History of the Earth

The age of the earth is estimated at 4.6 billion years. During the course of this immense period of time, the earth evolved from a cloud of cosmic dust, undergoing constant changes into what we know today.
The earliest era, called Precambrian, encompasses the time period from the very first beginnings of the earth to the time when the first rocks appeared. These rocks contained petrified residues from which it was possible to determine the age of one layer of rocks. No petrified remnants dating back to the Precambrian proper were found. It is not possible to provide any precise information concerning the formation of the rocks of that time because, since that time, the earth has been undergoing constant changes. It is possible to classify only certain processes of the origins of the mountains (most importantly in Northern America, Scotland, and Greenland - Laurentian Plateau formation).
The earth originated probably as a result of a formation of clusters of minuscule particles of cosmic dust and gases, where these were constantly bombarded and joined by new mass particles until finally the whole mass began circling around the sun as the third of a total of nine planets. In the beginning, the earth was hot and it dotted with countless volcanoes. It lacked oxygen and atmosphere. During that stage, life on Earth was impossible.
The earth was slowly cooling down, allowing water vapour and gases to escape and create an atmosphere. It started to rain and the oceans formed. It is probable that the first single-cell animals originated already 3.6 billion years ago, most likely by means of the spontaneous joining of molecules. Later on algae and bacteria evolved and, finally, the first multi-cell animals, such as articulates and cnidaria (coelenterata), which are made of simple cavities surrounded by soft tissue.
Approximately 4 billion years ago, the earth had a solid lithosphere. Later, about 3.9 billion years ago, water began to form on its surface. Immense earth masses originated some 3 billion years ago. In the past, their form and distribution had undergone changes. From a single giant continent encompassing the entire surface of the Earth there emerged the first two continents, which drifted away from each other and divided further.
The Precambrian era is followed by the Paleozoic era. Our knowledge concerning this period rests mainly on the evolution of animal life (fauna). Fossils were found in the rocks of the layers of the earth and, by determining their age, it was possible to estimate the age of the individual layers. The most important fossils of this kind are the trilobites and graptolites. However, within this immense time period, there already existed numerous mollusks, crustaceans, first vertebrates, etc., which evolved in ever greater diversity. By the end of the Paleozoic era there existed the first vertebrates - reptiles, which deposited their eggs on the ground. They evolved from the amphibians. In addition, during the Paleozoic era, the mountain range of Ural originated, uniting Europe and Asia into one continental block. The Appalachians also originated near the end of this era.
During the following era, the Mesozoic, which ended 66 million years ago, emerged the American Andes and Rocky Mountains. There occurred a mass extinction of approximately 90 percent of all animal species. For a long time, dinosaurs dominated life on Earth. They included inhabitants of the oceans, flying dinosaurs as well as land animals. However, they were not the only ones who were evolving. First birds and even small mammals began to appear. During the Jurassic period, i.e., the middle part of the Mesozoic era, a major portion of the European continental mass was flooded, but during the period that followed, the Cretaceous, this continent returned to its original form. These floods explain the fact that marine fossils were found in mountainous areas. When the dinosaurs and numerous reptiles became extinct and, with the dawn of the Tertiary period of the newest era of the Earth 65 million years ago, the Cenozoic, a great diversity of mammals began to appear. Some 55 million years ago horses, proboscideans, and artiodactyls (even-toed ungulates or hoofed mammals) first appeared, followed by (38 million years ago) the anthropoids (apes), hogs, deer, and other animals. The mountain ranges of Alps, Pyrenees, Caucasus, and Atlas emerged during this period.
During the Quaternary period of the Cenozoic period (following the Tertiary period), the global climate change occurred. Cold periods alternated with warm periods, giant glaciers and enormous inland masses of ice formed and, in the warm periods that followed, there were great floods as a consequence of the melting of these ice masses. Present-day human emerged at the end of this period.