A couple points: 1) There is no evidence that GMOs are harmful. So it's not that we "don't know definitely that they're harmful.' This isn't a "could be either one" scenario. Scientific evidence is strongly favoring one side here, and that's the GMOs don't cause harm side. 2) The second part of your sentence is asking for evidence that it isn't harmful. That's not how evidence works. We can only show there is no evidence that something is harmful, not the other way around.