Ergothioneine is an amino acid, one that is made by fungi and some bacteria. Humans can’t make it, but it is present in some tissues at fairly high level, and there is a specific transporter (SLC22A4). A more-active form of that gene has been favored by positive selection in Europeans: this could be related to a mostly-wheat diet, since wheat has very little ergothioneine.
A better ergothioneine transporter increases fitness by a few percent, else we wouldn’t have noticed the selection pressure. If logic worked in in biology, we could conclude that ergothioneine is good for you – a vitamin, even though we don’t understand what it does, or the negative consequences of a deficiency. At least some people must have such a deficiency – maybe we should look at people with knocked-out SLC22A4? And if we can’t find any, that says something, eh?
of course, anyone that decided to work this problem – one that we know for sure is relevant to human health – risks the ignominious fate of Adolf Windaus and Albert Szent-Györgyi.