everyday chemistry - Why do chalcogens (Group VI) stink so badly?

For more fun with Dr. Derek Lowe, see this for a primer: Things I Won't Work With - Carbon Diselenide. The short of it is that we carbon-based life forms generally like oxygen. However, move just one row down in the group, to sulfur, and things start going horribly wrong. Sulfur, and the thiol/mercaptan analogs of otherwise relatively benign hydrocarbons and alcohols, gives us the thrilling eaudeur of rotten eggs, burning tires, skunks, farts, etc.

Move one row further, to selenium, and abandon all hope ye who enter here; the tamest common example of selenium in our daily lives is garlic, an instantly recognizable scent, especially when your body starts excreting the selenium-containing essential oil back out of your pores. It's supposed to be the last word in mosquito repellent; down here in Texas the little biters apparently have a taste for Italian. Selenium analogs of thiols often defy chemists' descriptions of their olfactory power; one of Dr. Lowe's other selenium-based posts, on selenophenol (the selenium analog of thiophenol, which is bad enough), puts it thusly: "Imagine 6 skunks wrapped in rubber innertubes and the whole thing is set ablaze; that might approach the metaphysical stench of this material".

This poses the rather interesting question; why do things change so suddenly with this group? I'm willing to believe that all of them have similar chemical properties, and we as humans and other oxygen-respiring life forms differentiate oxygen from the rest of the group because oxygen's good for us while the others are bad. But, that borders on the simplistic, and poses more questions, like why we work so much better on oxygen than sulfur (there are a few bacterial species hanging out near deep-sea vents that just can't get enough of those sulfur gases, so obviously there's sulfur-based "organic" metabolism just as there is oxygen-based). I guess what I'm looking for is an explanation of why two chalcogens, both obeying the octet rule as Periods 2 and 3 tend to, are treated so differently by our sensibilities.

Answer

I think that there are two reasons: the sulfur is quite common in biochemistry and at the same time a lot of sulfur compounds are toxic. The bio-degradation of sulfur compounds mostly leads to some sulfides which very strongly bind transition metals, i.e. they can destroy/block Metalloenzymes. If you add some millions years of evolution it's a v.good feature to smell them (that would also work for small phosphines and amines). Your chances for food-poisoning are lower, chance for survival higher (Long Live "The selfish gene").