My theory of human evolution posits that many features of human nature began because they increased specialization and trade. One is language. Language began with single words, I assume. The use of single words began and grew because they helped the two sides of trade find each other. The first language, in other words, was the first advertising. Advertising has two sides: (a) saying what you have too much of and (b) saying what you have too little of.
Single words are still used this way. Stores are often adorned with single words that say what they sell. When you go to an unfamiliar store, you may use single words to find what you want (“thermometers?”). The use of single words to convey desires is clear in a paper by Alexander Graham Bell, which I learned about from Electric Universe by David Bodanis. Bell (the inventor of the telephone) was a teacher of the deaf and wrote a paper about teaching deaf children language. His method involved labeling objects around the house with their names. One of his students was a five-year-old boy:
One morning he came downstairs in high spirits, very anxious to play with his doll. He frantically beat his shoulder with his hands, but I could not understand what he meant. I produced a toy-horse; but that was not what he wanted. A table; still he was disappointed. . . . At last, in desperation, he went to the card-rack, and, after a moment’s consideration, pulled out the word “doll” and presented it to me.
For a different view of why language evolved, see this paper by Hauser, Chomsky and Fitch.