A study of CT scans of two early Jurassic period mammals reveals the brain evolution had three stages, the first of which was driven by olfaction. To read the original, go here. The next stage is characterized by an increase in tactile sensitivity from body hair and the third stage by skilled muscle movement using the senses (neuromuscular coordination).
Olfaction is different from the other senses in that every time we inhale, we detect odors whether we want to or not. Even mouth-breathing will not prevent detection of odors. A study from 2007 finds that there is a biological explanation for this: olfaction neurons are stimulated by both mechanical (air from respiration) and chemical (odors) stimuli. In the words of the authors from this study:
The mechanosensitivity of our olfactory neurons has two possible functions, suggest the investigators. The first is that it increases our ability to smell, enhancing the detection of odorous molecules in the air. The second is a peripheral drive in the brain to synchronize rhythmic activity, which is the concurrent firing of neurons in the olfactory bulb with breathing.
“The mechanosensitivity may increase the sensitivity of our nose, especially when stimulated by weak odors,” says Ma. “It helps the brain make better sense out of odor responses when it integrates airflow information. We still don’t know how it happens, but sniffing is essential for odor perception.”