@Theaetetus,
Well the inner monologue necessarily rose from the development of language. It wouldn't make sense to have an inner monologue without words forming it. Thus, the development of the inner monologue must be a fairly recent development (relatively speaking).
Yes, it isn't a very diffiult question is it? I just love to imagine prehistory, when more of man's thoughts were related to his immediate experience (i.e. no thoughts of molecules, bacteria, stars, UV radiation, other countries, complex natural processes, etc), so I was thinking about a time before the 'tyranny of words', as I like to all it. Anyway...
I actually forgot about visual thought, so I wonder did man before language still think in visual images, and are animals capable of thinking in image? It would seem so based on the reaction of pets to various stimuli, but then again this could just be a form of conditioning.
I think this raises an intersting question; 'what is thought?' What is the relation between conditioning and thought; is one causal of the other? My view is that thought is strictly a product of experiened stimuli and so an animal has thought as a man does, as he experienes stimuli. I think there is nothing essentially different between a man and a chimp, a dog, a mouse, an inset, a tree, a moss, a protists, a baterium and a plume of smoke in terms of thought, except of degree. As Nietzshe said, a crystal 'thinks' by maintaing its form. Anything whih has a struture, whih would be everything, is reactive. The division betwen living and non-living is arbitrary and only a mattter of complexity. I see no way around this idea unless one beleives in a magical soul, which, for no particular reason, we have and other structures do not.