At most you could say that fasting is not obviously harmful, because if it was, at some point someone would have noticed that all the fasting people get sick or die or whatever.
But it's a far jump from there to "very likely to be of use." The body is adapted to withstand all sorts of cultural practices that we no longer think are particularly medically useful, like haircuts, shaving, piercings, circumcision, branding, tattoos, foot binding, neck lengthening, bleeding (leeching), etc.
I dont believe there is evidence for any of those practices predating biologically modern humans. How far back to you have to go to find a life form that did not face constant fear of starvation?
But it's a far jump from there to "very likely to be of use." The body is adapted to withstand all sorts of cultural practices that we no longer think are particularly medically useful, like haircuts, shaving, piercings, circumcision, branding, tattoos, foot binding, neck lengthening, bleeding (leeching), etc.