As for phosphorylation, I've had some folks write to talk about the importance of phosphate cleavages for cellular energy production, and about the conformational effects of phosphorylation. All that's well taken - but I guess what I was getting at yesterday is that (for example) sulfation would seem to be a perfectly reasonable way to modify proteins. Why didn't life end up using it?

Perhaps the phosphate energy part is the key. That's such a basic mechanism that enzymes to handle phosphate groups must be archaic indeed. It could be that evolution just found a use for them, since they were there anyway, and that competing methods of post-transcriptional modification (like sulfation) never got off the ground. Of course, there's always glycosylation - wonder when that kicked in, evolutionarily?