maastrichtian stages, etc. At the same time, the zone is designated by the most characteristic type of fossil fauna. The zone covers, as a rule, only a certain part of the region and is developed over a smaller area than the deposits of the stage.
All subdivisions of the stratigraphic scale correspond to the geological sections in which these subdivisions were first identified. Therefore, such sections are reference, typical, and are called stratotypes, which contain only their own complex of organic remains, which determines the stratigraphic volume of a given stratotype. The determination of the relative age of any layers consists in comparing the discovered complex of organic remains in the studied layers with the complex of fossils in the stratotype of the corresponding division of the international geochronological scale, i.e. the age of the deposits is determined relative to the stratotype. That is why the paleontological method, despite its inherent shortcomings, remains the most important method for determining the geological age of rocks. Determining the relative age of, for example, the Devonian deposits only indicates that these deposits are younger than the Silurian, but older than the Carboniferous. However, it is impossible to establish the duration of the formation of Devonian deposits and give a conclusion about when (in absolute chronology) the accumulation of these deposits occurred. Only methods of absolute geochronology are able to answer this question.
Tab. 1. Geological table
| Era | Period | Epoch | Duration, Ma | Time from the beginning of the period to the present day, million years | Geological conditions | Vegetable world | Animal world |
| Cenozoic (time of mammals) | Quaternary | Modern | 0,011 | 0,011 | End of the last ice age. The climate is warm | The decline of woody forms, the flowering of herbaceous | Age of Man |
| Pleistocene | 1 | 1 | repeated glaciations. four ice ages | Extinction of many plant species | Extinction of large mammals. The origin of human society | ||
| Tertiary | Pliocene | 12 | 13 | The uplift of mountains in the west of North America continues. Volcanic activity | Decay of forests. Spread of meadows. flowering plants; development of monocots | The emergence of man from the great apes. Types of elephants, horses, camels, similar to modern | |
| Miocene | 13 | 25 | The Sierras and the Cascade Mountains formed. Volcanic activity in the northwestern United States. The climate is cool | The culminating period in the evolution of mammals. The first great apes | |||
| Oligocene | 11 | 30 | The continents are low. The climate is warm | Maximum distribution of forests. Strengthening the development of monocotyledonous flowering plants | Archaic mammals are dying out. The beginning of the development of anthropoids; ancestors of most extant genera of mammals | ||
| Eocene | 22 | 58 | The mountains are blurred. There are no inland seas. The climate is warm | Diverse and specialized placental mammals. Ungulates and carnivores flourish | |||
| Paleocene | 5 | 63 | Distribution of archaic mammals | ||||
| Alpine orogeny (minor destruction of fossils) | |||||||
| Mesozoic (time of reptiles) | Chalk | 72 | 135 | At the end of the period, the Andes, the Alps, the Himalayas, the Rocky Mountains are formed. Prior to this, inland seas and swamps. Deposition of writing chalk, shale | The first monocots. The first oak and maple forests. Decline of gymnosperms | Dinosaurs reach the highest development and die out. Toothed birds are dying out. Appearance of the first modern birds. Archaic mammals are common | |
| Yura | 46 | 181 | The continents are quite elevated. Shallow seas cover parts of Europe and the western United States | The value of dicots increases. Cycadophytes and conifers are common | The first toothed birds. Dinosaurs are large and specialized. Insectivorous marsupials | ||
| Triassic | 49 | 230 | Continents are elevated above sea level. Intensive development of arid climate conditions. Widespread continental deposits | The dominance of the gymnosperms, already beginning to decline. Extinction of seed ferns | The first dinosaurs, pterosaurs and egg-laying mammals. Extinction of primitive amphibians | ||
| Hercynian orogeny (some destruction of fossils) | |||||||
| Paleozoic (era of ancient life) | Permian | 50 | 280 | Continents are raised. Appalachian mountains formed. Dryness is getting worse. Glaciation in the southern hemisphere | Decline of club mosses and ferns | Many ancient animals are dying out. Animal reptiles and insects develop | |
| Upper and Middle Carboniferous | 40 | 320 | The continents are initially low-lying. Vast swamps in which coal was formed | Large forests of seed ferns and gymnosperms | The first reptiles. Insects are common. Distribution of ancient amphibians | ||
| Lower Carboniferous | 25 | 345 | The climate is initially warm and humid, later, due to the rise of the land, it becomes cooler. | Club mosses and fern-like plants dominate. Gymnosperms are spreading more and more | Sea lilies reach their highest development. Distribution of ancient sharks | ||
