What is the essence of the doctrine of the evolution of the living world on Earth?

The path of evolution presupposes the constant development and complication of organisms, their transition from simpler to complex. From the point of view of scientists, this process is associated with the fact that all creatures on the planet need to adapt to the constantly changing conditions of the surrounding world, cooling or warming of the climate, etc. So, initially, animals lived only in the aquatic environment, but then they went on land and learned how to get oxygen from the atmosphere. The appearance of man is also associated with the process of evolution, since he descended from animals.