Before we take step thinking that aliens visited earth , we must discribe the meaning of alien.Purely alien is an out-sider, means it didn't develop on earth from the beginning.and thinking about this more alien can be a small bacteria rather than those movie filmed aliens with such or so weird faces.So i believe aliens did visit earth but not at this times, but far back when our atmosphere was still at her diapers.Take this view of point to other directions