| Devonian | 60 | 405 | Inland seas are small. Land elevation; development of an arid climate. Glaciation | First forests. Land plants are well developed. First gymnosperms | The first amphibians. Abundance of lungfish and sharks | ||
| Silurus | 20 | 425 | Vast inland seas. Low-lying areas are getting drier as the land rises | The first reliable traces of land plants. Algae dominate | Marine arachnids dominate. The first (wingless) insects. Increased development of fish | ||
| Ordovician | 75 | 500 | Significant land sink. The climate is warm, even in the Arctic | Probably the first land plants appear. Abundance of seaweed | The first fish are probably freshwater. Abundance of corals and trilobites. Various clams | ||
| Cambrian | 100 | 600 | The continents are low, the climate is temperate. The most ancient rocks with abundant fossils | Seaweed | Trilobites and lechenopods dominate. The origin of most modern animal phyla | ||
| Second great orogeny (significant destruction of fossils) | |||||||
| Proterozoic | 1000 | 1600 | Intensive process of sedimentation. Later - volcanic activity. Erosion over large areas. Multiple glaciations | Primitive aquatic plants - algae, fungi | Various marine protozoa. By the end of the era - molluscs, worms and other marine invertebrates | ||
| First great mountain building (significant destruction of fossils) | |||||||
| archaeus | 2000 | 3600 | Significant volcanic activity. Weak sedimentation process. Erosion on large areas | Fossils are absent. Indirect evidence of the existence of living organisms in the form of deposits of organic matter in rocks | |||
The problem of determining the absolute age of rocks, the duration of the existence of the Earth has long occupied the minds of geologists, and attempts to solve it have been made many times, for which various phenomena and processes have been used. Early ideas about the absolute age of the Earth were curious. A contemporary of M. V. Lomonosov, the French naturalist Buffon determined the age of our planet at only 74,800 years. Other scientists gave different figures, not exceeding 400-500 million years. It should be noted here that all these attempts were doomed to failure in advance, since they proceeded from the constancy of the rates of processes, which, as is known, changed in the geological history of the Earth. And only in the first half of the XX century. there was a real opportunity to measure the truly absolute age of rocks, geological processes and the Earth as a planet.
| Tab.2. Isotopes used to determine absolute ages | ||
| parent isotope | Final product | Half-life, billion years |
| 147cm | 143 Nd+He | 106 |
| 238 U | 206 Pb+ 8 He | 4,46 |
| 235 U | 208 Pb+ 7 He | 0,70 |
| 232Th | 208 Pb+ 6 He | 14,00 |
| 87Rb | 87 Sr+β | 48,80 |
| 40K | 40 Ar+ 40 Ca | 1,30 |
| 14C | 14 N | 5730 years |
The notion of how life originated in the ancient eras of the Earth give us the fossil remains of organisms, but they are distributed in separate geological periods extremely uneven.
Geological periods
The era of the ancient life of the Earth includes 3 stages of the evolution of flora and fauna.
Archean era
Archean era- the oldest era in the history of existence. Its beginning takes a count of about 4 billion years ago. And the duration is 1 billion years. This is the beginning of the formation of the earth's crust as a result of the activity of volcanoes and air masses, sharp changes in temperature and pressure. There is a process of destruction of the primary mountains and the formation of sedimentary rocks.
The most ancient Archeozoic layers of the earth's crust are represented by highly altered, otherwise metamorphosed rocks, and therefore they do not contain noticeable remains of organisms.
But on this basis it is absolutely wrong to consider the archaeozoic a lifeless era: in the archaeozoic there were not only bacteria and algae, but also more complex organisms.
Proterozoic era
The first reliable traces of life in the form of extremely rare finds and poor quality preservation are found in Proterozoic, otherwise - the era of "primary life". The duration of the Proterozoic era is about 2 million years
Traces of crawling found in Proterozoic rocks annelids, sponge needles, shells of the simplest forms of brachiopods, arthropod remains.
Brachiopods, distinguished by an exceptional variety of forms, were widespread in the most ancient seas. They are found in the deposits of many periods, especially the next, the Paleozoic era.
Shell of the brachiopod "Horistites Moskmenzis" (ventral valve)
Only certain species of brachiopods have survived to this day. Most of the brachiopods had a shell with unequal valves: the ventral one, on which they lie or are attached to the seabed with the help of a "leg", was usually larger than the dorsal one. On this basis, in general, it is not difficult to recognize brachiopods.
An insignificant amount of fossil remains in the Proterozoic deposits is explained by the destruction of most of them as a result of a change (metamorphization) of the containing rock.
To judge how much life was represented in the Proterozoic, deposits help limestone, which then turned into marble. Limestones obviously owe their origin to a special type of bacteria that secreted carbonic lime.
