Chordates acquired in the course of historical development an internal skeleton, a neural tube, perfect muscles, a more perfect circulatory and excretory systems. They have adapted to breeding on land.
The most primitive ancient chordates, similar to modern lancelets, gave rise to fish.
One of the groups of ancient fish (close to the cross-finned) gave rise to terrestrial vertebrates – ancient amphibians, and they gave rise to ancient reptiles.
Ancient reptiles have spread widely on Earth. Ancient mammals and birds later descended from them.
In the process of the historical development of vertebrates, all their organ systems have been improved.
The integuments of the body changed.
The musculoskeletal system was improved.
The organs of movement became more complex.
A transition was made from a two-chambered heart and one circle of blood circulation (in fish) to a three-chamber (in amphibians and reptiles) and four-chamber (in birds and mammals) and two circles of blood circulation.
Primitive large-cell lungs evolved into lungs with a large respiratory surface.
The increase in the level of metabolism in birds and mammals has led to warm-bloodedness.
The nervous system was improved, primarily in the direction of the development of the brain.
Better thermoregulation, brain development, and other adaptations made it possible for birds and mammals to spread widely across the Earth.
The historical development of the animal world led to the emergence of the most highly developed vertebrates – primates, one of the branches of which gave rise to humans.