Since Heliocentrism was put forward by the Polish astronomer Copernicus in 1543, the discussion on the evolution of celestial bodies has broken through the shackles of religious theology and started a real scientific discussion on the origin of the earth and the solar system. 1644, R. Descartes put forward the first theory of the origin of the solar system in his book Principles of Philosophy. He believes that the sun, planets and satellites are formed in different sizes of eddies formed in the rotating motion of cosmic matter. A century later, Buffon put forward a second theory in 1745' s General and Special Natural History, arguing that a huge object, presumably a comet, collided with the sun, causing the sun's material to split into pieces and fly into space, forming the earth and planets. In fact, because the mass of comets is generally very small, it is impossible to knock out a lot of substances from the sun that are enough to form the earth and planets. In the 200 years after Buffon, many theories have been put forward, all of which tend to Descartes' "monism", that is, the sun and planets are condensed by the same primitive gas cloud; There is also a dualist view that planetary matter is separated from the sun. 1755, I Kant, the founder of the famous German classical philosophy, put forward the "nebula hypothesis". 1796, P.S. Laplace, a famous French mathematician and astronomer, independently put forward another nebula hypothesis about the origin of the solar system in his book On the Cosmic System. Because the theories of Laplace and Kant are consistent in basic arguments, later people call it "Kant-Laplace theory". Throughout the nineteenth century, this theory has been dominant in astronomy.
At the beginning of this century, because Kant-Laplace theory can't satisfactorily explain more and more observational facts in the solar system, "dualism" theory became popular again. 1900, American geologist T. C. Chamberlain put forward the theory of the origin of the solar system, which is called the "planetesimal theory". In the same year, Moreton developed this theory. He believes that once a star moves very close to the sun, it will cause huge tides on the front and back of the sun, thus throwing out a lot of substances, which will gradually condense into many solid lumps or particles, called planetesimals, and further aggregate into planets and satellites.
Modern research shows that Morton's theory is unconvincing because the stars in the universe are far apart and the possibility of colliding with each other is extremely small. Because the common feature of all catastrophe theories is that the origin of the solar system is attributed to some extremely accidental event, which lacks sufficient scientific basis. Dai Wensai, a famous astronomer in China, put forward a new theory of the origin of the solar system in 1979. He believes that the entire solar system is formed by the same primitive nebula. The main components of this nebula are gas and a small amount of solid dust. The original nebula began to rotate, and at the same time, due to its own gravity contraction, it formed a nebula disk, the middle part evolved into the sun, and the edge part formed a nebula and further accreted and evolved into a planet.
Generally speaking, there are more than 40 theories about the origin of the solar system. The catastrophe theory, which became popular rapidly at the beginning of this century, is a challenge to Kant-Laplace Nebula theory. The new nebula theory that appeared in the middle of this century is a more perfect theory to explain the origin of the solar system based on Kant-Laplace theory. People's understanding of the origin of the earth and the solar system is also deepening in this tortuous development process.
At this point, we can give the following possible conclusions about the material and way in which the primitive earth was formed. The material that formed the primitive earth is mainly the original material of the above-mentioned nebula disk, and its components are mainly hydrogen and helium, accounting for about 98% of the total mass. In addition, there are solid dust and matter thrown out by the early contraction and evolution stage of the sun. During the formation of the earth, due to the differentiation of substances, light substances are constantly separated from volatile substances such as hydrogen and helium, and are taken out of the solar system by solar light pressure and substances thrown out by the sun. Therefore, only when heavy matter or earth matter is condensed can the original earth gradually form and evolve into today's earth. Mercury, Venus and Mars, like the Earth, may have been formed in a similar way. Because they are close to the sun, they retain more heavy matter. And Jupiter, Saturn and other outer planets, because they are far away from the sun, still retain more light matter. Although there are still many speculations about the formation of the primitive earth, most researchers agree with Mr. Dai Wensai's conclusion that after the formation of the nebula disk, due to the effect of gravity and the instability of gravity, the materials in the nebula disk, including the dust layer, formed many protoasteroids or planetesimals due to collision accretion, and then gradually evolved into planets, in which the earth was born. It is estimated that the time required for the formation of the earth is about 1 billion years to 1 billion years. Planets closer to the sun (terrestrial planets) take a short time to form, while planets farther from the sun take a long time to form, even reaching hundreds of millions of years.
