When humans began to seek explanations of their existence, many people thought many different things. The descent of the human race from a tree was one belief. The unfolding of leaves and the expansion of branches were considered a symbol of life.
In some people's imaginations, trees were the homes of wood nymphs and fairies, demons and devils. To some, the world was believed to be born of a vast tree. This was the universe tree, or the world tree.
The world tree acted as a sort of "prop" to the world sky. Its leaves were the clouds and its fruits were the stars. This process gave birth to the closely related tree of knowledge and the tree of life.