According to an article I found today, languages tend to evolve in short bursts. The happens when groups of people coin lots of new words to describe their world.
Researchers have used computer programs normally used to study biologically evolution to study the development of basic vocabulary in 490 languages in Europe, Asia and Africa. They found that many new words appear in a short burst over a few generations when a new language starts to develop.
The new words could arise as way of differentiating new languages from related languages, and also to reinforce group identity. The researchers also suggest that small groups sometimes develop now forms of language based on linguistic idiosyncrasies of their founder members.