I was tickled by this study in this week's Science, where they determine exactly how many different smells can us humans distinguish.
They have a very interesting introduction, where they discuss how humans can distinguish up to 7.5 milion colors, and ~300,00 different tones. Up until this paper, it was widely cited that humans could distinguish ~10,000 odors, based off of rounding up of an upper limit found in one study. (Whaaaat, you can just round up like that in scientific literature?!).
To more accurately determine how many odors we can distinguish, the researchers made smell mixtures that consisted of 10, 20, or 30 different components. They varied the proportions of the components, and tested how well people were able to distinguish those variations.
Based on those results, and a bunch of math (I admit I zoned out at this part), they come to the pivotal conclusion:
"Mixtures that overlap by less than 51.17% can be discriminated by the majority of subjects, which means that humans can, on average, discriminate more than 1 trillion mixtures of 30 components."
Of course, there are caveats. They only tested this in 28 individuals, and there was a lot of variation. So who knows how accurate this is for different populations just yet. Still, the magnitude is astounding!
And we have pretty horrible senses of smells compared to many other animals. Imagine their smell resolution!