The evolution of animals is a process of their consistent and continuous historical development. The driving force behind evolution is natural selection - the survival of the fittest.
According to abiogenic hypotheses about the origin of life on Earth, the first step towards the origin of life on the planet was the synthesis of organic biopolymers. By means of chemical evolution, biopolymers passed to the first living organisms, which developed further along the principles of biological evolution. In the course of this historical development and complication, many forms of life have emerged.
The history of the Earth is subdivided into long time periods - eras: Catarchean, Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic. Paleontology, the science of ancient organisms of past geological eras, helps scientists to obtain data on the development of life on Earth. Fossil remains - shells of mollusks, teeth and scales of fish, shells of eggs, skeletons and other hard parts - are used to study organisms that lived on the planet tens, hundreds of millions of years ago.
It is believed that in the Archean ("ancient") era bacteria dominated the planet, the result of their vital activity was marble, graphite, limestone, etc. Remnants of cyanobacteria capable of oxygen-free photosynthesis were also found in the Archean deposits. At the end of the ancient era, living organisms, according to assumptions, were divided into prokaryotes and eukaryotes.
In the Proterozoic - the era of early life - living organisms continued to grow in complexity, and their ways of feeding and reproduction continued to improve. All life was concentrated in the aquatic environment and along the shores of reservoirs. A wide variety of coelenterates and sponges appeared among animals. Towards the end of the Proterozoic era, all types of invertebrates arose, and the first chordates were skullless. The sediments also contain the remains of worms, molluscs and arthropods. The lancelet is considered the only descendant of the era of early life that has survived to this day.
The Paleozoic is the era of "ancient life". It is distinguished by the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. At the beginning of the Paleozoic, the Cambrian, invertebrates appeared, covered with a hard skeleton built of chitin, calcium carbonate and phosphate, and silica. The fauna was predominantly represented by benthic organisms - coral polyps, sponges, worms, archecytes, echinoderms and arthropods. Trilobites - the oldest arthropods - have reached their greatest flourishing.
The Ordovician is characterized by the strongest flooding of the Earth and the appearance of many swamps. Arthropods and cephalopods were especially widespread during this period, but the first jawless vertebrates also appeared.
In the Silurian, animals and plants came to land. The first land animals were arachnids and centipedes, apparently descended from trilobites. In the Devonian period, primitive jaw-mouth fishes with a cartilaginous skeleton and covered with a shell arose. From them came sharks and cross-finned fish, and from the cross-finned fish, already capable of breathing atmospheric air, the first amphibians (ichthyostegs, stegocephals).
In the Carboniferous period, the period of marshes and vast swamp forests, amphibians flourished and the first insects appeared - cockroaches, dragonflies, coleoptera. Primitive reptiles also appeared, settling in drier places. In Perm, the climate became drier and cooler, which led to the extinction of trilobites, large molluscs, large fish, large insects and arachnids. Reptiles became the most numerous at this time. The ancestors of mammals appeared - therapsids.
In the Mesozoic, there are Triassic, Jurassic and Cretaceous periods. In the Triassic, many reptiles (turtles, ichthyosaurs, crocodiles, dinosaurs, plesiosaurs) and insects arose. At the end of the period, the first representatives of warm-blooded animals appeared. In the Jurassic period, dinosaurs reached their peak, and the first birds similar to reptiles appeared.
In the Cretaceous period, marsupials and placental mammals arose. At the end of the Cretaceous, there was a mass extinction of many animal species - dinosaurs, large reptiles, etc. Scientists attribute this to climate change and general cooling. Warm-blooded animals - birds and mammals - gained advantages in the struggle for survival, which flourished in the Cenozoic - the era of new life, consisting of the periods of the Paleogene, Neogene and Anthropogen.