Buddha gets his great revelation, that all is suffering and ends in death, while sitting under a Bo tree, also called a wisdom tree. Christ's cross, which in early Christianity was often represented as a tree, symbolizes liberation from death. Jonah learns that everything is born, flourishes briefly and then dies. It's the knowledge that we are all going to die. This is the beginning of all self-knowledge.
Life ends in death.
Where you take it from there depends on a lot of things, but everything starts at the same place. We don't even mention death in the kaddish, our prayer for the dead. But death is real, regardless of what we say, and the Torah makes that clear. The knowledge associated with the eating of the fruit of the Tree of Knowledge is the sensing of the possibility of one's own mortality. It's not that Adam and Eve would have lived forever had they not eaten, but that they would have had no awareness that death as a concept could ever apply to them.