The basis of the presumption is that if humans have been to exposed to something for 1000 years and we haven't noticed an effect yet, its less likely that we'll

The basis of the presumption is that if humans have been to exposed to something for 1000 years and we haven't noticed an effect yet, its less likely that we'll notice an effect in the future vs a substance for which we have no data about human exposure. Certainly many 'all-natural' chemicals can be highly toxic, but because we have a history of interacting with the plants that produce them, most of these harmful chemicals we already have some idea that they may be dangerous.