Reading The Unfolding of Language by Guy Deutscher, who describes how language might have evolved by looking at currently spoken languages and their relatively recent history.

Deutscher talks about the role of metaphor in the creation of language

“…tracing a stream of metaphors that runs right through language and flows from the concrete to the abstract.  In this contstant surge, the simplest and sturdiest of words are swepts along, one after another, and carried toward abstract meanings.  As these words drift downstream, they are bleached of their original vitality and turn into pale lifeless terms for abstract concepts — the substance from which the structure of language is formed.  And when at last the river sinks into the sea, these spent metaphrs are deposited, layer after layer, and so the structure of language grows, as a reef of dead metaphors.” (p. 118)

Even such abstract concepts, such as understanding, are rooted in metaphors of the physical world: “we talk of grasping the sense, catching the meaning, getting the point, following an explanation, cottoning on to an idea, seeing the difficulty, Are you with me?” (p. 128)  Deutscher describes how much of our language is rooted in metaphor, which only later through common use and “erosion” become words for abstract concepts in their own right.  It appears that this happens in all human languages, and the book lists examples from a wide array of modern and ancient language.