The presence of interlayers in the Proterozoic deposits of Karelia shungite, similar to anthracite coal, suggests that the initial material for its formation was the accumulation of algae and other organic residues.
In this distant time, the most ancient dry land was still not lifeless. In the vast expanses of still desert primary continents, bacteria settled. With the participation of these simple organisms, the weathering and loosening of the rocks that made up the most ancient earth's crust took place.
According to the Russian academician L. S. Berga(1876-1950), who studied how life originated in the ancient eras of the Earth, at that time soils had already begun to form - the basis for the further development of vegetation cover.
Palaeozoic
Deposits next in time, Paleozoic era, otherwise, the era of "ancient life", which began about 600 million years ago, differs sharply from the Proterozoic in the abundance and variety of forms even in the most ancient, Cambrian period.
Based on the study of the remains of organisms, it is possible to restore the following picture of the development of the organic world, characteristic of this era.
There are six periods of the Paleozoic era:
Cambrian period
Cambrian period was described for the first time in England, the county of Cambria, from where its name came from. During this period, all life was connected with water. These are red and blue-green algae, limestone algae. Algae released free oxygen, which made it possible for the development of organisms that consume it.
Careful study of blue-green Cambrian clays, which are clearly visible in the deep sections of the river valleys near St. Petersburg and especially in the coastal regions of Estonia, made it possible to establish in them (through a microscope) the presence plant spores.
This definitely suggests that some species that have existed in water since the earliest times of the development of life on our planet moved to land about 500 million years ago.
Among the organisms that inhabited the oldest Cambrian reservoirs, invertebrates were exceptionally widespread. Of the invertebrates, except for the smallest protozoa - rhizopods, were widely represented worms, brachiopods and arthropods.
Of the arthropods, these are primarily various insects, especially butterflies, beetles, flies, dragonflies. They appear much later. To the same type of animal world, in addition to insects, also belong arachnids and centipedes.
Among the most ancient arthropods, there were especially many trilobites, similar to modern wood lice, only much larger than them (up to 70 centimeters), and crustaceans, which sometimes reached impressive sizes.
Trilobites - representatives of the animal world of the most ancient seas In the body of a trilobite, three lobes are clearly distinguished, it is not for nothing that it is called so: in translation from the ancient Greek “trilobos” - three-lobed. Trilobites not only crawled along the bottom and burrowed into the silt, but could also swim.
Among the trilobites, generally medium-sized forms prevailed.
By definition of geologists, trilobites - "guiding fossils" - are characteristic of many deposits of the Paleozoic.
Fossils that prevail at a given geological time are called guiding fossils. From guide fossils, the age of the deposits in which they are found is usually easily determined. Trilobites reached their peak during the Ordovician and Silurian periods. They disappeared at the end of the Paleozoic era.
Ordovician period
Ordovician period characterized by a warmer and milder climate, as evidenced by the presence of limestone, shale and sandstone in the rock deposits. At this time, the area of the seas increases significantly.
This promotes the reproduction of large trilobites, from 50 to 70 cm long. Appear in the seas sea sponges, clams, and the first corals.
First corals Silurian
What did the Earth look like? Silurian? What changes have taken place on the primeval continents? Judging by the imprints on clay and other stone material, one can definitely say that at the end of the period, the first terrestrial vegetation appeared on the shores of water bodies.
The first plants of the Silurian period
These were small leafy plants, resembling rather sea brown algae, having neither roots nor leaves. The role of the leaves was played by green successively branching stems.
Psilophyte plants - naked plants The scientific name of these ancient progenitors of all terrestrial plants (psilophytes, otherwise - "naked plants", that is, plants without leaves) well conveys their distinctive features. (Translated from the ancient Greek "psilos" - bald, naked, and "phytos" - the trunk). Their roots were also undeveloped. Psilophytes grew on swampy marshy soils. An imprint in the rock (right) and a restored plant (left).
The inhabitants of the reservoirs of the Silurian period
From inhabitants maritime Silurian reservoirs It should be noted, apart from trilobites, corals and echinoderms - sea lilies, sea urchins and stars.
Sea lily "Acanthocrinus rex" Sea lilies, the remains of which were found in sediments, looked very little like predatory animals. Sea lily "Acanthocrinus-rex" means "spiny lily-king" in translation. The first word is formed from two Greek words: "acantha" - a prickly plant and "krinon" - a lily, the second Latin word "rex" - a king.
A huge number of species were represented by cephalopods and especially brachiopods. In addition to cephalopods, which had an inner shell, like belemnites, cephalopods with an external shell were widely used in the most ancient periods of the life of the Earth.
