> Those foods can be assumed safe because, by definition, they have been eaten by humans for hundreds of thousands of years

Even prior to GM foods, humans have been engineering food to change it's traits for longer than civilization, and essentially everything humans eat today is a product of such engineering. GM foods differ systematically from other forms fo engineering only on the degree of precision and control possible (which is far from perfect, but much greater than any other known technique.)

Basically nothing humans eat today has been eaten for hundreds of thousands of years.