The bible tells us that god created man, but that since man was bored using his right hand all the time god created a woman to play with man - and play they did. Since god was not too good on contraception the woman became pregnant and later had 2 sons. The sons grew up and also wanted to see some action, but since there were only 4 people in the world their choices were very limited - they could either hump their father, hump their mom, or hump each other. No wonder they got to fighting with each other.
People who believe the story about Adam and Eve must realize that incest is bad. Either all humanity is based on incest or the whole story is rubbish.