The shape of the shell was straight and curved in a spiral. The shell was successively divided into chambers. The body of the mollusk was placed in the largest outer chamber, the rest were filled with gas. A tube passed through the chambers - a siphon, which allowed the mollusk to regulate the amount of gas and, depending on this, float or sink to the bottom of the reservoir.
At present, of such cephalopods, only one ship with a coiled shell has been preserved. ship, or nautilus, which is the same thing, translated from Latin - an inhabitant of the warm sea.
The shells of some Silurian cephalopods, such as orthoceras (translated from the ancient Greek “straight horn”: from the words “orthoe” - straight and “keras” - horn), reached gigantic sizes and looked more like a straight two-meter pillar than a horn.
Limestones in which orthoceratites occur are called orthoceratite limestones. Square limestone slabs were widely used in pre-revolutionary St. Petersburg for sidewalks, and characteristic cuts of orthoceratite shells were often clearly visible on them.
A remarkable event of the Silurian time was the appearance in fresh and brackish water bodies of clumsy " armored fish”, which had an external bone shell and an unossified internal skeleton.
Their spinal column was answered by a cartilaginous cord - a chord. The shells did not have jaws and paired fins. They were poor swimmers and therefore stuck more to the bottom; their food was silt and small organisms.
Panther fish pterichthys The armored fish pterichthys was generally a poor swimmer and led a natural lifestyle.
It can be assumed that bothriolepis was already much more mobile than pterychthys.
Sea predators of the Silurian period
In later deposits, there are already remains marine predators close to sharks. Of these lower fish, which also had a cartilaginous skeleton, only teeth were preserved. Judging by the size of the teeth, for example, from the deposits of the Carboniferous age of the Moscow region, it can be concluded that these predators reached considerable sizes.
In the development of the animal world of our planet, the Silurian period is interesting not only because distant ancestors of fish appear in its reservoirs. At the same time, another equally important event took place: representatives of arachnids got out of the water onto land, among them ancient scorpions, still very close to shell scorpions.
Rakoscorpion inhabitants of shallow seas On the right, above, a predator armed with strange claws - pterygotus, reaching 3 meters, glory - eurypterus - up to 1 meter long.
Devonian
The land - the arena of the future life - gradually takes on new features, especially characteristic of the next, Devonian period. At this time, already woody vegetation appears, first in the form of low-growing shrubs and small trees, and then larger ones. Among the Devonian vegetation, we will meet well-known ferns, other plants will remind us of an elegant horsetail tree and green cords of club mosses, but not creeping along the ground, but proudly rising up.
Fern-like plants also appear in later Devonian deposits, which reproduced not by spores, but by seeds. These are seed ferns, occupying a transitional position between spore and seed plants.
Fauna of the Devonian period
Animal world seas Devonian period rich in brachiopods, corals and sea lilies; trilobites begin to play a secondary role.
Among the cephalopods, new forms appear, only not with a straight shell, like in Orthoceras, but with a spirally twisted one. They are called ammonites. They got their name from the Egyptian sun god Ammon, near the ruins of whose temple in Libya (in Africa) these characteristic fossils were first discovered.
In general appearance, they are difficult to confuse with other fossils, but at the same time, it is necessary to warn young geologists about how difficult it is to identify individual types of ammonites, the total number of which is not hundreds, but thousands.
Ammonites reached a particularly magnificent flourishing in the next, Mesozoic era. .
Significant development in the Devonian time received fish. Armored fish have shortened their bony shells, making them more mobile.
Some armored fish, such as the nine-meter giant dinichthys, were terrible predators (in Greek, “deinos” is terrible, terrible, and “ichthys” is fish).
The nine-meter dinichthys obviously posed a great threat to the inhabitants of the reservoirs.
In the Devonian reservoirs, there were also lobe-finned fish, from which the lungfish originated. This name is explained by the structural features of the paired fins: they are narrow and, in addition, sit on an axis covered with scales. In this feature, the lobe-finned fish differ, for example, from pike perch, perch and other bony fish called ray-finned fish.
The lobe-finned ancestors of bony fish, which appeared much later - at the end of the Triassic.
We would not even have an idea of how the loaf-finned fish actually looked like, which lived at least 300 million years ago, if it were not for the successful catches of the rarest specimens of their modern generation off the coast of South Africa in the middle of the 20th century.
They live, obviously, at considerable depths, which is why they come across so rarely to fishermen. The caught species was named coelacanth. It reached 1.5 meters in length.
In their organization, lungfish are close to the cross-finned fish. They have lungs corresponding to the swim bladder of a fish.
In their organization, lungfish are close to the cross-finned fish. They have lungs corresponding to the swim bladder of a fish.
