When the earth was new it was God's intention that all people and animals should live together in peace and harmony with God. This meant that no one would ever experience pain or cruelty. God created a special place called the Garden of Eden where his children and animals were to live. There were many trees in the garden, including two that were very special; the tree of the knowledge of good and evil and the tree of life. God also placed Adam and eve and all the animals in the garden. Adam and Eve recognized God and spoke to Him and He spoke to them Everything was perfect. Adam and Eve were allowed to eat the fruit from all of the trees except the tree of the knowledge of good and evil and the tree of life.
Evil must have existed outside of the garden, because one day a serpent came to Eve. He challenged her to eat fruit from the tree of the knowledge of good and evil. He told her that God knew that if she ate this fruit she would be just like God and know the difference between good and evil. Eve wanted to be wise and so she ate some of the fruit and gave some to Adam. He also ate the fruit.
When God realized that Adam and Eve had disobeyed him and that they now knew the difference between good and evil, He became worried that they would eat from the tree of life and live forever. To prevent this from happening God clothed Adam and Eve and sent them away from the Garden of Eden. They were now separated from God and experienced the evil that they now knew existed.
Churches often refer to this as original sin. This story explains how people today came to experience all of the hardship that exists. For further information about this story go to your Bible and read Genesis 3.
Some people say that this story has to be fiction. There was nothing written down until thousands of years after the story took place. Everything was by word of mouth, which, over time, can become confused. This is true, however, this story can be found in many other cultures, not just Christianity. The names are different but the content is the same. We know from our legal system that the more witnesses that agree about what they saw, the more credible the story becomes. The same is true here. The fact that many cultures record the same story says that it must be correct. Many witnesses saying the same thing mean it's true. Check it out.