How was the animal world born on Earth?

About 8 billion years ago, the earth was a hot ball. Gradually, releasing heat into space, it cools, and a hard crust forms on its surface, which has been continuously melted by millions of active volcanoes.

Earth formed as a planet about 4 billion years ago. Erupted by millions of volcanoes, the lava solidified on the surface, forming primary mountains and plateaus, continents and oceanic depressions.

The atmosphere cooled, as did the Earth itself, and heavy rains fell as a result. On the surface of the hot earth, they instantly turned into steam. Solid clouds surrounded the Earth, blocking the passage of sunlight that warmed the surface. The solid crust cools to form the earth, and the ocean basins fill with water. We do not know how the Earth was distributed then, but in any case there was nothing similar to the modern continents in shape or location. The atmosphere was sparse and consisted of swirls of hydrogen, carbon monoxide, ammonia, and methane. There may have been no oxygen at all. This mixture did not retain much of the sun’s ultraviolet radiation and reached The earth’s surface at an intensity that would be fatal to modern life forms. From time to time, monstrous thunderstorms broke out, spilling lightning over land and sea.

As a result of the interaction of a mixture of atmospheric gases with water vapor, electric discharges and ultraviolet rays, complex molecules appeared, including sugars, nucleic acids and amino acids, that is, the building blocks of proteins.

Millions of years passed, the concentration of these substances increased markedly, and the molecules began to interact, forming more and more complex substances. It is possible that some components could have been delivered from space by meteorites. Over time, among a huge variety of chemical compounds, there was one that became crucial for the further development of life. Bacteria appeared. This happened at the end of the Archean epoch, the epoch of the most ancient life.

Initially, the bacteria fed on a variety of carbon compounds that accumulated in the pristine seas for millions of years. But the more bacteria there were, the more quickly these reserves were depleted. It is obvious that the bacteria needed to find a different source of food. Unable to find ready-made food in the environment, the bacteria began to produce it in their own cells, using the energy of sunlight. So the process of photosynthesis was formed. For its flow, in particular, hydrogen is needed – a gas that was released in large quantities during volcanic eruptions.
As time went on, new forms of life emerged that could extract hydrogen from water. Their appearance had a decisive influence on the further development of life: by
when hydrogen is absorbed into the atmosphere, the second element of water, oxygen, is released. The organisms that performed this operation were somewhat more complex in structure than bacteria. This is how the simplest plants – blue-green algae-appeared. This name is explained by the fact that they looked like close relatives of green algae growing in ponds and swamps. The chlorophyll contained in them allowed blue-green algae to use water in the process of photosynthesis. The remains of blue-green algae were found in the oldest deposits of the earth’s crust.

The appearance of blue-green algae was the most important moment in the history of life on earth. The oxygen they released accumulated for hundreds of millions of years, creating The earth’s modern atmosphere, of which it makes up a significant part. The life of all living things depends on oxygen, since it is necessary for breathing. In addition, atmospheric oxygen forms a barrier, a layer of ozone that absorbs almost all of the ultraviolet part of solar radiation.
The next Archean era, the Proterozoic era, lasted 2 billion years. Its name means “era of primary life”.
In the Proterozoic era, single-celled organisms were formed-the simplest. The cells formed colonies as they grouped. From colonial single-celled organisms, whose cells began to perform various functions, the first multicellular organisms occurred. Some of them moved to a sedentary lifestyle and turned into sponge-like organisms. Others began to crawl, moving with the help of cilia. From them came the flatworms. Still others preserved a floating lifestyle, acquired a mouth, and gave rise to coelenterates.

Life was concentrated in the seas, and the land remained lifeless. Along the banks of reservoirs, soil-forming processes began as a result of the activity of bacteria and microscopic algae.

The next era in the development of the earth – the Paleozoic (era of ancient life) – began 600 million years ago. During the Paleozoic era, the organic world conquered the land. Among the animals were the first vertebrates among the plants and spore-bearing conifers.
Marine fauna has developed significantly. The first mollusks, corals, and brush-finned fish appeared. With the help of brush-like fins, the brush-finned fish were able to crawl. Their swim bladder was enriched with blood vessels and served as lungs. Thus, brush-finned fish could breathe air and crawl from lagoon to lagoon in search of food. Brush-finned fish were the ancestors of all land vertebrates.
The ancestors of modern sharks appeared in the oceans, and crustaceans, Scorpions, and insects of various sizes appeared in lakes. So the size of the wings of the ancient dragonflies – meganeura – reached one meter.
The first reptiles appeared. The most characteristic representative of the first reptiles – edaphosaurus. It looked like a huge lizard. Edaphosaurus was a herbivorous lizard and lived near swamps.

There were real predators-pelicosaurs. Among predatory reptiles, forms similar to modern wolves, hyenas, and Martens are developing.


The next era of Earth’s development is the Mesozoic. This is the “middle life era”. In the Mesozoic era, the modern outlines of continents and oceans, modern marine fauna and flora were gradually formed.

The first primitive bony fish appear. Powerful fins, well-developed dental apparatus, perfect shape, strong and light skeleton-all this contributed to the rapid spread of bony fish in the seas of our planet.
Amphibians were represented by stegocephals – sedentary animals with a small body, small limbs and a large head. They lay in the water waiting for their prey, and when the prey approached, they grabbed it.
Other amphibians went on land to hunt insects. These are mastodons. These animals, whose skulls reached one meter in length, in appearance resembled huge frogs.

Reptiles during this period are characterized by a significant variety – these are thecodonts, crocodiles, and dinosaurs. The largest of the dinosaurs was Brachiosaurus, which reached a length of 26 meters and weighed about 50 tons. It had columnar legs, a small head, and a thick, long neck. Brachiosaurs lived on the shores of lakes and fed on aquatic vegetation. Every day, the Brachiosaurus needed at least half a ton of green mass.
For the first time there are flying lizards-pterodactyls. The seas are home to Dolphin-like reptiles-ichthyosaurs.
By the end of the Mesozoic era, sharks gradually acquire a modern appearance. Crocodiles and turtles inhabit freshwater pools. On the territory of modern Europe, there are large lizards with long spines on their backs and huge pythons.
The most terrible predatory lizard was the Tyrannosaurus. It reached a length of 14 meters. It had a huge head (more than a meter in length) and large sharp teeth. The Tyrannosaurus moved on powerful hind legs, leaning on a thick tail. The step of the Tyrannosaurus was 4 meters.
The Cenozoic era-the” era of new life ” following the Mesozoic, began about 67 million years ago and continues into our time. In this era, the modern terrain, climate, atmosphere, animal and plant life, and man were formed.