Scientists have different opinions about whether the primitive earth was hot or cold. Based on the ancient theory of the origin of the earth, most people once thought that the earth was a molten body at first. After billions of years of geological evolution, the earth is still hot. The results of modern research tend to the theory of the origin of the earth's low temperature. Whether the early state of the earth is high or low is still controversial. However, regardless of the origin of high temperature or low temperature, the earth has generally experienced a stage from hot to cold. Because of the heat source inside the earth, this cooling process is extremely slow, and the earth is still in the process of continuous cooling until today.
Second, the evolution of the earth.
The basic outline of the earth's surface can be clearly divided into two parts, namely, the mainland and the ocean basin. The mainland is the highland of the earth's surface, and the ocean basin is a relatively low-lying area, which is filled with huge amounts of seawater. Continental basin and ocean basin together constitute the basic components of the earth's lithosphere. Therefore, the evolution of lithosphere, that is, the tectonic evolution of continental and marine basins. For the structure of the earth's interior, please refer to the section on the structure of the earth's sphere.
At present, most earth scientists have confirmed the phenomenon of continental drift, and agree that the structural distribution and changes of the earth, ocean and land are directly related to continental drift. The relatively hard lithospheric plate moves as a whole on the asthenosphere below it; Due to the relative movement of lithospheric plates, the continental drift has been caused, and the distribution of oceans and land on the earth today has been formed. The earth lithosphere can be divided into marine lithosphere and continental lithosphere. Generally speaking, the thickness of the former is half that of the latter, and the thickness of the marine lithosphere is very uneven, reaching 80 kilometers at the thickest place.
Most large earth plates are composed of continental lithosphere and marine lithosphere, but the huge Pacific plate is composed of a single marine lithosphere. The land area of the earth accounts for about 30% of the whole earth area, of which about 70% is distributed in the northern hemisphere, near the equator and in the mid-latitude area of the northern hemisphere, which is probably related to the depolarization movement of continental rocks caused by the earth's rotation.
On a global scale, almost all the continental crust islands distributed near the mainland are located on the east coast of the mainland, and some eastern edges of the mainland are surrounded by a series of colorful island groups composed of continental crust islands, forming a remarkable island arc protruding eastward. The distribution characteristics of global continental crust islands can be explained by the overall westward movement of lithospheric plates and the expansion of marginal seabed. For a long time, people have noticed that some continental structures on the surface can be pieced together, just like puzzles, especially between the west coast of Africa and the east coast of South America. This phenomenon can be explained by the direct rupture of continental lithosphere and the long-term drift of continental rocks. This is what we will introduce later, about Duto's suggestion that today's continent is formed by the fault drift of Laoya ancient land in the northern hemisphere and Gondwana land near Antarctica.
1966, H. W. Menard and others collected all the data about ocean depth at that time, and made statistics of ocean depth in the world again, and obtained that the global average land altitude was 0.875 km and the average ocean depth was 3.729 km. There is a tens of kilometers wide marginal zone submerged by seawater between the mainland and the ocean, including the continental shelf and continental slope, which together account for 10.9% of the earth's surface area. The difference between continental crust and oceanic crust is very obvious. The chemical composition of continental crust is mainly granite, while the rocks under ocean basin are mainly basalt or gabbro. Therefore, the whole crust can be divided into two types: continental silicon-aluminum crust and oceanic silicon-magnesium crust.
With regard to the origin of continents, geologist and geophysicist A. L. Du Toit put forward a model in his book Our Drifting Continent 1937, which holds that there are two primitive continents. If this model holds, then these two primitive continents are called Lanrasia and Gondwanaland); They are; In fact, as Wei Gena and others argued before, the global continent is just an ancient continent. Du Tote believes that two primitive continents were originally formed near the two poles of the earth, of which Laoya is in the north and Gondwana is in the south. After their formation, they gradually disintegrated and drifted to the position of today's continental block.
