Have I got a treat for you! Before getting to the special excitement, let me congratulate all of you adapationists for your outstanding performance in the last contest [Calling All Adaptationists]. You did a marvelous job of making up stories to explain homosexuality in humans. Rudyard Kipling would have been proud [Just-So Stories].
Your mission for today, should you choose to accept it, is to explain odorous urine (smelly pee).
Here's the data. Some people produce odorous urine when they eat asparagus. They can transform the chemical, asparagusic acid (1,2-dithiolane-4-carboxylic acid), into smelly compounds like methanethiol, dimethyl sulfide, dimethyl disulfide, bis(methylthio)methane, dimethyl sulfoxide, and dimethyl sulfone (Mitchell, 2001). Other people can eat asparagus 'till the cows come home and their urine remains as pleasantly smelling as usual. The allele for producing the smell is an autosomal dominant trait.
There's an additional complication. Some people can't smell the odorous urine. There's no linkage between excretors and perceivers (able to smell the urine). Some can excrete but not smell and some can excrete and smell. All permutations exist in the population.
This ability to excrete smelly compounds and the ability to smell them are both examples of visible phenotypes. They're not just some neutral variation hiding away in junk DNA. Therefore, many (most?) adaptationists will certainly insist that it has to have an adaptive role in human evolution.
This is untilled soil, as far as I can gather. For some reason the experts haven't published the explanation. At least there's nothing I could find after hunting on the internet. You could become famous if your story makes sense. Let's see what you can come up with. I'll get you started ....
Once upon a time there were naked hunter men running on the savannah while their gatherer wives collected asparagus in the deep water by the sea shore ....
Mitchell, S.C. (2001)
Food Idiosyncrasies: Beetroot and Asparagus. Drug Metabolism and Disposition 4(2):539-543.