How unusual the crossopterygians looked can be judged by a specimen, a coelacanth, caught in 1952 off the Comoros, west of the island of Madagascar. This fish, 1.5 liters long, weighed about 50 kg.
A descendant of ancient lungfish - the Australian ceratodus (translated from ancient Greek - horned tooth) - reaches two meters. He lives in drying up reservoirs and, as long as there is water in them, he breathes with gills, like all fish, but when the reservoir begins to dry out, he switches to pulmonary respiration.
Australian ceratodus - a descendant of ancient lungfish Its respiratory organs are the swim bladder, which has a cellular structure and is equipped with numerous blood vessels. In addition to ceratodus, two more species of lungfish are now known. One of them lives in Africa, and the other - in South America.
Transition of vertebrates from water to land
Table of transformation of amphibians.
ancient fish 
The first picture shows the oldest cartilaginous fish, diplocanthus (1). Below it is a primitive crossopterygian eusthenopteron (2), a putative, transitional form (3) is shown below. In the huge amphibian eogyrinus (about 4.5 m long), the limbs are still very weak (4), and only as they master the land lifestyle do they become a reliable support, for example, for overweight eriops, about 1.5 m long (5).
This table helps to understand how, as a result of a gradual change in the organs of movement (and respiration), aquatic organisms moved to land, how the fin of a fish was transformed into the limb of amphibians (4), and then reptiles (5). Along with this, the spine and skull of the animal change.
The appearance of the first wingless insects and terrestrial vertebrates belongs to the Devonian period. Hence, it can be assumed that it was at this time, and possibly even somewhat earlier, that the transition of vertebrates from water to land took place.
It was carried out through such fish, in which the swim bladder was changed, like that of lungfish, and the limbs, similar to fins, gradually turned into five-fingered ones, adapted to a terrestrial lifestyle.
Metopoposaurus still struggled to get out on land.
Therefore, the closest ancestors of the first terrestrial animals should be considered not lung-breathers, but precisely lobe-finned fish, adapted to breathing atmospheric air as a result of periodic drying of tropical reservoirs.
The connecting link between terrestrial vertebrates and the lobe-feathered ones is the ancient amphibians, or amphibians, united by the common name stegocephals. Translated from ancient Greek, stegocephaly means “covered heads”: from the words “stege” - roof and “kefale” - head. This name is given because the roof of the skull is an oversized shell of bones closely adjacent to each other.
There are five holes in the skull of the stegocephalus: two pairs of holes - eye and nasal, and one - for the parietal eye. In appearance, stegocephals somewhat resembled salamanders and often reached considerable sizes. They lived in swampy areas.
The remains of stegocephalians were sometimes found in the hollows of tree trunks, where they apparently hid from daylight. In the larval state, they breathed with gills, like modern amphibians.
Stegocephals found especially favorable conditions for their development in the next Carboniferous period.
Carboniferous period
Warm and humid climate, especially in the first half carboniferous period, favored the lush flourishing of terrestrial vegetation. Unseen coal forests, of course, were quite unlike modern ones.
Among those plants that about 275 million years ago settled in the swampy swampy expanses, giant tree-like horsetails and club mosses clearly stood out in their characteristic features.
Of tree-like horsetails, calamites were widely used, and of club mosses, giant lepidodendrons and graceful sigillaria, somewhat inferior to them in size, were widely used.
Well-preserved remnants of vegetation are often found in coal seams and overlying rocks, not only in the form of clear imprints of leaves and tree bark, but also whole stumps with roots and huge trunks turned into coal.
Based on these fossil remains, one can not only restore the general appearance of the plant, but also get acquainted with its internal structure, which is clearly visible under a microscope in the thinnest sections of the trunk, like a sheet of paper. Calamity derives its name from the Latin word "kalamus" - reed, reed.
Slender, hollow inside the trunks of calamites, ribbed and with transverse constrictions, like those of the well-known horsetails, rose in slender columns 20-30 meters from the ground.
Small narrow leaves, collected in rosettes on short stems, gave, perhaps, a certain resemblance to calamite with larch of the Siberian taiga, transparent in its elegant dress.
Nowadays, horsetails - field and forest - are distributed throughout the globe, except for Australia. In comparison with their distant ancestors, they seem to be miserable dwarfs, who, moreover, especially the field horsetail, enjoy a bad reputation with the farmer.
Horsetail is the worst weed, which is difficult to fight, as its rhizome goes deep into the ground and constantly gives new shoots.
Large species of horsetail - up to 10 meters in height are currently preserved only in the tropical forests of South America. However, these giants can only grow by leaning against neighboring trees, since they are only 2-3 centimeters across.
