It is not always obvious that metaphor has played this all-important function. But this is because the concrete metaphiers become hidden in phonemic change, leaving the words to exist on their own. Even such an unmetaphorical-sounding word as the verb 'to be' was generated from a metaphor. It comes from the Sanskrit bhu, to grow, or make grow, while the English forms 'am' and 'is' have evolved from the same root as the Sanskrit asmi, to breathe. It is something of a lovely surprise that the irregular conjugation of our most nondescript verb is thus a record of a time when man had no independent word for 'existence' and could only say that something 'grows' or that it breathes.
Book I, Chapter 2, p. 51 - The Origin of Consciousness in the Breakdown of the Bicameral Mind (1976)