"The human mind has evolved to wander, not only back and forth in time, but also into imaginary worlds, and into the minds of others. The extent to which this facility occurs in other species remains unknown, although it seems likely that the mind-wandering of humans is considerably more flexible than that of even our closest nonhuman relatives, chimpanzees and bonobos. One reason to suppose that this is so is that language appears to have evolved to allow us to share our mind-wanderings, and there is so far little evidence that anything resembling language exists in other species—at least with respect to its generativity and infinite range of expression. It is of course conceivable that chimpanzees have rich mental lives but lack the means to express their mental wanderings. 

The neurological underpinnings of mind wandering is the default network, identified largely through brain imaging in humans, but seemingly anatomically present in monkeys (Vincent et al., 2007). It is likely that primates have at least some degree of internal processing, independent of external input, but that the default network underwent considerable elaboration in our Homo ancestors."

Michael C. Corballis (2012)