Are There Lost Alien Civilizations in Our Past?

In the 1970s, there were a lot of theories floating about that aliens visited Earth and taught humans how to build cities and things. But Kurzgesagt looks at the idea from a different direction. All kinds of creatures and civilizations, including those from another world, could have come and gone and we just don't know about them because they existed too long ago, or their artifacts are something we wouldn't recognize.  

Of course, this is all just conjecture. This video explains how the vast timeline of the Earth makes it difficult to know what may have happened before humans came along, and how events beyond our knowledge can slip by us. It's an exercise in thinking about how little we actually know about the past. A couple hundred million years from now, another civilization may know nothing at all about us. Oh also, the last minute of this video is an ad.