1. Evolution of invertebrates.

The earliest animals on earth were unicellular protozoa.

Multicellular animals evolved from primitive unicellular animals. It is generally believed that multicellular animals originated from primitive flagellates, because many of them show the trend of multicellular development, such as Volvariella volvacea and Aeromonas. ?

Second, the evolution of vertebrates.

The evolution of vertebrates can be divided into three stages: evolution in water, evolution from water to land, evolution of amphibians and reptiles, evolution of birds and mammals.

1. The origin and evolution of fish.

Primitive cephalopods can be divided into two branches. A relatively primitive and jawless group, such as the ancient Ordovician Ankylosaurus, flourished in Silurian and Devonian, and their bodies were covered with thick bone shells, which were quickly eliminated because they could not adapt well. Few eels survive, such as lamprey fish and blind eels. Because they have no jaws, they have to live a semi-parasitic life. The other branch produced the upper and lower jaws, which could live on their own initiative and become the ancestors of fish.

2. The origin and evolution of amphibians and reptiles.

From aquatic organisms to terrestrial organisms, because the ancient total fin fish had inner nostrils and the structure of its lateral fins was similar to that of its five-toed limbs, it gradually became a primitive amphibian due to environmental changes. Earthworms are the ancestors of modern Anura, while another crustacean amphibian has evolved into modern Anura and Anpoda. ?

Reptiles were developed from archaeopteryx amphibians at the end of Carboniferous. Lizard (or Simonosaurus) has many characteristics of ancient amphibians, such as the neck is not obvious, but its occipital condyle, spine and appendages are similar to reptiles, which can be regarded as a type between amphibians and reptiles. Cup dragon seems to be the backbone of reptile ancestors, so all kinds of reptiles are direct or indirect descendants of Cup dragon. ?

3. The origin and evolution of birds and mammals?

Birds and reptiles have many similarities in morphology, such as dry skin, lack of glands, homologous feathers and scales, a occipital condyle in the skull, sheep eggs, in-vivo fertilization and so on. , can explain the genetic relationship between them.

Mammals originated earlier than birds and evolved from primitive reptiles in Paleozoic.

In biology, evolution refers to the change of genetic characteristics in a population between generations.

The so-called character refers to the expression of genes. In the process of reproduction, genes will be copied and passed on to future generations. Gene mutation can change traits, and then lead to genetic variation among individuals. Due to species migration or horizontal gene transfer between species, new traits will be transmitted in the population with genes. When these genetic variations are influenced by non-random natural selection or random genetic drift and become more common or no longer rare in the population, it means that evolution has taken place. In short, the essence of evolution is: the change of population gene frequency.