Lepidodendrons and sigillaria occupied a prominent place among the Carboniferous vegetation.
Although in appearance they did not look like modern club mosses, they nevertheless resembled them in one of their characteristic features. The powerful trunks of lepidodendrons, reaching 40 meters in height, with a diameter of up to two meters, were covered with a distinct pattern of fallen leaves.
These leaves, while the plant was still young, sat on the trunk in the same way as its small green scales - leaves - sit on the club moss. As the tree grows, the leaves age and fall off. From these scaly leaves, the giants of the coal forests - lepidodendrons, otherwise - "scaly trees" (from the Greek words: "lepis" - scales and "dendron" - tree) got their name.
Traces of fallen leaves on the bark of sigillaria had a slightly different shape. They differed from lepidodendrons in their smaller height and greater slenderness of the trunk, branching only at the very top and ending in two huge bunches of hard leaves, each meter long.
Acquaintance with the Carboniferous vegetation will be incomplete if we do not also mention cordaites, which are close to conifers in terms of wood structure. These were tall (up to 30 meters), but relatively thin-stemmed trees.
Cordaites derive their name from the Latin elephant "cor" - heart, since the seed of the plant had a heart-shaped shape. These beautiful trees were crowned with a lush crown of ribbon-like leaves (up to 1 meter in length).
Judging by the structure of the wood, the trunks of the coal giants still did not have the strength that is inherent in the bulk of modern trees. Their bark was much stronger than wood, hence the general fragility of the plant, weak resistance to fracture.
Strong winds and especially storms broke trees, felled huge forest tracts, and new lush growth again grew from the swampy soil to replace them ... The felled wood served as the source material from which powerful layers of coal were later formed.
Lepidodendrons, otherwise - scaly trees, reached enormous sizes.
It is not correct to attribute the formation of coal only to the Carboniferous period, since coals also occur in other geological systems.
For example, the oldest Donetsk coal basin was formed in the Carboniferous time. The Karaganda basin is the same age as it.
As for the largest Kuznetsk basin, it only in an insignificant part belongs to the Carboniferous system, and mainly to the Permian and Jurassic systems.
One of the largest basins - "Zapolyarnaya Kochegarka" - the richest Pechora basin, was also formed mainly in the Permian and, to a lesser extent, in the Carboniferous.
Flora and fauna of the Carboniferous period
For marine sediments carboniferous period representatives of the simplest animals from the class rhizopods. The most typical were fusulins (from the Latin word "fuzus" - "spindle") and schwagerins, which served as the source material for the formation of strata of fusulin and schwagerin limestones.
Carboniferous rhizomes: 1 - fuzulina; 2 - schwagerin Carboniferous rhizomes - fuzulina (1) and schwagerina (2) are enlarged 16 times.
Elongated, like grains of wheat, fuzulins and almost spherical schwagerins are clearly visible on the limestones of the same name. Corals and brachiopods have been luxuriantly developed, giving many guiding forms.
The most widespread were the genus productus (translated from Latin - “stretched”) and spirifer (translated from the same language - “carrying a spiral”, which supported the soft “legs” of the animal).
Trilobites that dominated in previous periods are much less common, but on land, other representatives of arthropods - long-legged spiders, scorpions, huge centipedes (up to 75 centimeters in length) and especially giant-shaped insects, similar to dragonflies, with a span of "wings" up to 75 centimeters! The largest modern butterflies in New Guinea and Australia reach a wingspan of 26 centimeters.
Ancient coal dragonfly The oldest coal dragonfly seems to be an exorbitant giant compared to the modern one.
Judging by the fossil remains, sharks have noticeably multiplied in the seas.
Amphibians, firmly entrenched on land in the Carboniferous, go through a further path of development. The dryness of the climate, which increased at the end of the Carboniferous period, gradually forces the ancient amphibians to move away from the aquatic lifestyle and move predominantly to a terrestrial existence.
These organisms, transitional to a new way of life, already laid their eggs on land, and did not spawn in the water, like amphibians. The offspring hatched from the eggs acquired such features that sharply distinguished it from the progenitors.
The body was covered, like a shell, with scale-like outgrowths of the skin, protecting the body from moisture loss through evaporation. So reptiles, or reptiles, separated from amphibians (amphibians). In the next, Mesozoic era, they conquered land, water and air.
Permian period
The last period of the Paleozoic - Permian- in duration was much shorter than the Carboniferous. It should be noted, in addition, the great changes that have taken place on the ancient geographical map of the world - land, as confirmed by geological research, receives a significant predominance over the sea.
