As this process is repeated tens of thousands or hundreds of thousands of times, the structure and shape of the body of living things change. This is called "evolution. Human beings evolved over a long period of time to become the way they are today.
So, why do living things evolve?　 In a word, it is because genes change. Genes act like a blueprint for the creation of a living organism's body. Our bodies are made up of many small cells, and genes are contained within those cells. Genes are passed on from parent to child, just as when a copy is made. However, when genes are copied, sometimes mistakes are made, and genes that are different from those of the parents are passed on to the child. If the genes are different, it is only natural that the offspring will be different from the parents.
Thus, it is believed that living organisms evolve through changes in their genes. It is thought that when monkeys evolved into humans, such genetic copying errors occurred many times over millions of years, resulting in changes in body characteristics.