For a long time, people have tried to understand how the existing variety of plants and animals appeared on Earth. Gradually, in various sciences (paleontology, comparative morphology, embryology, etc.), facts have accumulated indicating that living organisms on Earth have changed and are changing in the process of their development.
During excavations in various layers of the earth’s crust, fossilized remains of animals, bones and whole skeletons were found, which were very different from the existing ones. Studying the remains of extinct animals, paleontologists (lat. “Paleos” – ancient) found that some species combined in their structure the features of animals of different taxonomic groups, occupying an intermediate position between them.
For example, stegocephals had a number of well-defined features of fish and amphibians.
Signs of fish:
- streamlined body;
- there was a rudimentary caudal fin;
- lateral line organs;
- similar structure of the vertebrae;
- the skull retains the rudimentary bones of the operculum;
- the nostrils, like in lungfish, are displaced to the edge of the mouth.
Signs of amphibians:
- the structure of the shoulder and pelvic girdle;
- limbs of the terrestrial type.
The tooth-toothed lizards had signs of reptiles and mammals.
Over time, paleontologists have identified many intermediate, or transitional, forms, indicating the relationship of animals, the signs of which they combined.
The facts of comparative morphology (from the Greek “morphe” – form) also spoke about the historical change in the animal world. When comparing vertebrates of different taxonomic groups, a general structure plan was revealed. It turned out that in fish, amphibians, reptiles and other vertebrates, the skeleton consists of the skull, spine, girdles of the limbs, the digestive system – of the mouth, pharynx, esophagus, stomach and intestines; nervous system – from the brain and spinal cord and nerves extending from them.
These and other signs of similarity can only be explained by the kinship of all vertebrates with each other and by descent from common ancestors.
The science of the embryonic development of animals – embryology (from the Greek “embryo” – the embryo) – revealed a striking similarity between vertebrates at the early stages of embryo development.
For example, even the embryos of animals have, at an early stage of development, the rudiments of gill slits, as in fish embryos.
Based on the facts of paleontology, comparative morphology, embryology and other sciences, scientists have proposed various hypotheses for the historical development of the animal world, or evolution, of animals.
Most scientists argued that evolution is progressive, from lower forms to higher ones. At the same time, it remained unclear for a long time why low-organized animals (hydras, worms) and highly organized animals (birds, mammals) exist at the same time.
The answer to this question was given by the English scientist Charles Darwin (XIX century), who substantiated the theory of the evolution of the organic world. He explained that not only animals with complex structures survived in the struggle for existence, but also primitive ones, if they had and developed reliable adaptations to their environment.