Death. This term had never been mentioned until Genesis 2:16,17 when God directly told the first man and woman, Adam and Eve, that if they ate from the tree of knowledge of good and bad, they would violate God's command and would result in death. As a result of Adam's sin, all of humanity suffered because of his disobedience and caused death to spread to all men.
Sin started in heaven with Lucifer. But yes here on earth sin entered though one man Adam and the results of sin brought death to all who are born. But for those who receive it they can have life again though our Lord And Savior Jesus Christ.
No. Death, decay and such came with sin. The world was undying and eternal until sin entered the world. A and E could easily have exisyed on earth for thousands or even millions of years before they ushered sin and death into the world.
Death has been around as long as life. The concept of "sin" was invented by the Abrahamic religions and has nothing to do with the fact that everything that lives, dies. There is a vast amount of scientific evidence for death, but no evidence for the story of "original sin".
The idea that we were suppose to live forever is absurd but let's look at the christian outlook at it. Adam and Eve were suppose to be sinless and would of, according to christian beliefs lived forever. But that only was applicable to humans not animals, so there was death and decay prior to the sin of Adam, and he would have known about death.
Now saying that, Adam and Eve is a myth and never happened because humans have been around for much much longer then 6000 years, and before that there were neanderthals and other human species. There have been humans in one type or another for well over 2 million years.