As early as the end of 19, geologist E. Suess realized that the geological structures of the continents in the southern hemisphere of the earth were very similar, and merged them into an ancient continent for research, and called it Gondwana continent, which originated from a standard stratigraphic area name (Gondwana) in eastern and central India. Gondwana includes South America, Africa, Madagascar, Arabia, India, Sri Lanka, Antarctica, Australia and New Zealand. They were all formed in the same geological era, and there are the same plant fossils in the rock layers, which are called Gondwana rocks. The main evidence used by Dutot to prove the existence and drift of Laocrow and Gondwana comes from geology, paleontology and paleoclimatology. According to the data accumulated for more than 30 years, Gondwana's theory is proved to be basically correct.
Laoyazhou is a combination of Europe, Asia and North America. Even now, these continents are not far apart. Laoya ancient land has a complicated history of formation and evolution, which is mainly formed by merging several ancient landmasses, including ancient North America landmass, ancient Europe landmass, ancient Siberia landmass and ancient China landmass. In the late Paleozoic (about 300 million years ago), these ancient landmasses were gradually disturbed and collided, and gradually closed from the early and middle Carboniferous to the Permian (that is, 200 million to 270 million years ago). Paleogeology, paleoclimate and paleontology data show that Laoya ancient land was in the middle and low latitudes during Carboniferous-Permian. After the Mesozoic era (that is, the latest 1 billion-200 million years), the Lauya continent gradually fractured and disintegrated, leading to the expansion and formation of the North Atlantic. The study shows that the formation and distribution of the new orogenic belt in the world is the structural result of the drift of Laoya and Gondwana faults. In this process, the law of uneven westward movement and off-pole movement of continental rocks is very obvious. Generally speaking, the Laoya ancient land was once located in the middle and high latitudes of the northern hemisphere, and Gondwana ancient land was once located near the South Pole of the southern hemisphere. The two continents are separated by an area called the ancient Mediterranean (also known as the Tethys geosyncline).
Before Dutt (1937) put forward the theory of Laoya and Gondwana, as early as 19 12, Wei Gena put forward the theory that there is only one primitive continent on the earth, which is called joint ancient land. Wei Gena believes that it was formed in the Carboniferous period (about 220-270 million years ago). Wei Gena took the United ancient land as the starting point of his description of continental drift. However, according to people's current understanding, the unified ancient land proposed by Wei Gena is by no means a primitive continent. Although there are still a large number of people who agree with the idea of uniting ancient lands, their maps of ancient lands restoration are quite different from those put forward by Wei Gena. On the contrary, it is a little close to Dutt's theory of the distribution of two ancient lands.
Continental drift and plate movement in the past 200 million years have been confirmed and widely recognized. However, some people speculate that the plate movement probably started as early as 3 billion years ago, and the plate movement speed is different in different geological periods. Continents are constantly colliding and splicing, and they are also constantly breaking and separating. Many collisions of continental rocks have formed folded mountains, which are connected to form a new continent, and the seabed expands to form a new ocean basin. Therefore, it is very difficult to accurately restore the so-called "drifting before drifting" in the mainland more than 200 million years ago. The age of the earth is 4.6 billion years old. At present, the oldest rock on the earth is known to be 3.7 billion years old, and its distribution area is quite small. In this way, from 4.6 billion to 3.7 billion years, about 900 million years were completely lacking in geological data. In addition, the geological records on the earth 2.5 billion years ago are also very limited, which brings many difficulties to the study of the early history of the earth. Therefore, until now, we have not yet had a unified theory about the early history of the earth.
