3 Answers

Maybe aliens are not flesh and blood!

Maybe their ships move faster than light does and there's always the worm holes that can jump them from solar system to solar system. You need to watch more sci-fi. :)

Aliens on Earth ??? ALERT ALERT RED ALERT !!!

10,000 B.C.
Along the Chinese-Tibetan border, the Dropa Stones indicate what some believe to be an ancient alien crash-landing. While exploring some curious caves in the Himalayas, an archeological professor and his students came across small tombs, which held the skeletons of several 4' 4" creatures. There were no gravestones but instead hundreds of foot-long disks with holes in the center and grooves spiraling out from the holes. Beijing's Dr. Tsum Um determined the grooves contained hieroglyphics that told a tale of a group, the Dropas, who crashed on earth. The Dropas attempted to befriend local tribes but were instead killed by the primitive humans who shunned them for their inhuman appearance.

http://www.trutv.com/conspiracy/paranormal/history-of-aliens/gallery.html?curPhoto=4