"...Q: So what evidence should make us question these early assumptions?
A: A unique study a few years ago calculated that that we could tell a trillion different odors apart. [In another study], researchers laid out a smell trail on a field at [University of California,] Berkeley. They blindfolded undergraduate subjects and gave them earmuffs so they couldn't use any senses other than smell (see video, below). They found the students were perfectly capable of following the trail laid out in the field. The study didn’t test humans and dogs head to head—I think the dogs might win depending on the odor—but what it shows is the limits of human smell tracking haven't really been investigated...."