You know what the alternatives are, right? either we have two mouths, or we speak with our nostrils and the silent, gaping hole below only unhinges for the consumption of organic matter.
Thinking about how smell might have developed is kind of cool. Some mutation allowing cells to detect smell (something modern science is kind of puzzled by) then just sending those signals to the brain and saying, you figure it out. I'd bet there is some other property of matter that exists that we have no ability to detect.
Isn't smell just particulate matter hitting receptors not dissimilar to how taste receptors send taste signals to the brain? I thought science had this stuff nailed down now?