From the wiki, humans detect at 0.4 ppb and it appears sharks are at about 1 ppm. I can't find a proper source, but several articles say about a teaspoon of blood to an olympic swimming pool. https://www.sharktrust.org/shark-senses
Savannah hominids obviously needed to run to the parking and put up the tops on their convertibles as soon as they scented rain?
By etymology, I'd consider magma to be a good referent for petrichor. Or at least the salty deposits left along contact surfaces of strike-slip faults?
I imagine the evolutionary benefit of the sensitivity is in avoiding drinking stagnant water, where the compound is also present: https://en.wikipedia.org/wiki/Geosmin
The "more people drown in deserts than die of thirst" factoid is not entirely made up; flash floods are incredibly deadly, especially so in areas where it doesn't rain much. Modern humans are quite good at surviving such events but I can definitely see (early) mammals struggle to survive them.
Are individual scent receptors evolutionary preserved, or is there enough "cross-encoding" either at the genetic level or the receptor level itself that preservation of a small number of survival-correlated scent receptors will, coincidentally, cause preservation of other receptors as well?