Yes, clearly language started simple, and then became more complex. Some case/verb endings can clearly be identified as formerly being words of meaning, which then became used as grammatical particles, which then became context specific and absorbed as endings. Then analogy created more endings.
So there has been a process of language starting simple, gaining the greater complexity required for communication in more complex social/cooperative situations (cooperative hunting seems to be sociallly complex enough, if one looks at the mind-bogglingly complex Inuit language), and then later, in some cases, being simplified. Simplification seems to be favoured by lingua franca situations.
This much is uncontroversial. What is more controversial is my suggestion that at some stage in their development, all languages became really complex. I don't really have any proof. I just suspect that when we were all at the same stage of social development as pre-Colombian Inuit, ie, small tribes of hunter gatherers, we all needed more subtle languages, and the easiest way was to become more complex. It took later larger social structures to refine subtle, but less complex, languages out of that.
Edited by: iviehoff