The history of the evolution of the Earth, stretching over many millennia, began essentially from scratch. With the age of the earth's crust about 4.6 billion years, the first signs of life on its barren surface appeared a billion years after its formation. But it took another 3 billion years before the undoubted remains of living beings began to be found in the planet's fossil record.
Evolution timeline
Based on the study of the earth's crust, scientists distinguish three main geological eras that replaced the long awakened Precambrian: Paleozoic (translated from Greek. "Ancient life"), Mesozoic ("middle life"), Cenozoic ("new life"). Each era is subdivided into several periods, and the Cenozoic is also divided into a number of eras.
Although the question of the origin of life is still a subject of controversy and speculation, this topic seriously attracted the attention of scientists and philosophers only after the publication of Darwin's theory of evolution in 1859. Today's paleontologists, possessing modern techniques that allow very accurate measurements, were able to confirm many of Darwin's ingenious assumptions. Especially great progress has been made in the field of determining the age of fossil remains.
During its earliest period (4 billion years) - the Precambrian - the Earth was apparently devoid of life. But even when there was still no oxygen in the atmosphere, the ancient ocean was already accumulating the main components of future life. The first living things were bacteria and unicellular algae, and their appearance, dating back about 3.5 billion years ago, was a turning point in the history of the Earth - it became habitable.
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Following the early "soft-bodied" forms, Cambrian inhabitants, equipped with a hard skeleton, appeared. They also form the basis of the earliest paleontological finds, among which trilobites predominate. The first fish-like vertebrates developed only in the Ordovician. And by the time of the appearance of the early jaw-toothed fish, at the end of the Silurian, marine plants begin to master the coast.
The first terrestrial inhabitants
By the beginning of the Devonian period, living things already lived in the sea and on land. It was a time of great geological transformations: the earth's firmament either rose or formed deep depressions, which eventually led to the formation of high mountain ranges. At the same time, the sea repeatedly conquered the land and retreated again, leaving behind deposits of silt rich in organic matter. And when a lush carpet of vegetation covered the previously bare rocks, the first insects appeared. Later, pulmonary fish emerged from sea waters onto land, and by the end of the Devonian the first amphibians (amphibians) appeared.
In the Carboniferous, the evolution of reptiles (reptiles) began. The representatives of this new group of animals had a more perfect brain and a more progressive general organization in comparison with their amphibian ancestors. In addition, they no longer needed an aquatic environment to lay their eggs.
The ancestors of reptiles - cotylosaurs - served as a source for many new forms, including the most important for us group of animal-like reptiles characteristic of the Permian period. Some of these reptiles that lived in the deserts, later in the Triassic, gave rise to the first mammals.
It is curious that the thecodonts - a group of rather small, but very successful reptiles, later gave the largest dinosaurs that ever lived on Earth. The long-necked vegetarians of the late Jurassic and early Cretaceous period, the 25-meter Diplodocus and Braehiosaurus, weighed over 50 tons. But not all dinosaurs were giants. For example, the carnivorous Podokesaurus was about the size of a chicken.
In modern evolutionary constructions, it is generally accepted that dinosaurs and other large reptiles, including flying pterosaurs, were warm-blooded animals that resembled mammals rather than reptiles in their behavior. From them, in fact, such warm-blooded animals as birds originated, apparently the direct descendants of one of the two orders of dinosaurs. The first mammals of that time were probably monotremes, or oviparous mammals.
The decline of the reptile era
By the end of the Mesozoic, major geological transformations changed the face of the Earth. Gradually, the huge single continent broke up into several separate land areas. Has experienced significant upheavals and evolutionary development of the dominant groups: for reasons not fully understood, dinosaurs and their outlandish compatriots - large sea and flying reptiles - died out. Their disappearance is associated with the onset of a new, Cenozoic, era - the heyday of mammals.
The evolutionary path of mammals ended with the appearance about 1 million years ago of the species Homo erectus - Homo erectus. As you can see, it took nature almost 4 billion years for the first man to appear on our planet.