You are all blasphemous fools! The Bible "clearly" states exactly what happened in the Garden of Eden and The Great Flood. Note my capitalization to make them all seem exceedingly important.
IMO, they were all sitting around tripping one evening by a campfire. Noah looked over and saw (hallucinated) some water. Ignoring the paranoia that normally accompanies his frequent "trips," he immediately feared for his life and started gathering animals, both male and female, and boarding them up in this wooden ship.
Unfortunately for the animals, Noah forgot that the circle of life dictates that some animals will eat other animals. After coming down from their high, they quickly opened up the Arc to ensure the animals were ok. Imagine their horror when they saw the slaughter that had taken place inside the Arc. It just so happens that the only animals to survive were the lions and the birds.
Noah and his groupies spent the afternoon dismantling the Arc and cleaning the blood. Noah knew they would need to explain what happened to everyone else. So, to get the inspiration for the story that became "The Great Flood," they tripped again that night.
And so, the story of Noah lived on through the centuries in the world's most well-known collection of tall tales, the Bible.
The End.