The origin of the earth can also be traced back to the origin of elements and chemical molecules related to life. Therefore, the origin of life should start from the beginning of the formation of the universe. The so-called "Big Bang" produced carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur and other major elements that make up life. About 6.6 billion years ago, there was a big explosion in the Milky Way. After a long time of gluing, its fragments and loose matter formed the solar system about 4.6 billion years ago. As a member of the solar system, the earth was also formed 4.6 billion years ago. Then, the ice-cold nebula material released a lot of gravitational potential energy, which was converted into kinetic energy and heat energy, leading to an increase in temperature, and the radioactive heat energy of the elements inside the earth also increased, so the original earth was in a molten state. In the process of high temperature earth rotation, the substances in it are differentiated, the heavy elements settle to the center and condense and nucleate, and the lighter substances constitute the mantle and crust, and gradually the circle structure appears. This process takes a long time. The original crust appeared about 3.8 billion years ago, which is consistent with the age of most rocks on the surface of the moon. The origin and evolution of life are closely related to that of the universe. The constituent elements of life, such as carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur, come from the evolution of elements after the Big Bang. The data show that the chemical evolution in the pre-biological stage is not limited to the earth, and the products of chemical evolution exist widely in the universe. In interstellar evolution, some biomolecules, such as amino acids, purines and pyrimidines, may be formed in interstellar dust or condensed nebulae, and then under certain conditions, biopolymers such as peptides and polynucleotides are produced on the planet surface. Through several transitional forms of pre-biological evolution, the most primitive biological system, that is, life with original cell structure, was finally formed on the earth. At this point, the evolution of life began, and countless complex life forms have been produced on the earth until today. 3.8 billion years ago, a stable land mass was formed on the earth, and various evidences show that the liquid hydrosphere is hot and even boiling. Some extremely thermophilic archaea and methanogens may be closest to the oldest life forms on earth, and their metabolic patterns may be chemical and inorganic autotrophs. 3.5 billion years ago, microorganisms in the Wallavo group in Western Australia may be the earliest evidence of life on earth. The appearance of the primitive crust marks that the earth has entered the era of geological development from the era of astronomical planets, and life with primitive cell structure has gradually formed. However, for a long time, there were not many creatures. It was not until the Cambrian 540 million years ago that a large number of metazoans with shells appeared, so the geological age after the Cambrian was called Phanerozoic. In the Middle Ages, it was also very popular in the West that God described in the Bible created everything in seven days. Today, the origin of life is not as described in these ancient legends or myths, but shows that human beings have devoted great enthusiasm and attention to the mystery of the origin of life for a long time. But how should the origin of life happen? How does science explore this eternal mystery? What progress have we made? What other problems remain unsolved? First, the origin of life, the first mystery is the time of life, the time of origin. In the Western Middle Ages, people believed in the story of God creating man in the Bible. 1650, an Irish archbishop calculated the exact time of God's creation according to the description in the Bible, and another priest even calculated the creation time more accurately to 9: 00 a.m. on June 23, 4004 BC. In other words, the origin of life is 6000 years ago, which is certainly not true, but what is it? That's a really scientific answer. How does science answer the time of the origin of life? That is to say, to answer with fossils, fossils are preserved in rocks. We know that after the death of living things, their remains are preserved in rocks under appropriate conditions. We call it a fossil. Rock formations formed in geological history, like a chronicle, are buried deep in these rocks, and older biological fossils are preserved at the bottom of the rock formations. So far, we have found that the oldest biological fossils are rocks from western Australia, about 3.5 billion years ago. These fossils are similar to modern cyanobacteria. They are some primitive life, invisible to the naked eye. Its size is only a few microns to tens of microns, so we can say that the origin of life is not later than 3.5 billion years. At the same time, we know that the earth was formed about 4.6 billion years ago. With these two data, we can see the age of the origin of life, which can be roughly defined between 4.6 billion and 3.5 billion years. Today, with the development of science, geologists believe that in the early days of the earth's formation, the earth was hit by a large number of asteroids and meteorites, which was not suitable for life. It was not so much that there was life on the earth at that time as that it was destroying life, so the origin of life on the earth was not earlier than 4 billion years. In addition, carbon was found in rocks in Greenland 3.85 billion years ago. As we know, there are two kinds of carbon, one is inorganic carbon and the other is organic carbon. In addition, this kind of carbon can be divided into heavy carbon and light carbon, so we can infer the source of these carbons according to the ratio of light carbon to heavy carbon in this kind of carbon. According to the isotope analysis of carbon, scientists speculate that it is organic carbon and comes from organisms. That is to say, in this way, we have greatly shortened the time of the origin of life, that is, between 4 billion and 3.8 billion years, from the origin of life on earth to the present, it is an endless evolutionary history of life.