The image of the “tree of life” was a common one in the near east, crossing many peoples. It symbolised fertility, life, growth, health, sustainability etc…It occurs in literature, and art.
According to The New Jerome Biblical Commentary “a tree of life: A common image in ancient mythology and a recurring metaphor in Prov (cf.11:30, 13:12, 15:4). It recalls the tree of Gen 2:9, which symbolised the human yearning for immortality; here it represents sustenance and healing for a happy life” (pg 456).
Now, you want to hear my own whacky theory?
I reckon the tree of life in the garden of Eden is of the type of Christ, it is a prophecy of the true fountainhead of immortality.