One question we often here is... if aliens were here in the ancient past... where is their technology. Not one piece of ancient technology has ever been found (putting aside the famous out of place artifacts). I'm not sure this disqualifies the notion off hand. But it's a serious question.... where is the technology that brought them here?
Obviously, there is more than one species of et visitor who has cruised the Earth. I'm of the notion that the 'teachers', or the ones who from the heavens came. Greeted them with open arms, and probably had an appearance very much the same as our own. They seemed to have been more like missionaries. Teaching different civilizations how to plant a seed, raise cattle, canal building, and construction. As well as the all important... faith. But they didn't do this while their HDD5 Star cruiser was hovering over the village. Do you follow? They came as missionaries. Left here. Because that's what they wanted.
In almost every one of these cultures there is also the explicitly implied intent for 'A Return'.
I don't think they are the ones buzzing our skies in discs, or giant chevrons. The teachers seem to have been here during a time when the Earth was less 'discovered'. About 50,000 years ago seems right. But could be as little as 8 or 9 thousand years ago.
They took the daughters of men as wives.... How many woman you know would lay down with a grey???? no matter how nice his toupee was. That ain't going to happened without some eyes getting gouged out.
So they weren't greys.... they may not have been human but very humanoid in appearance. All the commentators of the past 30 years on this subject constantly try to get us to think of alien life as possibly being totally different than life here on earth.... We might encounter a vapor that is intelligent life in the future (I think Sagan said that once). When almost all examples of life on earth follow the same simple bilateral laws, and sacred geometry. I can't for the life of me think of reason it wouldn't do the very same things on other planets.
I think most everyone is over the whole.... life started here on Earth.