Plants of the Permian period
The climate of the northern continents of the Upper Permian was dry and sharply continental. Sandy deserts are widely distributed in places, as evidenced by the composition and reddish hue of the rocks that make up the Permian suite.
This time was marked by the gradual extinction of the giants of the coal forests, the development of plants close to conifers, and the appearance of cycads and ginkgos, which became widespread in the Mesozoic.
Cycad plants have a spherical and tuberous stem immersed in the soil, or, conversely, a powerful columnar trunk up to 20 meters high, with a lush rosette of large pinnate leaves. In appearance, cycad plants resemble the modern sago palm of tropical forests in the Old and New Worlds.
Sometimes they form impenetrable thickets, especially on the flooded banks of the rivers of New Guinea and the Malay Archipelago (Greater Sunda Islands, Lesser Sunda, Moluccas and Philippine). Nutritious flour and cereals (sago) are made from the soft core of the palm tree, which contains starch.
Forest of sigiliaria Sago bread and porridge are the daily food of millions of inhabitants of the Malay Archipelago. The sago palm is widely used in residential construction and for household products.
Another very peculiar plant - ginkgo is also interesting because in the wild it has survived only in some places in southern China. Ginkgo has been carefully bred near Buddhist temples since time immemorial.
Ginkgo was brought to Europe in the middle of the 18th century. Now it is found in park culture in many places, including ours on the Black Sea coast. Ginkgo is a large tree up to 30-40 meters in height and up to two meters thick, in general it resembles a poplar, and in its youth it looks more like some conifers.
Branch of modern ginkgo biloba with fruits The leaves are petiolate, like those of aspen, have a fan-shaped plate with fan-shaped venation without transverse bridges and an incision in the middle. Leaves fall in winter. The fruit, a fragrant drupe like a cherry, is edible in the same way as the seeds. In Europe and Siberia, ginkgo disappeared during the Ice Age.
Cordaites, conifers, cycads and ginkgo belong to the group of gymnosperms (since their seeds lie open).
Angiosperms - monocotyledonous and dicotyledonous - appear somewhat later.
Fauna of the Permian period
Among the aquatic organisms that inhabited the Permian seas, ammonites stood out noticeably. Many groups of marine invertebrates, such as trilobites, some corals, and most brachiopods, have become extinct.
Permian period characterized by the development of reptiles. The so-called animal-like lizards deserve special attention. Although they possessed some features characteristic of mammals, such as teeth and skeletal features, they still retained a primitive structure that brings them closer to stegocephals (from which reptiles originated).
The animal-like Permian lizards differed in significant sizes. The sedentary herbivorous pareiasaurus reached two and a half meters in length, and the formidable predator with the teeth of a tiger, in other words, the "animal-toothed lizard" - foreigners, was even larger - about three meters.
Pareiasaurus, translated from ancient Greek, means “cheeky lizard”: from the words “pareia” - cheek and “sauros” - lizard, lizard; the animal-toothed lizard of foreigners is named so in memory of the famous geologist - prof. A. A. Inostrantseva (1843-1919).
The richest finds of the remains of these animals from the ancient life of the Earth are associated with the name of the enthusiastic geologist prof. V. P. Amalitsky(1860-1917). This persistent researcher, not receiving the necessary support from the treasury, nevertheless achieved remarkable results in his work. Instead of a well-deserved summer vacation, he, along with his wife, who shared all the hardships with him, went in a boat with two rowers in search of the remains of animal-like lizards.
Persistently, for four years he conducted his research on the Sukhona, the Northern Dvina and other rivers. Finally, he managed to make discoveries of exceptional value for world science on the Northern Dvina, not far from the town of Kotlas.
Here, in the coastal cliff of the river, in thick lentils of sand and sandstone, among striped rukhlyak, concretions of bones of ancient animals (concretions - stone accumulations) were found. Gatherings of only one year of work of geologists took two freight cars during transportation.
Subsequent developments of these bone-bearing accumulations further enriched the information about Permian reptiles.
Finding site of Permian lizards Location of Perm pangolins discovered by professor V. P. Amalitsky in 1897. The right bank of the Malaya Severnaya Dvina River near the village of Efimovka, near the town of Kotlas.
The richest collections taken out from here amount to tens of tons, and the skeletons collected from them represent the richest collection in the Paleontological Museum of the Academy of Sciences, which has no equal in any museum in the world.
Among the ancient animal-like Permian reptiles, the original three-meter predator Dimetrodon stood out, otherwise it was “two-dimensional” in length and height (from the ancient Greek words: “di” - twice and “metron” - measure).
