yes aliens helped build the pyramids and and visited earth many times many years ago. on december 21 2012 the world will not end but the earth will go into a golden age and will may see the aliens come and we will find out who we really are. you heard it first from me here just remember!!!
I think it's arrogant to believe that in the vast universe we are the only living creatures. On the other hand nothing says that they are more or less advanced than us and why of all the places out there would they have visited Earth? I think chances are slim.
I'm DEFINITELY convinced that aliens exist, the universe is simply TOO large to not have any other life out there. But, I think they could have possibly visited before, but I'm not entirely convinced. Not any of that modern day UFO/area 51 bullshit, if they visited they visited a long long time ago.