The big if as far as alien visitors goes is time. in rough numbers the Earth is about 4.5b years old, life first appeared about a billion years after that, and the first real human civilization was only 6,000 years ago, so there very well could have been visitors during that window before or after life here first emerged, but the chances that it happened only since the first civilization appeared, is vanishingly small. OR, they keep coming back, OR multiple visitors from multiple parts of the universe at multiple times. I'd love to know.