Granting that most humans are anything but "intelligent" (crafty, sly, scheming, 'learned', cunning, etc; yes. Intelligent, not) you may be onto something. In that case, "like Topsy, it just grew."
OTOH, the few intelligent humans around would be the 'intelligence' behind the evolution of human language.
Back to square one in the argument. That is the square where everyone squares off at each other, rather than displaying intelligence.
AFAIK, language DEVOLVES, as the common people use & abuse it, reducing it to the simplest, crudest form possible to still communicate the barest modicum of everyday drivel.
For "proof" look at emails and SMS-speak, and listen to Liberal talking...
Have you read "Human Devolution" by any chance?