I admit I've just read fleetingly about it, but I thought the idea was that the diet humans had existed on for the longest time should be the most healthy, because that's the diet our bodies have had most time to evolve and adapt to. Or have I got it wrong?

I have a few quibbles with this, because evolution doesn't necessarily favour the best possible state of health, it just weeds out the worst cases. So as long as you stayed healthy enough long enough to have as many kids as the Joneses, fat or skinny, your genes were in the running, so to speak

Testing it out gradually and updating it with modern knowledge sounds smart. I like Veronicas name for it!