The living species which existed in simple forms earlier were changed into complex forms with the accumulation of gradual changes in them.
The changing environment leads to changes in the living species.
These changes resulted in the descendant species being quite different from the ancestor species.
Meaning of 'evolution':
The word evolution was derived from a Latin word called 'evolvere' which means to ‘unfold’ or unroll'.
The term refers to the changes that can be brought about in an orderly fashion in organisms that can result in the descendants being entirely different from the ancestors.