How can one species eventually change into something completely different over the course of millions of years?

One species eventually changes into something completely different over the course of millions of years. This can be described using the evolution theory. The evolutionary process of species shows how one population of a species changes over time to the point where that population is distinct and can no longer interbreed with the parent population. Hence one species is changed into another over o period of time. Speciation is an evolutionary process by which a new species comes into being.

This theory of evolution was proposed by Charles Darwin.

the generally accepted parts of modern cell theory include: all known living things are made up of one or more cells. all living cells arise from pre-existing cells by division. the cell is the fundamental unit of structure and function in all living organisms.