Beastlike Dimetrodon Its characteristic feature is the unusually long processes of the vertebrae, forming a high ridge (up to 80 centimeters) on the back of the animal, which were apparently connected by a skin membrane. In addition to predators, this group of reptiles also included plant- or mollusk-eating forms, also of very considerable size. The fact that they ate mollusks can be judged by the arrangement of teeth suitable for crushing and grinding shells. (No ratings yet)
The origin of life on Earth took place about 3.8 billion years ago, when the formation of the earth's crust ended. Scientists have found that the first living organisms appeared in the aquatic environment, and only after a billion years did the first creatures come to the surface of the land.
The formation of terrestrial flora was facilitated by the formation of organs and tissues in plants, the ability to reproduce by spores. Animals also evolved significantly and adapted to life on land: internal fertilization, the ability to lay eggs, and pulmonary respiration appeared. An important stage of development was the formation of the brain, conditioned and unconditioned reflexes, survival instincts. The further evolution of animals provided the basis for the formation of humanity.
The division of the history of the Earth into eras and periods gives an idea of the features of the development of life on the planet in different time periods. Scientists identify particularly significant events in the formation of life on Earth in separate periods of time - eras, which are divided into periods.
There are five eras:
- Archean;
- Proterozoic;
- Paleozoic;
- Mesozoic;
- Cenozoic.
The Archean era began about 4.6 billion years ago, when the planet Earth only began to form and there were no signs of life on it. The air contained chlorine, ammonia, hydrogen, the temperature reached 80 °, the radiation level exceeded the permissible limits, under such conditions the origin of life was impossible.
It is believed that about 4 billion years ago our planet collided with a celestial body, and the result was the formation of the Earth's satellite - the Moon. This event became significant in the development of life, stabilized the axis of rotation of the planet, contributed to the purification of water structures. As a result, the first life originated in the depths of the oceans and seas: protozoa, bacteria and cyanobacteria.
The Proterozoic era lasted from about 2.5 billion years to 540 million years ago. Remains of unicellular algae, mollusks, annelids were found. Soil is starting to form.
The air at the beginning of the era was not yet saturated with oxygen, but in the process of life, the bacteria that inhabit the seas began to release more and more O 2 into the atmosphere. When the amount of oxygen was at a stable level, many creatures took a step in evolution and switched to aerobic respiration.
The Paleozoic era includes six periods.
Cambrian period(530 - 490 million years ago) is characterized by the emergence of representatives of all types of plants and animals. The oceans were inhabited by algae, arthropods, mollusks, and the first chordates (Haikouihthys) appeared. The land remained uninhabited. The temperature remained high.
Ordovician period(490 - 442 million years ago). The first settlements of lichens appeared on land, and the megalograpt (a representative of arthropods) began to come ashore to lay eggs. Vertebrates, corals, sponges continue to develop in the thickness of the ocean.
Silurian(442 - 418 million years ago). Plants come to land, and rudiments of lung tissue form in arthropods. The formation of the bone skeleton in vertebrates is completed, sensory organs appear. Mountain building is underway, different climatic zones are being formed.
Devonian(418 - 353 million years ago). The formation of the first forests, mainly ferns, is characteristic. Bone and cartilaginous organisms appear in water bodies, amphibians began to land on land, new organisms are formed - insects.
Carboniferous period(353 - 290 million years ago). The appearance of amphibians, the sinking of the continents, at the end of the period there was a significant cooling, which led to the extinction of many species.
Permian period(290 - 248 million years ago). The earth is inhabited by reptiles, therapsids appeared - the ancestors of mammals. The hot climate led to the formation of deserts, where only resistant ferns and some conifers could survive.
The Mesozoic era is divided into 3 periods:
Triassic(248 - 200 million years ago). The development of gymnosperms, the appearance of the first mammals. The division of land into continents.
Jurassic period(200 - 140 million years ago). The emergence of angiosperms. The emergence of the ancestors of birds.
Cretaceous period(140 - 65 million years ago). Angiosperms (flowering) became the dominant group of plants. The development of higher mammals, real birds.
The Cenozoic era consists of three periods:
Lower Tertiary period or Paleogene(65 - 24 million years ago). The disappearance of most cephalopods, lemurs and primates appear, later parapithecus and dryopithecus. The development of the ancestors of modern mammalian species - rhinos, pigs, rabbits, etc.
Upper Tertiary or Neogene(24 - 2.6 million years ago). Mammals inhabit land, water and air. The emergence of Australopithecus - the first ancestors of humans. During this period, the Alps, the Himalayas, the Andes were formed.
Quaternary or Anthropogene(2.6 million years ago - today). A significant event of the period is the appearance of man, first Neanderthals, and soon Homo sapiens. The flora and fauna have acquired modern features.
