Charles Darwin wrote in The Descent of Man:
If no organic being excepting man had possessed any mental power, or if his powers had been of a wholly different nature from those of the lower animals, then we should never have been able toHow convince ourselves that our high faculties had been gradually developed.
Justin Smith elaborates:
If it were not for animals, we would misunderstand our higher faculties; we would take them to have popped into being suddenly and miraculously. We would take ourselves to be angels, and we angels would be deprived of the bulk of our metaphors, which is to say of the bulk of our language, as well as of any bearings or sense of situatedness within any broader community of beings. We would be lonely, inarticulate angels. Instead we are animals that won't shut up ('rational animals', is how this has sometimes been put), and, whether we care to admit it or not, we are not alone.
The Dish 