Math. The moment Og realized his neighbor Zug had 3 more pelts than Og did, it was on like Donkey Kong.
"And the LORD God commanded the man, “You are free to eat from any tree in the garden; but you must not eat from the tree of the knowledge of good and evil, for when you eat of it you will surely die.”
'Here are eleventy billion trees just don't eat from this ONE'. Math.
But they had to log the food and what they kept in the cave thus writing.
About the Tree... mankind has always done something that required him to be sent to the naughty corner. That is where the concept of time came about. :) How much longer do we have to wander the desert?