The history of animal evolution, what is the evolutionary process of animals?

At present, there are about 1.5 million known animal species, distributed in all seas and lands on the earth, including mountains, grasslands, deserts, forests, farmland, waters and poles, becoming an inseparable part of nature.

The earliest ancestors of animals are thought to be sponges, which have lived on Earth for at least 5.6 billion years, and sponge fossils dating back about 500 million years have also been discovered.

Amphibians such as frogs and toads were the first vertebrates to land land. Although amphibians have been able to land on land, they are not completely free from the constraints of the water environment, and must also lay eggs in the water to reproduce and spend their childhood.

Continuing from primitive amphibians, reptiles emerged. After the emergence of land, they diverged in all directions, leading to the emergence of higher species of birds and mammals. When mammals such as cats, dogs, rabbits, apes, tigers, etc., continued to develop further, humans finally appeared.

In short, the evolutionary process of animals can be summarized as: from simple to complex, from low to high, from aquatic to terrestrial. Some amphibians evolved into primitive reptiles, and some reptiles evolved into primitive birds and mammals.

The structure of various animals gradually became more complex, and the living environment gradually changed from water to land, and finally completely adapted to life on land.

About 5 billion years ago, the Earth was born. At first, it was a blazing fireball that was lifeless at all. It took a long time for the Earth's temperature to drop before life began to appear.

Through long-term geological and biological investigations, scientists have discovered that living things are constantly evolving. The evolution of animals has evolved step by step from simple to complex, from low to high, from aquatic to terrestrial. The lowest protozoa began to appear, all of which were unicellular and mostly lived in water.

Later, it slowly becomes a multicellular coelenterate and cannot leave the water. Later, it evolved into annelids and arthropods, one more complex than the other. The body of the annelids begins to be segmented, but there are no appendages, they can only squirm, not crawl.

After a long period of evolution, it gradually became an arthropod, not only with segmented bodies, but also with appendages with segments, which were very agile and had the ability to adapt to dry environments. But it was still invertebrates, and it was only a few hundred million years ago that marine vertebrates appeared. From fish to amphibians, reptiles, birds and mammals, their structure is more complex than the other, and their living habits have changed from aquatic to land, and their physiology has also undergone a series of changes.

Evidence of animal evolution has also been found in vertebrates. For example, the wings of birds, the skin membranes of bats, and the fins of whales are all very different in appearance and function, but through dissection and comparison of their internal structure, they are basically the same. The bones of the forelimbs, all of which have humerus, forearm, carpal, metacarpal and phalanges, indicate that their ancestors are in common.

Later, due to the change of living environment, different evolutions took place, the forelimbs of birds and bats became flying organs, and the forelimbs of whales became swimming organs.

The evolution of animals has certain internal and external factors, and it evolves in a certain direction. The heredity and variation of the animal organism are the internal causes of evolution and play a decisive role, and the objective environment is the indispensable condition for evolution and evolution. For example, domestic chickens are changed from original chickens, and domestic chickens retain some characteristics of original chickens, which is genetic.

But the egg production increases, and the weight increases, which is different from the original chicken, which is the variation. This variation is subject to long-term selection, both artificial and natural. People need chickens that lay more eggs and weigh more, so they keep them.

Chickens with few eggs and light weight are killed, so that the targeted selection is carried out from generation to generation, and excellent chicken breeds are selected. At the same time, animals live in nature and must fight against nature. In the struggle, the winner survives, and the loser is naturally eliminated, which is natural selection.

In the natural world, animals have evolved and perfected through continuous struggle and adaptation, forming a wide variety of animal worlds.

Pay more attention to the human and natural and animal world programs, and you will understand a lot.